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| author | Arnold D. Robbins <arnold@skeeve.com> | 2010-07-16 12:47:28 +0300 |
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| committer | Arnold D. Robbins <arnold@skeeve.com> | 2010-07-16 12:47:28 +0300 |
| commit | 6719bb6e1c5576e857ab6fc121ec31a75161a3e7 (patch) | |
| tree | 97cba951750ceb73899e48490dbb33674e5b29e1 /doc/gawk.info | |
| parent | 558ba97bdeac5a68bb9248a5c4cdf2feeb24e771 (diff) | |
| download | egawk-6719bb6e1c5576e857ab6fc121ec31a75161a3e7.tar.gz egawk-6719bb6e1c5576e857ab6fc121ec31a75161a3e7.tar.bz2 egawk-6719bb6e1c5576e857ab6fc121ec31a75161a3e7.zip | |
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diff --git a/doc/gawk.info b/doc/gawk.info new file mode 100644 index 00000000..680fbab3 --- /dev/null +++ b/doc/gawk.info @@ -0,0 +1,18232 @@ +This is gawk.info, produced by makeinfo version 4.0 from gawk.texi. + +INFO-DIR-SECTION Programming Languages +START-INFO-DIR-ENTRY +* Gawk: (gawk.info). A Text Scanning and Processing Language. +END-INFO-DIR-ENTRY + + This file documents `awk', a program that you can use to select +particular records in a file and perform operations upon them. + + This is Edition 1.0.1 of `The GNU Awk User's Guide', for the +3.0.1 version of the GNU implementation of AWK. + + Copyright (C) 1989, 1991, 92, 93, 96 Free Software Foundation, Inc. + + Permission is granted to make and distribute verbatim copies of this +manual provided the copyright notice and this permission notice are +preserved on all copies. + + Permission is granted to copy and distribute modified versions of +this manual under the conditions for verbatim copying, provided that +the entire resulting derived work is distributed under the terms of a +permission notice identical to this one. + + Permission is granted to copy and distribute translations of this +manual into another language, under the above conditions for modified +versions, except that this permission notice may be stated in a +translation approved by the Foundation. + + +File: gawk.info, Node: Top, Next: Preface, Prev: (dir), Up: (dir) + +General Introduction +******************** + + This file documents `awk', a program that you can use to select +particular records in a file and perform operations upon them. + + This is Edition 1.0.1 of `The GNU Awk User's Guide', +for the 3.0.1 version of the GNU implementation +of AWK. + +* Menu: + +* Preface:: What this Info file is about; brief + history and acknowledgements. +* What Is Awk:: What is the `awk' language; using this + Info file. +* Getting Started:: A basic introduction to using `awk'. How + to run an `awk' program. Command line + syntax. +* One-liners:: Short, sample `awk' programs. +* Regexp:: All about matching things using regular + expressions. +* Reading Files:: How to read files and manipulate fields. +* Printing:: How to print using `awk'. Describes the + `print' and `printf' statements. + Also describes redirection of output. +* Expressions:: Expressions are the basic building blocks of + statements. +* Patterns and Actions:: Overviews of patterns and actions. +* Statements:: The various control statements are described + in detail. +* Built-in Variables:: Built-in Variables +* Arrays:: The description and use of arrays. Also + includes array-oriented control statements. +* Built-in:: The built-in functions are summarized here. +* User-defined:: User-defined functions are described in + detail. +* Invoking Gawk:: How to run `gawk'. +* Library Functions:: A Library of `awk' Functions. +* Sample Programs:: Many `awk' programs with complete + explanations. +* Language History:: The evolution of the `awk' language. +* Gawk Summary:: `gawk' Options and Language Summary. +* Installation:: Installing `gawk' under various operating + systems. +* Notes:: Something about the implementation of + `gawk'. +* Glossary:: An explanation of some unfamiliar terms. +* Copying:: Your right to copy and distribute `gawk'. +* Index:: Concept and Variable Index. + +* History:: The history of `gawk' and `awk'. +* Manual History:: Brief history of the GNU project and this + Info file. +* Acknowledgements:: Acknowledgements. +* This Manual:: Using this Info file. Includes sample + input files that you can use. +* Conventions:: Typographical Conventions. +* Sample Data Files:: Sample data files for use in the `awk' + programs illustrated in this Info file. +* Names:: What name to use to find `awk'. +* Running gawk:: How to run `gawk' programs; includes + command line syntax. +* One-shot:: Running a short throw-away `awk' program. +* Read Terminal:: Using no input files (input from terminal + instead). +* Long:: Putting permanent `awk' programs in + files. +* Executable Scripts:: Making self-contained `awk' programs. +* Comments:: Adding documentation to `gawk' programs. +* Very Simple:: A very simple example. +* Two Rules:: A less simple one-line example with two rules. +* More Complex:: A more complex example. +* Statements/Lines:: Subdividing or combining statements into + lines. +* Other Features:: Other Features of `awk'. +* When:: When to use `gawk' and when to use other + things. +* Regexp Usage:: How to Use Regular Expressions. +* Escape Sequences:: How to write non-printing characters. +* Regexp Operators:: Regular Expression Operators. +* GNU Regexp Operators:: Operators specific to GNU software. +* Case-sensitivity:: How to do case-insensitive matching. +* Leftmost Longest:: How much text matches. +* Computed Regexps:: Using Dynamic Regexps. +* Records:: Controlling how data is split into records. +* Fields:: An introduction to fields. +* Non-Constant Fields:: Non-constant Field Numbers. +* Changing Fields:: Changing the Contents of a Field. +* Field Separators:: The field separator and how to change it. +* Basic Field Splitting:: How fields are split with single characters or + simple strings. +* Regexp Field Splitting:: Using regexps as the field separator. +* Single Character Fields:: Making each character a separate field. +* Command Line Field Separator:: Setting `FS' from the command line. +* Field Splitting Summary:: Some final points and a summary table. +* Constant Size:: Reading constant width data. +* Multiple Line:: Reading multi-line records. +* Getline:: Reading files under explicit program control + using the `getline' function. +* Getline Intro:: Introduction to the `getline' function. +* Plain Getline:: Using `getline' with no arguments. +* Getline/Variable:: Using `getline' into a variable. +* Getline/File:: Using `getline' from a file. +* Getline/Variable/File:: Using `getline' into a variable from a + file. +* Getline/Pipe:: Using `getline' from a pipe. +* Getline/Variable/Pipe:: Using `getline' into a variable from a + pipe. +* Getline Summary:: Summary Of `getline' Variants. +* Print:: The `print' statement. +* Print Examples:: Simple examples of `print' statements. +* Output Separators:: The output separators and how to change them. +* OFMT:: Controlling Numeric Output With `print'. +* Printf:: The `printf' statement. +* Basic Printf:: Syntax of the `printf' statement. +* Control Letters:: Format-control letters. +* Format Modifiers:: Format-specification modifiers. +* Printf Examples:: Several examples. +* Redirection:: How to redirect output to multiple files and + pipes. +* Special Files:: File name interpretation in `gawk'. + `gawk' allows access to inherited file + descriptors. +* Close Files And Pipes:: Closing Input and Output Files and Pipes. +* Constants:: String, numeric, and regexp constants. +* Scalar Constants:: Numeric and string constants. +* Regexp Constants:: Regular Expression constants. +* Using Constant Regexps:: When and how to use a regexp constant. +* Variables:: Variables give names to values for later use. +* Using Variables:: Using variables in your programs. +* Assignment Options:: Setting variables on the command line and a + summary of command line syntax. This is an + advanced method of input. +* Conversion:: The conversion of strings to numbers and vice + versa. +* Arithmetic Ops:: Arithmetic operations (`+', `-', + etc.) +* Concatenation:: Concatenating strings. +* Assignment Ops:: Changing the value of a variable or a field. +* Increment Ops:: Incrementing the numeric value of a variable. +* Truth Values:: What is ``true'' and what is ``false''. +* Typing and Comparison:: How variables acquire types, and how this + affects comparison of numbers and strings with + `<', etc. +* Boolean Ops:: Combining comparison expressions using boolean + operators `||' (``or''), `&&' + (``and'') and `!' (``not''). +* Conditional Exp:: Conditional expressions select between two + subexpressions under control of a third + subexpression. +* Function Calls:: A function call is an expression. +* Precedence:: How various operators nest. +* Pattern Overview:: What goes into a pattern. +* Kinds of Patterns:: A list of all kinds of patterns. +* Regexp Patterns:: Using regexps as patterns. +* Expression Patterns:: Any expression can be used as a pattern. +* Ranges:: Pairs of patterns specify record ranges. +* BEGIN/END:: Specifying initialization and cleanup rules. +* Using BEGIN/END:: How and why to use BEGIN/END rules. +* I/O And BEGIN/END:: I/O issues in BEGIN/END rules. +* Empty:: The empty pattern, which matches every record. +* Action Overview:: What goes into an action. +* If Statement:: Conditionally execute some `awk' + statements. +* While Statement:: Loop until some condition is satisfied. +* Do Statement:: Do specified action while looping until some + condition is satisfied. +* For Statement:: Another looping statement, that provides + initialization and increment clauses. +* Break Statement:: Immediately exit the innermost enclosing loop. +* Continue Statement:: Skip to the end of the innermost enclosing + loop. +* Next Statement:: Stop processing the current input record. +* Nextfile Statement:: Stop processing the current file. +* Exit Statement:: Stop execution of `awk'. +* User-modified:: Built-in variables that you change to control + `awk'. +* Auto-set:: Built-in variables where `awk' gives you + information. +* ARGC and ARGV:: Ways to use `ARGC' and `ARGV'. +* Array Intro:: Introduction to Arrays +* Reference to Elements:: How to examine one element of an array. +* Assigning Elements:: How to change an element of an array. +* Array Example:: Basic Example of an Array +* Scanning an Array:: A variation of the `for' statement. It + loops through the indices of an array's + existing elements. +* Delete:: The `delete' statement removes an element + from an array. +* Numeric Array Subscripts:: How to use numbers as subscripts in + `awk'. +* Uninitialized Subscripts:: Using Uninitialized variables as subscripts. +* Multi-dimensional:: Emulating multi-dimensional arrays in + `awk'. +* Multi-scanning:: Scanning multi-dimensional arrays. +* Calling Built-in:: How to call built-in functions. +* Numeric Functions:: Functions that work with numbers, including + `int', `sin' and `rand'. +* String Functions:: Functions for string manipulation, such as + `split', `match', and + `sprintf'. +* I/O Functions:: Functions for files and shell commands. +* Time Functions:: Functions for dealing with time stamps. +* Definition Syntax:: How to write definitions and what they mean. +* Function Example:: An example function definition and what it + does. +* Function Caveats:: Things to watch out for. +* Return Statement:: Specifying the value a function returns. +* Options:: Command line options and their meanings. +* Other Arguments:: Input file names and variable assignments. +* AWKPATH Variable:: Searching directories for `awk' programs. +* Obsolete:: Obsolete Options and/or features. +* Undocumented:: Undocumented Options and Features. +* Known Bugs:: Known Bugs in `gawk'. +* Portability Notes:: What to do if you don't have `gawk'. +* Nextfile Function:: Two implementations of a `nextfile' + function. +* Assert Function:: A function for assertions in `awk' + programs. +* Round Function:: A function for rounding if `sprintf' does + not do it correctly. +* Ordinal Functions:: Functions for using characters as numbers and + vice versa. +* Join Function:: A function to join an array into a string. +* Mktime Function:: A function to turn a date into a timestamp. +* Gettimeofday Function:: A function to get formatted times. +* Filetrans Function:: A function for handling data file transitions. +* Getopt Function:: A function for processing command line + arguments. +* Passwd Functions:: Functions for getting user information. +* Group Functions:: Functions for getting group information. +* Library Names:: How to best name private global variables in + library functions. +* Clones:: Clones of common utilities. +* Cut Program:: The `cut' utility. +* Egrep Program:: The `egrep' utility. +* Id Program:: The `id' utility. +* Split Program:: The `split' utility. +* Tee Program:: The `tee' utility. +* Uniq Program:: The `uniq' utility. +* Wc Program:: The `wc' utility. +* Miscellaneous Programs:: Some interesting `awk' programs. +* Dupword Program:: Finding duplicated words in a document. +* Alarm Program:: An alarm clock. +* Translate Program:: A program similar to the `tr' utility. +* Labels Program:: Printing mailing labels. +* Word Sorting:: A program to produce a word usage count. +* History Sorting:: Eliminating duplicate entries from a history + file. +* Extract Program:: Pulling out programs from Texinfo source + files. +* Simple Sed:: A Simple Stream Editor. +* Igawk Program:: A wrapper for `awk' that includes files. +* V7/SVR3.1:: The major changes between V7 and System V + Release 3.1. +* SVR4:: Minor changes between System V Releases 3.1 + and 4. +* POSIX:: New features from the POSIX standard. +* BTL:: New features from the Bell Laboratories + version of `awk'. +* POSIX/GNU:: The extensions in `gawk' not in POSIX + `awk'. +* Command Line Summary:: Recapitulation of the command line. +* Language Summary:: A terse review of the language. +* Variables/Fields:: Variables, fields, and arrays. +* Fields Summary:: Input field splitting. +* Built-in Summary:: `awk''s built-in variables. +* Arrays Summary:: Using arrays. +* Data Type Summary:: Values in `awk' are numbers or strings. +* Rules Summary:: Patterns and Actions, and their component + parts. +* Pattern Summary:: Quick overview of patterns. +* Regexp Summary:: Quick overview of regular expressions. +* Actions Summary:: Quick overview of actions. +* Operator Summary:: `awk' operators. +* Control Flow Summary:: The control statements. +* I/O Summary:: The I/O statements. +* Printf Summary:: A summary of `printf'. +* Special File Summary:: Special file names interpreted internally. +* Built-in Functions Summary:: Built-in numeric and string functions. +* Time Functions Summary:: Built-in time functions. +* String Constants Summary:: Escape sequences in strings. +* Functions Summary:: Defining and calling functions. +* Historical Features:: Some undocumented but supported ``features''. +* Gawk Distribution:: What is in the `gawk' distribution. +* Getting:: How to get the distribution. +* Extracting:: How to extract the distribution. +* Distribution contents:: What is in the distribution. +* Unix Installation:: Installing `gawk' under various versions + of Unix. +* Quick Installation:: Compiling `gawk' under Unix. +* Configuration Philosophy:: How it's all supposed to work. +* VMS Installation:: Installing `gawk' on VMS. +* VMS Compilation:: How to compile `gawk' under VMS. +* VMS Installation Details:: How to install `gawk' under VMS. +* VMS Running:: How to run `gawk' under VMS. +* VMS POSIX:: Alternate instructions for VMS POSIX. +* PC Installation:: Installing and Compiling `gawk' on MS-DOS + and OS/2 +* Atari Installation:: Installing `gawk' on the Atari ST. +* Atari Compiling:: Compiling `gawk' on Atari +* Atari Using:: Running `gawk' on Atari +* Amiga Installation:: Installing `gawk' on an Amiga. +* Bugs:: Reporting Problems and Bugs. +* Other Versions:: Other freely available `awk' + implementations. +* Compatibility Mode:: How to disable certain `gawk' extensions. +* Additions:: Making Additions To `gawk'. +* Adding Code:: Adding code to the main body of `gawk'. +* New Ports:: Porting `gawk' to a new operating system. +* Future Extensions:: New features that may be implemented one day. +* Improvements:: Suggestions for improvements by volunteers. + + To Miriam, for making me complete. + + + To Chana, for the joy you bring us. + + + To Rivka, for the exponential increase. + + + To Nachum, for the added dimension. + + +File: gawk.info, Node: Preface, Next: What Is Awk, Prev: Top, Up: Top + +Preface +******* + + This Info file teaches you about the `awk' language and how you can +use it effectively. You should already be familiar with basic system +commands, such as `cat' and `ls',(1) and basic shell facilities, such +as Input/Output (I/O) redirection and pipes. + + Implementations of the `awk' language are available for many +different computing environments. This Info file, while describing the +`awk' language in general, also describes a particular implementation +of `awk' called `gawk' (which stands for "GNU Awk"). `gawk' runs on a +broad range of Unix systems, ranging from 80386 PC-based computers, up +through large scale systems, such as Crays. `gawk' has also been ported +to MS-DOS and OS/2 PC's, Atari and Amiga micro-computers, and VMS. + +* Menu: + +* History:: The history of `gawk' and `awk'. +* Manual History:: Brief history of the GNU project and this + Info file. +* Acknowledgements:: Acknowledgements. + + ---------- Footnotes ---------- + + (1) These commands are available on POSIX compliant systems, as well +as on traditional Unix based systems. If you are using some other +operating system, you still need to be familiar with the ideas of I/O +redirection and pipes. + + +File: gawk.info, Node: History, Next: Manual History, Prev: Preface, Up: Preface + +History of `awk' and `gawk' +=========================== + + The name `awk' comes from the initials of its designers: Alfred V. +Aho, Peter J. Weinberger, and Brian W. Kernighan. The original version +of `awk' was written in 1977 at AT&T Bell Laboratories. In 1985 a new +version made the programming language more powerful, introducing +user-defined functions, multiple input streams, and computed regular +expressions. This new version became generally available with Unix +System V Release 3.1. The version in System V Release 4 added some new +features and also cleaned up the behavior in some of the "dark corners" +of the language. The specification for `awk' in the POSIX Command +Language and Utilities standard further clarified the language based on +feedback from both the `gawk' designers, and the original Bell Labs +`awk' designers. + + The GNU implementation, `gawk', was written in 1986 by Paul Rubin +and Jay Fenlason, with advice from Richard Stallman. John Woods +contributed parts of the code as well. In 1988 and 1989, David +Trueman, with help from Arnold Robbins, thoroughly reworked `gawk' for +compatibility with the newer `awk'. Current development focuses on bug +fixes, performance improvements, standards compliance, and +occasionally, new features. + + +File: gawk.info, Node: Manual History, Next: Acknowledgements, Prev: History, Up: Preface + +The GNU Project and This Book +============================= + + The Free Software Foundation (FSF) is a non-profit organization +dedicated to the production and distribution of freely distributable +software. It was founded by Richard M. Stallman, the author of the +original Emacs editor. GNU Emacs is the most widely used version of +Emacs today. + + The GNU project is an on-going effort on the part of the Free +Software Foundation to create a complete, freely distributable, POSIX +compliant computing environment. (GNU stands for "GNU's not Unix".) +The FSF uses the "GNU General Public License" (or GPL) to ensure that +source code for their software is always available to the end user. A +copy of the GPL is included for your reference (*note GNU GENERAL +PUBLIC LICENSE: Copying.). The GPL applies to the C language source +code for `gawk'. + + As of this writing (1995), the only major component of the GNU +environment still uncompleted is the operating system kernel, and work +proceeds apace on that. A shell, an editor (Emacs), highly portable +optimizing C, C++, and Objective-C compilers, a symbolic debugger, and +dozens of large and small utilities (such as `gawk'), have all been +completed and are freely available. + + Until the GNU operating system is released, the FSF recommends the +use of Linux, a freely distributable, Unix-like operating system for +80386 and other systems. There are many books on Linux. One freely +available one is `Linux Installation and Getting Started', by Matt +Welsh. Many Linux distributions are available, often in computer +stores or bundled on CD-ROM with books about Linux. Also, the FSF +provides a Linux distribution ("Debian"); contact them for more +information. *Note Getting the `gawk' Distribution: Getting, for the +FSF's contact information. (There are two other freely available, +Unix-like operating systems for 80386 and other systems, NetBSD and +FreeBSD. Both are based on the 4.4-Lite Berkeley Software Distribution, +and both use recent versions of `gawk' for their versions of `awk'.) + + This Info file itself has gone through several previous, preliminary +editions. I started working on a preliminary draft of `The GAWK +Manual', by Diane Close, Paul Rubin, and Richard Stallman in the fall +of 1988. It was around 90 pages long, and barely described the +original, "old" version of `awk'. After substantial revision, the first +version of the `The GAWK Manual' to be released was Edition 0.11 Beta in +October of 1989. The manual then underwent more substantial revision +for Edition 0.13 of December 1991. David Trueman, Pat Rankin, and +Michal Jaegermann contributed sections of the manual for Edition 0.13. +That edition was published by the FSF as a bound book early in 1992. +Since then there have been several minor revisions, notably Edition +0.14 of November 1992 that was published by the FSF in January of 1993, +and Edition 0.16 of August 1993. + + Edition 1.0 of `The GNU Awk User's Guide' represents a significant +re-working of `The GAWK Manual', with much additional material. The +FSF and I agree that I am now the primary author. I also felt that it +needed a more descriptive title. + + `The GNU Awk User's Guide' will undoubtedly continue to evolve. An +electronic version comes with the `gawk' distribution from the FSF. If +you find an error in this Info file, please report it! *Note Reporting +Problems and Bugs: Bugs, for information on submitting problem reports +electronically, or write to me in care of the FSF. + + +File: gawk.info, Node: Acknowledgements, Prev: Manual History, Up: Preface + +Acknowledgements +================ + + I would like to acknowledge Richard M. Stallman, for his vision of a +better world, and for his courage in founding the FSF and starting the +GNU project. + + The initial draft of `The GAWK Manual' had the following +acknowledgements: + + Many people need to be thanked for their assistance in producing + this manual. Jay Fenlason contributed many ideas and sample + programs. Richard Mlynarik and Robert Chassell gave helpful + comments on drafts of this manual. The paper `A Supplemental + Document for `awk'' by John W. Pierce of the Chemistry Department + at UC San Diego, pinpointed several issues relevant both to `awk' + implementation and to this manual, that would otherwise have + escaped us. + + The following people provided many helpful comments on Edition 0.13 +of `The GAWK Manual': Rick Adams, Michael Brennan, Rich Burridge, Diane +Close, Christopher ("Topher") Eliot, Michael Lijewski, Pat Rankin, +Miriam Robbins, and Michal Jaegermann. + + The following people provided many helpful comments for Edition 1.0 +of `The GNU Awk User's Guide': Karl Berry, Michael Brennan, Darrel +Hankerson, Michal Jaegermann, Michael Lijewski, and Miriam Robbins. +Pat Rankin, Michal Jaegermann, Darrel Hankerson and Scott Deifik +updated their respective sections for Edition 1.0. + + Robert J. Chassell provided much valuable advice on the use of +Texinfo. He also deserves special thanks for convincing me _not_ to +title this Info file `How To Gawk Politely'. Karl Berry helped +significantly with the TeX part of Texinfo. + + David Trueman deserves special credit; he has done a yeoman job of +evolving `gawk' so that it performs well, and without bugs. Although +he is no longer involved with `gawk', working with him on this project +was a significant pleasure. + + Scott Deifik, Darrel Hankerson, Kai Uwe Rommel, Pat Rankin, and +Michal Jaegermann (in no particular order) are long time members of the +`gawk' "crack portability team." Without their hard work and help, +`gawk' would not be nearly the fine program it is today. It has been +and continues to be a pleasure working with this team of fine people. + + Jeffrey Friedl provided invaluable help in tracking down a number of +last minute problems with regular expressions in `gawk' 3.0. + + David and I would like to thank Brian Kernighan of Bell Labs for +invaluable assistance during the testing and debugging of `gawk', and +for help in clarifying numerous points about the language. We could +not have done nearly as good a job on either `gawk' or its +documentation without his help. + + I would like to thank Marshall and Elaine Hartholz of Seattle, and +Dr. Bert and Rita Schreiber of Detroit for large amounts of quiet +vacation time in their homes, which allowed me to make significant +progress on this Info file and on `gawk' itself. Phil Hughes of SSC +contributed in a very important way by loaning me his laptop Linux +system, not once, but twice, allowing me to do a lot of work while away +from home. + + Finally, I must thank my wonderful wife, Miriam, for her patience +through the many versions of this project, for her proof-reading, and +for sharing me with the computer. I would like to thank my parents for +their love, and for the grace with which they raised and educated me. +I also must acknowledge my gratitude to G-d, for the many opportunities +He has sent my way, as well as for the gifts He has given me with which +to take advantage of those opportunities. + + + +Arnold Robbins +Atlanta, Georgia +January, 1996 + + +File: gawk.info, Node: What Is Awk, Next: Getting Started, Prev: Preface, Up: Top + +Introduction +************ + + If you are like many computer users, you would frequently like to +make changes in various text files wherever certain patterns appear, or +extract data from parts of certain lines while discarding the rest. To +write a program to do this in a language such as C or Pascal is a +time-consuming inconvenience that may take many lines of code. The job +may be easier with `awk'. + + The `awk' utility interprets a special-purpose programming language +that makes it possible to handle simple data-reformatting jobs with +just a few lines of code. + + The GNU implementation of `awk' is called `gawk'; it is fully upward +compatible with the System V Release 4 version of `awk'. `gawk' is +also upward compatible with the POSIX specification of the `awk' +language. This means that all properly written `awk' programs should +work with `gawk'. Thus, we usually don't distinguish between `gawk' +and other `awk' implementations. + + Using `awk' you can: + + * manage small, personal databases + + * generate reports + + * validate data + + * produce indexes, and perform other document preparation tasks + + * even experiment with algorithms that can be adapted later to other + computer languages + +* Menu: + +* This Manual:: Using this Info file. Includes sample + input files that you can use. +* Conventions:: Typographical Conventions. +* Sample Data Files:: Sample data files for use in the `awk' + programs illustrated in this Info file. + + +File: gawk.info, Node: This Manual, Next: Conventions, Prev: What Is Awk, Up: What Is Awk + +Using This Book +=============== + + The term `awk' refers to a particular program, and to the language +you use to tell this program what to do. When we need to be careful, +we call the program "the `awk' utility" and the language "the `awk' +language." The term `gawk' refers to a version of `awk' developed as +part the GNU project. The purpose of this Info file is to explain both +the `awk' language and how to run the `awk' utility. + + The main purpose of the Info file is to explain the features of +`awk', as defined in the POSIX standard. It does so in the context of +one particular implementation, `gawk'. While doing so, it will also +attempt to describe important differences between `gawk' and other +`awk' implementations. Finally, any `gawk' features that are not in +the POSIX standard for `awk' will be noted. + + The term "`awk' program" refers to a program written by you in the +`awk' programming language. + + *Note Getting Started with `awk': Getting Started, for the bare +essentials you need to know to start using `awk'. + + Some useful "one-liners" are included to give you a feel for the +`awk' language (*note Useful One Line Programs: One-liners.). + + Many sample `awk' programs have been provided for you (*note A +Library of `awk' Functions: Library Functions.; also *note Practical +`awk' Programs: Sample Programs.). + + The entire `awk' language is summarized for quick reference in *Note +`gawk' Summary: Gawk Summary. Look there if you just need to refresh +your memory about a particular feature. + + If you find terms that you aren't familiar with, try looking them up +in the glossary (*note Glossary::). + + Most of the time complete `awk' programs are used as examples, but in +some of the more advanced sections, only the part of the `awk' program +that illustrates the concept being described is shown. + + While this Info file is aimed principally at people who have not been +exposed to `awk', there is a lot of information here that even the `awk' +expert should find useful. In particular, the description of POSIX +`awk', and the example programs in *Note A Library of `awk' Functions: +Library Functions, and *Note Practical `awk' Programs: Sample Programs, +should be of interest. + +Dark Corners +------------ + + Until the POSIX standard (and `The Gawk Manual'), many features of +`awk' were either poorly documented, or not documented at all. +Descriptions of such features (often called "dark corners") are noted +in this Info file with "(d.c.)". They also appear in the index under +the heading "dark corner." + + +File: gawk.info, Node: Conventions, Next: Sample Data Files, Prev: This Manual, Up: What Is Awk + +Typographical Conventions +========================= + + This Info file is written using Texinfo, the GNU documentation +formatting language. A single Texinfo source file is used to produce +both the printed and on-line versions of the documentation. This +section briefly documents the typographical conventions used in Texinfo. + + Examples you would type at the command line are preceded by the +common shell primary and secondary prompts, `$' and `>'. Output from +the command is preceded by the glyph "-|". This typically represents +the command's standard output. Error messages, and other output on the +command's standard error, are preceded by the glyph "error-->". For +example: + + $ echo hi on stdout + -| hi on stdout + $ echo hello on stderr 1>&2 + error--> hello on stderr + + Characters that you type at the keyboard look `like this'. In +particular, there are special characters called "control characters." +These are characters that you type by holding down both the `CONTROL' +key and another key, at the same time. For example, a `Control-d' is +typed by first pressing and holding the `CONTROL' key, next pressing +the `d' key, and finally releasing both keys. + + +File: gawk.info, Node: Sample Data Files, Prev: Conventions, Up: What Is Awk + +Data Files for the Examples +=========================== + + Many of the examples in this Info file take their input from two +sample data files. The first, called `BBS-list', represents a list of +computer bulletin board systems together with information about those +systems. The second data file, called `inventory-shipped', contains +information about shipments on a monthly basis. In both files, each +line is considered to be one "record". + + In the file `BBS-list', each record contains the name of a computer +bulletin board, its phone number, the board's baud rate(s), and a code +for the number of hours it is operational. An `A' in the last column +means the board operates 24 hours a day. A `B' in the last column +means the board operates evening and weekend hours, only. A `C' means +the board operates only on weekends. + + aardvark 555-5553 1200/300 B + alpo-net 555-3412 2400/1200/300 A + barfly 555-7685 1200/300 A + bites 555-1675 2400/1200/300 A + camelot 555-0542 300 C + core 555-2912 1200/300 C + fooey 555-1234 2400/1200/300 B + foot 555-6699 1200/300 B + macfoo 555-6480 1200/300 A + sdace 555-3430 2400/1200/300 A + sabafoo 555-2127 1200/300 C + + The second data file, called `inventory-shipped', represents +information about shipments during the year. Each record contains the +month of the year, the number of green crates shipped, the number of +red boxes shipped, the number of orange bags shipped, and the number of +blue packages shipped, respectively. There are 16 entries, covering +the 12 months of one year and four months of the next year. + + Jan 13 25 15 115 + Feb 15 32 24 226 + Mar 15 24 34 228 + Apr 31 52 63 420 + May 16 34 29 208 + Jun 31 42 75 492 + Jul 24 34 67 436 + Aug 15 34 47 316 + Sep 13 55 37 277 + Oct 29 54 68 525 + Nov 20 87 82 577 + Dec 17 35 61 401 + + Jan 21 36 64 620 + Feb 26 58 80 652 + Mar 24 75 70 495 + Apr 21 70 74 514 + + If you are reading this in GNU Emacs using Info, you can copy the +regions of text showing these sample files into your own test files. +This way you can try out the examples shown in the remainder of this +document. You do this by using the command `M-x write-region' to copy +text from the Info file into a file for use with `awk' (*Note +Miscellaneous File Operations: (emacs)Misc File Ops, for more +information). Using this information, create your own `BBS-list' and +`inventory-shipped' files, and practice what you learn in this Info +file. + + If you are using the stand-alone version of Info, see *Note +Extracting Programs from Texinfo Source Files: Extract Program, for an +`awk' program that will extract these data files from `gawk.texi', the +Texinfo source file for this Info file. + + +File: gawk.info, Node: Getting Started, Next: One-liners, Prev: What Is Awk, Up: Top + +Getting Started with `awk' +************************** + + The basic function of `awk' is to search files for lines (or other +units of text) that contain certain patterns. When a line matches one +of the patterns, `awk' performs specified actions on that line. `awk' +keeps processing input lines in this way until the end of the input +files are reached. + + Programs in `awk' are different from programs in most other +languages, because `awk' programs are "data-driven"; that is, you +describe the data you wish to work with, and then what to do when you +find it. Most other languages are "procedural"; you have to describe, +in great detail, every step the program is to take. When working with +procedural languages, it is usually much harder to clearly describe the +data your program will process. For this reason, `awk' programs are +often refreshingly easy to both write and read. + + When you run `awk', you specify an `awk' "program" that tells `awk' +what to do. The program consists of a series of "rules". (It may also +contain "function definitions", an advanced feature which we will +ignore for now. *Note User-defined Functions: User-defined.) Each +rule specifies one pattern to search for, and one action to perform +when that pattern is found. + + Syntactically, a rule consists of a pattern followed by an action. +The action is enclosed in curly braces to separate it from the pattern. +Rules are usually separated by newlines. Therefore, an `awk' program +looks like this: + + PATTERN { ACTION } + PATTERN { ACTION } + ... + +* Menu: + +* Names:: What name to use to find `awk'. +* Running gawk:: How to run `gawk' programs; includes + command line syntax. +* Very Simple:: A very simple example. +* Two Rules:: A less simple one-line example with two rules. +* More Complex:: A more complex example. +* Statements/Lines:: Subdividing or combining statements into + lines. +* Other Features:: Other Features of `awk'. +* When:: When to use `gawk' and when to use other + things. + + +File: gawk.info, Node: Names, Next: Running gawk, Prev: Getting Started, Up: Getting Started + +A Rose By Any Other Name +======================== + + The `awk' language has evolved over the years. Full details are +provided in *Note The Evolution of the `awk' Language: Language History. +The language described in this Info file is often referred to as "new +`awk'." + + Because of this, many systems have multiple versions of `awk'. Some +systems have an `awk' utility that implements the original version of +the `awk' language, and a `nawk' utility for the new version. Others +have an `oawk' for the "old `awk'" language, and plain `awk' for the +new one. Still others only have one version, usually the new one.(1) + + All in all, this makes it difficult for you to know which version of +`awk' you should run when writing your programs. The best advice we +can give here is to check your local documentation. Look for `awk', +`oawk', and `nawk', as well as for `gawk'. Chances are, you will have +some version of new `awk' on your system, and that is what you should +use when running your programs. (Of course, if you're reading this +Info file, chances are good that you have `gawk'!) + + Throughout this Info file, whenever we refer to a language feature +that should be available in any complete implementation of POSIX `awk', +we simply use the term `awk'. When referring to a feature that is +specific to the GNU implementation, we use the term `gawk'. + + ---------- Footnotes ---------- + + (1) Often, these systems use `gawk' for their `awk' implementation! + + +File: gawk.info, Node: Running gawk, Next: Very Simple, Prev: Names, Up: Getting Started + +How to Run `awk' Programs +========================= + + There are several ways to run an `awk' program. If the program is +short, it is easiest to include it in the command that runs `awk', like +this: + + awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ... + +where PROGRAM consists of a series of patterns and actions, as +described earlier. (The reason for the single quotes is described +below, in *Note One-shot Throw-away `awk' Programs: One-shot.) + + When the program is long, it is usually more convenient to put it in +a file and run it with a command like this: + + awk -f PROGRAM-FILE INPUT-FILE1 INPUT-FILE2 ... + +* Menu: + +* One-shot:: Running a short throw-away `awk' program. +* Read Terminal:: Using no input files (input from terminal + instead). +* Long:: Putting permanent `awk' programs in + files. +* Executable Scripts:: Making self-contained `awk' programs. +* Comments:: Adding documentation to `gawk' programs. + + +File: gawk.info, Node: One-shot, Next: Read Terminal, Prev: Running gawk, Up: Running gawk + +One-shot Throw-away `awk' Programs +---------------------------------- + + Once you are familiar with `awk', you will often type in simple +programs the moment you want to use them. Then you can write the +program as the first argument of the `awk' command, like this: + + awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ... + +where PROGRAM consists of a series of PATTERNS and ACTIONS, as +described earlier. + + This command format instructs the "shell", or command interpreter, +to start `awk' and use the PROGRAM to process records in the input +file(s). There are single quotes around PROGRAM so that the shell +doesn't interpret any `awk' characters as special shell characters. +They also cause the shell to treat all of PROGRAM as a single argument +for `awk' and allow PROGRAM to be more than one line long. + + This format is also useful for running short or medium-sized `awk' +programs from shell scripts, because it avoids the need for a separate +file for the `awk' program. A self-contained shell script is more +reliable since there are no other files to misplace. + + *Note Useful One Line Programs: One-liners, presents several short, +self-contained programs. + + As an interesting side point, the command + + awk '/foo/' FILES ... + +is essentially the same as + + egrep foo FILES ... + + +File: gawk.info, Node: Read Terminal, Next: Long, Prev: One-shot, Up: Running gawk + +Running `awk' without Input Files +--------------------------------- + + You can also run `awk' without any input files. If you type the +command line: + + awk 'PROGRAM' + +then `awk' applies the PROGRAM to the "standard input", which usually +means whatever you type on the terminal. This continues until you +indicate end-of-file by typing `Control-d'. (On other operating +systems, the end-of-file character may be different. For example, on +OS/2 and MS-DOS, it is `Control-z'.) + + For example, the following program prints a friendly piece of advice +(from Douglas Adams' `The Hitchhiker's Guide to the Galaxy'), to keep +you from worrying about the complexities of computer programming +(`BEGIN' is a feature we haven't discussed yet). + + $ awk "BEGIN { print \"Don't Panic!\" }" + -| Don't Panic! + + This program does not read any input. The `\' before each of the +inner double quotes is necessary because of the shell's quoting rules, +in particular because it mixes both single quotes and double quotes. + + This next simple `awk' program emulates the `cat' utility; it copies +whatever you type at the keyboard to its standard output. (Why this +works is explained shortly.) + + $ awk '{ print }' + Now is the time for all good men + -| Now is the time for all good men + to come to the aid of their country. + -| to come to the aid of their country. + Four score and seven years ago, ... + -| Four score and seven years ago, ... + What, me worry? + -| What, me worry? + Control-d + + +File: gawk.info, Node: Long, Next: Executable Scripts, Prev: Read Terminal, Up: Running gawk + +Running Long Programs +--------------------- + + Sometimes your `awk' programs can be very long. In this case it is +more convenient to put the program into a separate file. To tell `awk' +to use that file for its program, you type: + + awk -f SOURCE-FILE INPUT-FILE1 INPUT-FILE2 ... + + The `-f' instructs the `awk' utility to get the `awk' program from +the file SOURCE-FILE. Any file name can be used for SOURCE-FILE. For +example, you could put the program: + + BEGIN { print "Don't Panic!" } + +into the file `advice'. Then this command: + + awk -f advice + +does the same thing as this one: + + awk "BEGIN { print \"Don't Panic!\" }" + +which was explained earlier (*note Running `awk' without Input Files: +Read Terminal.). Note that you don't usually need single quotes around +the file name that you specify with `-f', because most file names don't +contain any of the shell's special characters. Notice that in +`advice', the `awk' program did not have single quotes around it. The +quotes are only needed for programs that are provided on the `awk' +command line. + + If you want to identify your `awk' program files clearly as such, +you can add the extension `.awk' to the file name. This doesn't affect +the execution of the `awk' program, but it does make "housekeeping" +easier. + + +File: gawk.info, Node: Executable Scripts, Next: Comments, Prev: Long, Up: Running gawk + +Executable `awk' Programs +------------------------- + + Once you have learned `awk', you may want to write self-contained +`awk' scripts, using the `#!' script mechanism. You can do this on +many Unix systems(1) (and someday on the GNU system). + + For example, you could update the file `advice' to look like this: + + #! /bin/awk -f + + BEGIN { print "Don't Panic!" } + +After making this file executable (with the `chmod' utility), you can +simply type `advice' at the shell, and the system will arrange to run +`awk'(2) as if you had typed `awk -f advice'. + + $ advice + -| Don't Panic! + +Self-contained `awk' scripts are useful when you want to write a +program which users can invoke without their having to know that the +program is written in `awk'. + + Some older systems do not support the `#!' mechanism. You can get a +similar effect using a regular shell script. It would look something +like this: + + : The colon ensures execution by the standard shell. + awk 'PROGRAM' "$@" + + Using this technique, it is _vital_ to enclose the PROGRAM in single +quotes to protect it from interpretation by the shell. If you omit the +quotes, only a shell wizard can predict the results. + + The `"$@"' causes the shell to forward all the command line +arguments to the `awk' program, without interpretation. The first +line, which starts with a colon, is used so that this shell script will +work even if invoked by a user who uses the C shell. (Not all older +systems obey this convention, but many do.) + + ---------- Footnotes ---------- + + (1) The `#!' mechanism works on Linux systems, Unix systems derived +from Berkeley Unix, System V Release 4, and some System V Release 3 +systems. + + (2) The line beginning with `#!' lists the full file name of an +interpreter to be run, and an optional initial command line argument to +pass to that interpreter. The operating system then runs the +interpreter with the given argument and the full argument list of the +executed program. The first argument in the list is the full file name +of the `awk' program. The rest of the argument list will either be +options to `awk', or data files, or both. + + +File: gawk.info, Node: Comments, Prev: Executable Scripts, Up: Running gawk + +Comments in `awk' Programs +-------------------------- + + A "comment" is some text that is included in a program for the sake +of human readers; it is not really part of the program. Comments can +explain what the program does, and how it works. Nearly all +programming languages have provisions for comments, because programs are +typically hard to understand without their extra help. + + In the `awk' language, a comment starts with the sharp sign +character, `#', and continues to the end of the line. The `#' does not +have to be the first character on the line. The `awk' language ignores +the rest of a line following a sharp sign. For example, we could have +put the following into `advice': + + # This program prints a nice friendly message. It helps + # keep novice users from being afraid of the computer. + BEGIN { print "Don't Panic!" } + + You can put comment lines into keyboard-composed throw-away `awk' +programs also, but this usually isn't very useful; the purpose of a +comment is to help you or another person understand the program at a +later time. + + +File: gawk.info, Node: Very Simple, Next: Two Rules, Prev: Running gawk, Up: Getting Started + +A Very Simple Example +===================== + + The following command runs a simple `awk' program that searches the +input file `BBS-list' for the string of characters: `foo'. (A string +of characters is usually called a "string". The term "string" is +perhaps based on similar usage in English, such as "a string of +pearls," or, "a string of cars in a train.") + + awk '/foo/ { print $0 }' BBS-list + +When lines containing `foo' are found, they are printed, because +`print $0' means print the current line. (Just `print' by itself means +the same thing, so we could have written that instead.) + + You will notice that slashes, `/', surround the string `foo' in the +`awk' program. The slashes indicate that `foo' is a pattern to search +for. This type of pattern is called a "regular expression", and is +covered in more detail later (*note Regular Expressions: Regexp.). The +pattern is allowed to match parts of words. There are single-quotes +around the `awk' program so that the shell won't interpret any of it as +special shell characters. + + Here is what this program prints: + + $ awk '/foo/ { print $0 }' BBS-list + -| fooey 555-1234 2400/1200/300 B + -| foot 555-6699 1200/300 B + -| macfoo 555-6480 1200/300 A + -| sabafoo 555-2127 1200/300 C + + In an `awk' rule, either the pattern or the action can be omitted, +but not both. If the pattern is omitted, then the action is performed +for _every_ input line. If the action is omitted, the default action +is to print all lines that match the pattern. + + Thus, we could leave out the action (the `print' statement and the +curly braces) in the above example, and the result would be the same: +all lines matching the pattern `foo' would be printed. By comparison, +omitting the `print' statement but retaining the curly braces makes an +empty action that does nothing; then no lines would be printed. + + +File: gawk.info, Node: Two Rules, Next: More Complex, Prev: Very Simple, Up: Getting Started + +An Example with Two Rules +========================= + + The `awk' utility reads the input files one line at a time. For +each line, `awk' tries the patterns of each of the rules. If several +patterns match then several actions are run, in the order in which they +appear in the `awk' program. If no patterns match, then no actions are +run. + + After processing all the rules (perhaps none) that match the line, +`awk' reads the next line (however, *note The `next' Statement: Next +Statement., and also *note The `nextfile' Statement: Nextfile +Statement.). This continues until the end of the file is reached. + + For example, the `awk' program: + + /12/ { print $0 } + /21/ { print $0 } + +contains two rules. The first rule has the string `12' as the pattern +and `print $0' as the action. The second rule has the string `21' as +the pattern and also has `print $0' as the action. Each rule's action +is enclosed in its own pair of braces. + + This `awk' program prints every line that contains the string `12' +_or_ the string `21'. If a line contains both strings, it is printed +twice, once by each rule. + + This is what happens if we run this program on our two sample data +files, `BBS-list' and `inventory-shipped', as shown here: + + $ awk '/12/ { print $0 } + > /21/ { print $0 }' BBS-list inventory-shipped + -| aardvark 555-5553 1200/300 B + -| alpo-net 555-3412 2400/1200/300 A + -| barfly 555-7685 1200/300 A + -| bites 555-1675 2400/1200/300 A + -| core 555-2912 1200/300 C + -| fooey 555-1234 2400/1200/300 B + -| foot 555-6699 1200/300 B + -| macfoo 555-6480 1200/300 A + -| sdace 555-3430 2400/1200/300 A + -| sabafoo 555-2127 1200/300 C + -| sabafoo 555-2127 1200/300 C + -| Jan 21 36 64 620 + -| Apr 21 70 74 514 + +Note how the line in `BBS-list' beginning with `sabafoo' was printed +twice, once for each rule. + + +File: gawk.info, Node: More Complex, Next: Statements/Lines, Prev: Two Rules, Up: Getting Started + +A More Complex Example +====================== + + Here is an example to give you an idea of what typical `awk' +programs do. This example shows how `awk' can be used to summarize, +select, and rearrange the output of another utility. It uses features +that haven't been covered yet, so don't worry if you don't understand +all the details. + + ls -lg | awk '$6 == "Nov" { sum += $5 } + END { print sum }' + + This command prints the total number of bytes in all the files in the +current directory that were last modified in November (of any year). +(In the C shell you would need to type a semicolon and then a backslash +at the end of the first line; in a POSIX-compliant shell, such as the +Bourne shell or Bash, the GNU Bourne-Again shell, you can type the +example as shown.) + + The `ls -lg' part of this example is a system command that gives you +a listing of the files in a directory, including file size and the date +the file was last modified. Its output looks like this: + + -rw-r--r-- 1 arnold user 1933 Nov 7 13:05 Makefile + -rw-r--r-- 1 arnold user 10809 Nov 7 13:03 gawk.h + -rw-r--r-- 1 arnold user 983 Apr 13 12:14 gawk.tab.h + -rw-r--r-- 1 arnold user 31869 Jun 15 12:20 gawk.y + -rw-r--r-- 1 arnold user 22414 Nov 7 13:03 gawk1.c + -rw-r--r-- 1 arnold user 37455 Nov 7 13:03 gawk2.c + -rw-r--r-- 1 arnold user 27511 Dec 9 13:07 gawk3.c + -rw-r--r-- 1 arnold user 7989 Nov 7 13:03 gawk4.c + +The first field contains read-write permissions, the second field +contains the number of links to the file, and the third field +identifies the owner of the file. The fourth field identifies the group +of the file. The fifth field contains the size of the file in bytes. +The sixth, seventh and eighth fields contain the month, day, and time, +respectively, that the file was last modified. Finally, the ninth field +contains the name of the file. + + The `$6 == "Nov"' in our `awk' program is an expression that tests +whether the sixth field of the output from `ls -lg' matches the string +`Nov'. Each time a line has the string `Nov' for its sixth field, the +action `sum += $5' is performed. This adds the fifth field (the file +size) to the variable `sum'. As a result, when `awk' has finished +reading all the input lines, `sum' is the sum of the sizes of files +whose lines matched the pattern. (This works because `awk' variables +are automatically initialized to zero.) + + After the last line of output from `ls' has been processed, the +`END' rule is executed, and the value of `sum' is printed. In this +example, the value of `sum' would be 80600. + + These more advanced `awk' techniques are covered in later sections +(*note Overview of Actions: Action Overview.). Before you can move on +to more advanced `awk' programming, you have to know how `awk' +interprets your input and displays your output. By manipulating fields +and using `print' statements, you can produce some very useful and +impressive looking reports. + + +File: gawk.info, Node: Statements/Lines, Next: Other Features, Prev: More Complex, Up: Getting Started + +`awk' Statements Versus Lines +============================= + + Most often, each line in an `awk' program is a separate statement or +separate rule, like this: + + awk '/12/ { print $0 } + /21/ { print $0 }' BBS-list inventory-shipped + + However, `gawk' will ignore newlines after any of the following: + + , { ? : || && do else + +A newline at any other point is considered the end of the statement. +(Splitting lines after `?' and `:' is a minor `gawk' extension. The +`?' and `:' referred to here is the three operand conditional +expression described in *Note Conditional Expressions: Conditional Exp.) + + If you would like to split a single statement into two lines at a +point where a newline would terminate it, you can "continue" it by +ending the first line with a backslash character, `\'. The backslash +must be the final character on the line to be recognized as a +continuation character. This is allowed absolutely anywhere in the +statement, even in the middle of a string or regular expression. For +example: + + awk '/This regular expression is too long, so continue it\ + on the next line/ { print $1 }' + +We have generally not used backslash continuation in the sample programs +in this Info file. Since in `gawk' there is no limit on the length of +a line, it is never strictly necessary; it just makes programs more +readable. For this same reason, as well as for clarity, we have kept +most statements short in the sample programs presented throughout the +Info file. Backslash continuation is most useful when your `awk' +program is in a separate source file, instead of typed in on the +command line. You should also note that many `awk' implementations are +more particular about where you may use backslash continuation. For +example, they may not allow you to split a string constant using +backslash continuation. Thus, for maximal portability of your `awk' +programs, it is best not to split your lines in the middle of a regular +expression or a string. + + *Caution: backslash continuation does not work as described above +with the C shell.* Continuation with backslash works for `awk' +programs in files, and also for one-shot programs _provided_ you are +using a POSIX-compliant shell, such as the Bourne shell or Bash, the +GNU Bourne-Again shell. But the C shell (`csh') behaves differently! +There, you must use two backslashes in a row, followed by a newline. +Note also that when using the C shell, _every_ newline in your awk +program must be escaped with a backslash. To illustrate: + + % awk 'BEGIN { \ + ? print \\ + ? "hello, world" \ + ? }' + -| hello, world + +Here, the `%' and `?' are the C shell's primary and secondary prompts, +analogous to the standard shell's `$' and `>'. + + `awk' is a line-oriented language. Each rule's action has to begin +on the same line as the pattern. To have the pattern and action on +separate lines, you _must_ use backslash continuation--there is no +other way. + + Note that backslash continuation and comments do not mix. As soon as +`awk' sees the `#' that starts a comment, it ignores _everything_ on +the rest of the line. For example: + + $ gawk 'BEGIN { print "dont panic" # a friendly \ + > BEGIN rule + > }' + error--> gawk: cmd. line:2: BEGIN rule + error--> gawk: cmd. line:2: ^ parse error + +Here, it looks like the backslash would continue the comment onto the +next line. However, the backslash-newline combination is never even +noticed, since it is "hidden" inside the comment. Thus, the `BEGIN' is +noted as a syntax error. + + When `awk' statements within one rule are short, you might want to +put more than one of them on a line. You do this by separating the +statements with a semicolon, `;'. + + This also applies to the rules themselves. Thus, the previous +program could have been written: + + /12/ { print $0 } ; /21/ { print $0 } + +*Note:* the requirement that rules on the same line must be separated +with a semicolon was not in the original `awk' language; it was added +for consistency with the treatment of statements within an action. + + +File: gawk.info, Node: Other Features, Next: When, Prev: Statements/Lines, Up: Getting Started + +Other Features of `awk' +======================= + + The `awk' language provides a number of predefined, or built-in +variables, which your programs can use to get information from `awk'. +There are other variables your program can set to control how `awk' +processes your data. + + In addition, `awk' provides a number of built-in functions for doing +common computational and string related operations. + + As we develop our presentation of the `awk' language, we introduce +most of the variables and many of the functions. They are defined +systematically in *Note Built-in Variables::, and *Note Built-in +Functions: Built-in. + + +File: gawk.info, Node: When, Prev: Other Features, Up: Getting Started + +When to Use `awk' +================= + + You might wonder how `awk' might be useful for you. Using utility +programs, advanced patterns, field separators, arithmetic statements, +and other selection criteria, you can produce much more complex output. +The `awk' language is very useful for producing reports from large +amounts of raw data, such as summarizing information from the output of +other utility programs like `ls'. (*Note A More Complex Example: More +Complex.) + + Programs written with `awk' are usually much smaller than they would +be in other languages. This makes `awk' programs easy to compose and +use. Often, `awk' programs can be quickly composed at your terminal, +used once, and thrown away. Since `awk' programs are interpreted, you +can avoid the (usually lengthy) compilation part of the typical +edit-compile-test-debug cycle of software development. + + Complex programs have been written in `awk', including a complete +retargetable assembler for eight-bit microprocessors (*note Glossary::, +for more information) and a microcode assembler for a special purpose +Prolog computer. However, `awk''s capabilities are strained by tasks of +such complexity. + + If you find yourself writing `awk' scripts of more than, say, a few +hundred lines, you might consider using a different programming +language. Emacs Lisp is a good choice if you need sophisticated string +or pattern matching capabilities. The shell is also good at string and +pattern matching; in addition, it allows powerful use of the system +utilities. More conventional languages, such as C, C++, and Lisp, offer +better facilities for system programming and for managing the complexity +of large programs. Programs in these languages may require more lines +of source code than the equivalent `awk' programs, but they are easier +to maintain and usually run more efficiently. + + +File: gawk.info, Node: One-liners, Next: Regexp, Prev: Getting Started, Up: Top + +Useful One Line Programs +************************ + + Many useful `awk' programs are short, just a line or two. Here is a +collection of useful, short programs to get you started. Some of these +programs contain constructs that haven't been covered yet. The +description of the program will give you a good idea of what is going +on, but please read the rest of the Info file to become an `awk' expert! + + Most of the examples use a data file named `data'. This is just a +placeholder; if you were to use these programs yourself, you would +substitute your own file names for `data'. + + Since you are reading this in Info, each line of the example code is +enclosed in quotes, to represent text that you would type literally. +The examples themselves represent shell commands that use single quotes +to keep the shell from interpreting the contents of the program. When +reading the examples, focus on the text between the open and close +quotes. + +`awk '{ if (length($0) > max) max = length($0) }' +` END { print max }' data' + This program prints the length of the longest input line. + +`awk 'length($0) > 80' data' + This program prints every line that is longer than 80 characters. + The sole rule has a relational expression as its pattern, and has + no action (so the default action, printing the record, is used). + +`expand data | awk '{ if (x < length()) x = length() }' +` END { print "maximum line length is " x }'' + This program prints the length of the longest line in `data'. The + input is processed by the `expand' program to change tabs into + spaces, so the widths compared are actually the right-margin + columns. + +`awk 'NF > 0' data' + This program prints every line that has at least one field. This + is an easy way to delete blank lines from a file (or rather, to + create a new file similar to the old file but from which the blank + lines have been deleted). + +`awk 'BEGIN { for (i = 1; i <= 7; i++)' +` print int(101 * rand()) }'' + This program prints seven random numbers from zero to 100, + inclusive. + +`ls -lg FILES | awk '{ x += $5 } ; END { print "total bytes: " x }'' + This program prints the total number of bytes used by FILES. + +`ls -lg FILES | awk '{ x += $5 }' +` END { print "total K-bytes: " (x + 1023)/1024 }'' + This program prints the total number of kilobytes used by FILES. + +`awk -F: '{ print $1 }' /etc/passwd | sort' + This program prints a sorted list of the login names of all users. + +`awk 'END { print NR }' data' + This program counts lines in a file. + +`awk 'NR % 2 == 0' data' + This program prints the even numbered lines in the data file. If + you were to use the expression `NR % 2 == 1' instead, it would + print the odd numbered lines. + + +File: gawk.info, Node: Regexp, Next: Reading Files, Prev: One-liners, Up: Top + +Regular Expressions +******************* + + A "regular expression", or "regexp", is a way of describing a set of +strings. Because regular expressions are such a fundamental part of +`awk' programming, their format and use deserve a separate chapter. + + A regular expression enclosed in slashes (`/') is an `awk' pattern +that matches every input record whose text belongs to that set. + + The simplest regular expression is a sequence of letters, numbers, or +both. Such a regexp matches any string that contains that sequence. +Thus, the regexp `foo' matches any string containing `foo'. Therefore, +the pattern `/foo/' matches any input record containing the three +characters `foo', _anywhere_ in the record. Other kinds of regexps let +you specify more complicated classes of strings. + +* Menu: + +* Regexp Usage:: How to Use Regular Expressions. +* Escape Sequences:: How to write non-printing characters. +* Regexp Operators:: Regular Expression Operators. +* GNU Regexp Operators:: Operators specific to GNU software. +* Case-sensitivity:: How to do case-insensitive matching. +* Leftmost Longest:: How much text matches. +* Computed Regexps:: Using Dynamic Regexps. + + +File: gawk.info, Node: Regexp Usage, Next: Escape Sequences, Prev: Regexp, Up: Regexp + +How to Use Regular Expressions +============================== + + A regular expression can be used as a pattern by enclosing it in +slashes. Then the regular expression is tested against the entire text +of each record. (Normally, it only needs to match some part of the +text in order to succeed.) For example, this prints the second field +of each record that contains the three characters `foo' anywhere in it: + + $ awk '/foo/ { print $2 }' BBS-list + -| 555-1234 + -| 555-6699 + -| 555-6480 + -| 555-2127 + + Regular expressions can also be used in matching expressions. These +expressions allow you to specify the string to match against; it need +not be the entire current input record. The two operators, `~' and +`!~', perform regular expression comparisons. Expressions using these +operators can be used as patterns or in `if', `while', `for', and `do' +statements. (*Note Control Statements in Actions: Statements.) + +`EXP ~ /REGEXP/' + This is true if the expression EXP (taken as a string) is matched + by REGEXP. The following example matches, or selects, all input + records with the upper-case letter `J' somewhere in the first + field: + + $ awk '$1 ~ /J/' inventory-shipped + -| Jan 13 25 15 115 + -| Jun 31 42 75 492 + -| Jul 24 34 67 436 + -| Jan 21 36 64 620 + + So does this: + + awk '{ if ($1 ~ /J/) print }' inventory-shipped + +`EXP !~ /REGEXP/' + This is true if the expression EXP (taken as a character string) + is _not_ matched by REGEXP. The following example matches, or + selects, all input records whose first field _does not_ contain + the upper-case letter `J': + + $ awk '$1 !~ /J/' inventory-shipped + -| Feb 15 32 24 226 + -| Mar 15 24 34 228 + -| Apr 31 52 63 420 + -| May 16 34 29 208 + ... + + When a regexp is written enclosed in slashes, like `/foo/', we call +it a "regexp constant", much like `5.27' is a numeric constant, and +`"foo"' is a string constant. + + +File: gawk.info, Node: Escape Sequences, Next: Regexp Operators, Prev: Regexp Usage, Up: Regexp + +Escape Sequences +================ + + Some characters cannot be included literally in string constants +(`"foo"') or regexp constants (`/foo/'). You represent them instead +with "escape sequences", which are character sequences beginning with a +backslash (`\'). + + One use of an escape sequence is to include a double-quote character +in a string constant. Since a plain double-quote would end the string, +you must use `\"' to represent an actual double-quote character as a +part of the string. For example: + + $ awk 'BEGIN { print "He said \"hi!\" to her." }' + -| He said "hi!" to her. + + The backslash character itself is another character that cannot be +included normally; you write `\\' to put one backslash in the string or +regexp. Thus, the string whose contents are the two characters `"' and +`\' must be written `"\"\\"'. + + Another use of backslash is to represent unprintable characters such +as tab or newline. While there is nothing to stop you from entering +most unprintable characters directly in a string constant or regexp +constant, they may look ugly. + + Here is a table of all the escape sequences used in `awk', and what +they represent. Unless noted otherwise, all of these escape sequences +apply to both string constants and regexp constants. + +`\\' + A literal backslash, `\'. + +`\a' + The "alert" character, `Control-g', ASCII code 7 (BEL). + +`\b' + Backspace, `Control-h', ASCII code 8 (BS). + +`\f' + Formfeed, `Control-l', ASCII code 12 (FF). + +`\n' + Newline, `Control-j', ASCII code 10 (LF). + +`\r' + Carriage return, `Control-m', ASCII code 13 (CR). + +`\t' + Horizontal tab, `Control-i', ASCII code 9 (HT). + +`\v' + Vertical tab, `Control-k', ASCII code 11 (VT). + +`\NNN' + The octal value NNN, where NNN are one to three digits between `0' + and `7'. For example, the code for the ASCII ESC (escape) + character is `\033'. + +`\xHH...' + The hexadecimal value HH, where HH are hexadecimal digits (`0' + through `9' and either `A' through `F' or `a' through `f'). Like + the same construct in ANSI C, the escape sequence continues until + the first non-hexadecimal digit is seen. However, using more than + two hexadecimal digits produces undefined results. (The `\x' + escape sequence is not allowed in POSIX `awk'.) + +`\/' + A literal slash (necessary for regexp constants only). You use + this when you wish to write a regexp constant that contains a + slash. Since the regexp is delimited by slashes, you need to + escape the slash that is part of the pattern, in order to tell + `awk' to keep processing the rest of the regexp. + +`\"' + A literal double-quote (necessary for string constants only). You + use this when you wish to write a string constant that contains a + double-quote. Since the string is delimited by double-quotes, you + need to escape the quote that is part of the string, in order to + tell `awk' to keep processing the rest of the string. + + In `gawk', there are additional two character sequences that begin +with backslash that have special meaning in regexps. *Note Additional +Regexp Operators Only in `gawk': GNU Regexp Operators. + + In a string constant, what happens if you place a backslash before +something that is not one of the characters listed above? POSIX `awk' +purposely leaves this case undefined. There are two choices. + + * Strip the backslash out. This is what Unix `awk' and `gawk' both + do. For example, `"a\qc"' is the same as `"aqc"'. + + * Leave the backslash alone. Some other `awk' implementations do + this. In such implementations, `"a\qc"' is the same as if you had + typed `"a\\qc"'. + + In a regexp, a backslash before any character that is not in the +above table, and not listed in *Note Additional Regexp Operators Only +in `gawk': GNU Regexp Operators, means that the next character should +be taken literally, even if it would normally be a regexp operator. +E.g., `/a\+b/' matches the three characters `a+b'. + + For complete portability, do not use a backslash before any +character not listed in the table above. + + Another interesting question arises. Suppose you use an octal or +hexadecimal escape to represent a regexp metacharacter (*note Regular +Expression Operators: Regexp Operators.). Does `awk' treat the +character as literal character, or as a regexp operator? + + It turns out that historically, such characters were taken literally +(d.c.). However, the POSIX standard indicates that they should be +treated as real metacharacters, and this is what `gawk' does. However, +in compatibility mode (*note Command Line Options: Options.), `gawk' +treats the characters represented by octal and hexadecimal escape +sequences literally when used in regexp constants. Thus, `/a\52b/' is +equivalent to `/a\*b/'. + + To summarize: + + 1. The escape sequences in the table above are always processed first, + for both string constants and regexp constants. This happens very + early, as soon as `awk' reads your program. + + 2. `gawk' processes both regexp constants and dynamic regexps (*note + Using Dynamic Regexps: Computed Regexps.), for the special + operators listed in *Note Additional Regexp Operators Only in + `gawk': GNU Regexp Operators. + + 3. A backslash before any other character means to treat that + character literally. + + +File: gawk.info, Node: Regexp Operators, Next: GNU Regexp Operators, Prev: Escape Sequences, Up: Regexp + +Regular Expression Operators +============================ + + You can combine regular expressions with the following characters, +called "regular expression operators", or "metacharacters", to increase +the power and versatility of regular expressions. + + The escape sequences described in *Note Escape Sequences::, are +valid inside a regexp. They are introduced by a `\'. They are +recognized and converted into the corresponding real characters as the +very first step in processing regexps. + + Here is a table of metacharacters. All characters that are not +escape sequences and that are not listed in the table stand for +themselves. + +`\' + This is used to suppress the special meaning of a character when + matching. For example: + + \$ + + matches the character `$'. + +`^' + This matches the beginning of a string. For example: + + ^@chapter + + matches the `@chapter' at the beginning of a string, and can be + used to identify chapter beginnings in Texinfo source files. The + `^' is known as an "anchor", since it anchors the pattern to + matching only at the beginning of the string. + + It is important to realize that `^' does not match the beginning of + a line embedded in a string. In this example the condition is not + true: + + if ("line1\nLINE 2" ~ /^L/) ... + +`$' + This is similar to `^', but it matches only at the end of a string. + For example: + + p$ + + matches a record that ends with a `p'. The `$' is also an anchor, + and also does not match the end of a line embedded in a string. + In this example the condition is not true: + + if ("line1\nLINE 2" ~ /1$/) ... + +`.' + The period, or dot, matches any single character, _including_ the + newline character. For example: + + .P + + matches any single character followed by a `P' in a string. Using + concatenation we can make a regular expression like `U.A', which + matches any three-character sequence that begins with `U' and ends + with `A'. + + In strict POSIX mode (*note Command Line Options: Options.), `.' + does not match the NUL character, which is a character with all + bits equal to zero. Otherwise, NUL is just another character. + Other versions of `awk' may not be able to match the NUL character. + +`[...]' + This is called a "character list". It matches any _one_ of the + characters that are enclosed in the square brackets. For example: + + [MVX] + + matches any one of the characters `M', `V', or `X' in a string. + + Ranges of characters are indicated by using a hyphen between the + beginning and ending characters, and enclosing the whole thing in + brackets. For example: + + [0-9] + + matches any digit. Multiple ranges are allowed. E.g., the list + `[A-Za-z0-9]' is a common way to express the idea of "all + alphanumeric characters." + + To include one of the characters `\', `]', `-' or `^' in a + character list, put a `\' in front of it. For example: + + [d\]] + + matches either `d', or `]'. + + This treatment of `\' in character lists is compatible with other + `awk' implementations, and is also mandated by POSIX. The regular + expressions in `awk' are a superset of the POSIX specification for + Extended Regular Expressions (EREs). POSIX EREs are based on the + regular expressions accepted by the traditional `egrep' utility. + + "Character classes" are a new feature introduced in the POSIX + standard. A character class is a special notation for describing + lists of characters that have a specific attribute, but where the + actual characters themselves can vary from country to country + and/or from character set to character set. For example, the + notion of what is an alphabetic character differs in the USA and + in France. + + A character class is only valid in a regexp _inside_ the brackets + of a character list. Character classes consist of `[:', a keyword + denoting the class, and `:]'. Here are the character classes + defined by the POSIX standard. + + `[:alnum:]' + Alphanumeric characters. + + `[:alpha:]' + Alphabetic characters. + + `[:blank:]' + Space and tab characters. + + `[:cntrl:]' + Control characters. + + `[:digit:]' + Numeric characters. + + `[:graph:]' + Characters that are printable and are also visible. (A space + is printable, but not visible, while an `a' is both.) + + `[:lower:]' + Lower-case alphabetic characters. + + `[:print:]' + Printable characters (characters that are not control + characters.) + + `[:punct:]' + Punctuation characters (characters that are not letter, + digits, control characters, or space characters). + + `[:space:]' + Space characters (such as space, tab, and formfeed, to name a + few). + + `[:upper:]' + Upper-case alphabetic characters. + + `[:xdigit:]' + Characters that are hexadecimal digits. + + For example, before the POSIX standard, to match alphanumeric + characters, you had to write `/[A-Za-z0-9]/'. If your character + set had other alphabetic characters in it, this would not match + them. With the POSIX character classes, you can write + `/[[:alnum:]]/', and this will match _all_ the alphabetic and + numeric characters in your character set. + + Two additional special sequences can appear in character lists. + These apply to non-ASCII character sets, which can have single + symbols (called "collating elements") that are represented with + more than one character, as well as several characters that are + equivalent for "collating", or sorting, purposes. (E.g., in + French, a plain "e" and a grave-accented "e`" are equivalent.) + + Collating Symbols + A "collating symbol" is a multi-character collating element + enclosed in `[.' and `.]'. For example, if `ch' is a + collating element, then `[[.ch.]]' is a regexp that matches + this collating element, while `[ch]' is a regexp that matches + either `c' or `h'. + + Equivalence Classes + An "equivalence class" is a locale-specific name for a list of + characters that are equivalent. The name is enclosed in `[=' + and `=]'. For example, the name `e' might be used to + represent all of "e," "e`," and "e'." In this case, `[[=e]]' + is a regexp that matches any of `e', `e'', or `e`'. + + These features are very valuable in non-English speaking locales. + + *Caution:* The library functions that `gawk' uses for regular + expression matching currently only recognize POSIX character + classes; they do not recognize collating symbols or equivalence + classes. + +`[^ ...]' + This is a "complemented character list". The first character after + the `[' _must_ be a `^'. It matches any characters _except_ those + in the square brackets. For example: + + [^0-9] + + matches any character that is not a digit. + +`|' + This is the "alternation operator", and it is used to specify + alternatives. For example: + + ^P|[0-9] + + matches any string that matches either `^P' or `[0-9]'. This + means it matches any string that starts with `P' or contains a + digit. + + The alternation applies to the largest possible regexps on either + side. In other words, `|' has the lowest precedence of all the + regular expression operators. + +`(...)' + Parentheses are used for grouping in regular expressions as in + arithmetic. They can be used to concatenate regular expressions + containing the alternation operator, `|'. For example, + `@(samp|code)\{[^}]+\}' matches both `@code{foo}' and + `@samp{bar}'. (These are Texinfo formatting control sequences.) + +`*' + This symbol means that the preceding regular expression is to be + repeated as many times as necessary to find a match. For example: + + ph* + + applies the `*' symbol to the preceding `h' and looks for matches + of one `p' followed by any number of `h's. This will also match + just `p' if no `h's are present. + + The `*' repeats the _smallest_ possible preceding expression. + (Use parentheses if you wish to repeat a larger expression.) It + finds as many repetitions as possible. For example: + + awk '/\(c[ad][ad]*r x\)/ { print }' sample + + prints every record in `sample' containing a string of the form + `(car x)', `(cdr x)', `(cadr x)', and so on. Notice the escaping + of the parentheses by preceding them with backslashes. + +`+' + This symbol is similar to `*', but the preceding expression must be + matched at least once. This means that: + + wh+y + + would match `why' and `whhy' but not `wy', whereas `wh*y' would + match all three of these strings. This is a simpler way of + writing the last `*' example: + + awk '/\(c[ad]+r x\)/ { print }' sample + +`?' + This symbol is similar to `*', but the preceding expression can be + matched either once or not at all. For example: + + fe?d + + will match `fed' and `fd', but nothing else. + +`{N}' +`{N,}' +`{N,M}' + One or two numbers inside braces denote an "interval expression". + If there is one number in the braces, the preceding regexp is + repeated N times. If there are two numbers separated by a comma, + the preceding regexp is repeated N to M times. If there is one + number followed by a comma, then the preceding regexp is repeated + at least N times. + + `wh{3}y' + matches `whhhy' but not `why' or `whhhhy'. + + `wh{3,5}y' + matches `whhhy' or `whhhhy' or `whhhhhy', only. + + `wh{2,}y' + matches `whhy' or `whhhy', and so on. + + Interval expressions were not traditionally available in `awk'. + As part of the POSIX standard they were added, to make `awk' and + `egrep' consistent with each other. + + However, since old programs may use `{' and `}' in regexp + constants, by default `gawk' does _not_ match interval expressions + in regexps. If either `--posix' or `--re-interval' are specified + (*note Command Line Options: Options.), then interval expressions + are allowed in regexps. + + In regular expressions, the `*', `+', and `?' operators, as well as +the braces `{' and `}', have the highest precedence, followed by +concatenation, and finally by `|'. As in arithmetic, parentheses can +change how operators are grouped. + + If `gawk' is in compatibility mode (*note Command Line Options: +Options.), character classes and interval expressions are not available +in regular expressions. + + The next node discusses the GNU-specific regexp operators, and +provides more detail concerning how command line options affect the way +`gawk' interprets the characters in regular expressions. + + +File: gawk.info, Node: GNU Regexp Operators, Next: Case-sensitivity, Prev: Regexp Operators, Up: Regexp + +Additional Regexp Operators Only in `gawk' +========================================== + + GNU software that deals with regular expressions provides a number of +additional regexp operators. These operators are described in this +section, and are specific to `gawk'; they are not available in other +`awk' implementations. + + Most of the additional operators are for dealing with word matching. +For our purposes, a "word" is a sequence of one or more letters, digits, +or underscores (`_'). + +`\w' + This operator matches any word-constituent character, i.e. any + letter, digit, or underscore. Think of it as a short-hand for + `[[:alnum:]_]'. + +`\W' + This operator matches any character that is not word-constituent. + Think of it as a short-hand for `[^[:alnum:]_]'. + +`\<' + This operator matches the empty string at the beginning of a word. + For example, `/\<away/' matches `away', but not `stowaway'. + +`\>' + This operator matches the empty string at the end of a word. For + example, `/stow\>/' matches `stow', but not `stowaway'. + +`\y' + This operator matches the empty string at either the beginning or + the end of a word (the word boundar*y*). For example, `\yballs?\y' + matches either `ball' or `balls' as a separate word. + +`\B' + This operator matches the empty string within a word. In other + words, `\B' matches the empty string that occurs between two + word-constituent characters. For example, `/\Brat\B/' matches + `crate', but it does not match `dirty rat'. `\B' is essentially + the opposite of `\y'. + + There are two other operators that work on buffers. In Emacs, a +"buffer" is, naturally, an Emacs buffer. For other programs, the +regexp library routines that `gawk' uses consider the entire string to +be matched as the buffer. + + For `awk', since `^' and `$' always work in terms of the beginning +and end of strings, these operators don't add any new capabilities. +They are provided for compatibility with other GNU software. + +`\`' + This operator matches the empty string at the beginning of the + buffer. + +`\'' + This operator matches the empty string at the end of the buffer. + + In other GNU software, the word boundary operator is `\b'. However, +that conflicts with the `awk' language's definition of `\b' as +backspace, so `gawk' uses a different letter. + + An alternative method would have been to require two backslashes in +the GNU operators, but this was deemed to be too confusing, and the +current method of using `\y' for the GNU `\b' appears to be the lesser +of two evils. + + The various command line options (*note Command Line Options: +Options.) control how `gawk' interprets characters in regexps. + +No options + In the default case, `gawk' provide all the facilities of POSIX + regexps and the GNU regexp operators described in *Note Regular + Expression Operators: Regexp Operators. However, interval + expressions are not supported. + +`--posix' + Only POSIX regexps are supported, the GNU operators are not special + (e.g., `\w' matches a literal `w'). Interval expressions are + allowed. + +`--traditional' + Traditional Unix `awk' regexps are matched. The GNU operators are + not special, interval expressions are not available, and neither + are the POSIX character classes (`[[:alnum:]]' and so on). + Characters described by octal and hexadecimal escape sequences are + treated literally, even if they represent regexp metacharacters. + +`--re-interval' + Allow interval expressions in regexps, even if `--traditional' has + been provided. + + +File: gawk.info, Node: Case-sensitivity, Next: Leftmost Longest, Prev: GNU Regexp Operators, Up: Regexp + +Case-sensitivity in Matching +============================ + + Case is normally significant in regular expressions, both when +matching ordinary characters (i.e. not metacharacters), and inside +character sets. Thus a `w' in a regular expression matches only a +lower-case `w' and not an upper-case `W'. + + The simplest way to do a case-independent match is to use a character +list: `[Ww]'. However, this can be cumbersome if you need to use it +often; and it can make the regular expressions harder to read. There +are two alternatives that you might prefer. + + One way to do a case-insensitive match at a particular point in the +program is to convert the data to a single case, using the `tolower' or +`toupper' built-in string functions (which we haven't discussed yet; +*note Built-in Functions for String Manipulation: String Functions.). +For example: + + tolower($1) ~ /foo/ { ... } + +converts the first field to lower-case before matching against it. +This will work in any POSIX-compliant implementation of `awk'. + + Another method, specific to `gawk', is to set the variable +`IGNORECASE' to a non-zero value (*note Built-in Variables::). When +`IGNORECASE' is not zero, _all_ regexp and string operations ignore +case. Changing the value of `IGNORECASE' dynamically controls the case +sensitivity of your program as it runs. Case is significant by default +because `IGNORECASE' (like most variables) is initialized to zero. + + x = "aB" + if (x ~ /ab/) ... # this test will fail + + IGNORECASE = 1 + if (x ~ /ab/) ... # now it will succeed + + In general, you cannot use `IGNORECASE' to make certain rules +case-insensitive and other rules case-sensitive, because there is no way +to set `IGNORECASE' just for the pattern of a particular rule. To do +this, you must use character lists or `tolower'. However, one thing +you can do only with `IGNORECASE' is turn case-sensitivity on or off +dynamically for all the rules at once. + + `IGNORECASE' can be set on the command line, or in a `BEGIN' rule +(*note Other Command Line Arguments: Other Arguments.; also *note +Startup and Cleanup Actions: Using BEGIN/END.). Setting `IGNORECASE' +from the command line is a way to make a program case-insensitive +without having to edit it. + + Prior to version 3.0 of `gawk', the value of `IGNORECASE' only +affected regexp operations. It did not affect string comparison with +`==', `!=', and so on. Beginning with version 3.0, both regexp and +string comparison operations are affected by `IGNORECASE'. + + Beginning with version 3.0 of `gawk', the equivalences between +upper-case and lower-case characters are based on the ISO-8859-1 (ISO +Latin-1) character set. This character set is a superset of the +traditional 128 ASCII characters, that also provides a number of +characters suitable for use with European languages. + + The value of `IGNORECASE' has no effect if `gawk' is in +compatibility mode (*note Command Line Options: Options.). Case is +always significant in compatibility mode. + + +File: gawk.info, Node: Leftmost Longest, Next: Computed Regexps, Prev: Case-sensitivity, Up: Regexp + +How Much Text Matches? +====================== + + Consider the following example: + + echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }' + + This example uses the `sub' function (which we haven't discussed yet, +*note Built-in Functions for String Manipulation: String Functions.) +to make a change to the input record. Here, the regexp `/a+/' indicates +"one or more `a' characters," and the replacement text is `<A>'. + + The input contains four `a' characters. What will the output be? +In other words, how many is "one or more"--will `awk' match two, three, +or all four `a' characters? + + The answer is, `awk' (and POSIX) regular expressions always match +the leftmost, _longest_ sequence of input characters that can match. +Thus, in this example, all four `a' characters are replaced with `<A>'. + + $ echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }' + -| <A>bcd + + For simple match/no-match tests, this is not so important. But when +doing regexp-based field and record splitting, and text matching and +substitutions with the `match', `sub', `gsub', and `gensub' functions, +it is very important. *Note Built-in Functions for String +Manipulation: String Functions, for more information on these functions. +Understanding this principle is also important for regexp-based record +and field splitting (*note How Input is Split into Records: Records., +and also *note Specifying How Fields are Separated: Field Separators.). + + +File: gawk.info, Node: Computed Regexps, Prev: Leftmost Longest, Up: Regexp + +Using Dynamic Regexps +===================== + + The right hand side of a `~' or `!~' operator need not be a regexp +constant (i.e. a string of characters between slashes). It may be any +expression. The expression is evaluated, and converted if necessary to +a string; the contents of the string are used as the regexp. A regexp +that is computed in this way is called a "dynamic regexp". For example: + + BEGIN { identifier_regexp = "[A-Za-z_][A-Za-z_0-9]+" } + $0 ~ identifier_regexp { print } + +sets `identifier_regexp' to a regexp that describes `awk' variable +names, and tests if the input record matches this regexp. + + *Caution:* When using the `~' and `!~' operators, there is a +difference between a regexp constant enclosed in slashes, and a string +constant enclosed in double quotes. If you are going to use a string +constant, you have to understand that the string is in essence scanned +_twice_; the first time when `awk' reads your program, and the second +time when it goes to match the string on the left-hand side of the +operator with the pattern on the right. This is true of any string +valued expression (such as `identifier_regexp' above), not just string +constants. + + What difference does it make if the string is scanned twice? The +answer has to do with escape sequences, and particularly with +backslashes. To get a backslash into a regular expression inside a +string, you have to type two backslashes. + + For example, `/\*/' is a regexp constant for a literal `*'. Only +one backslash is needed. To do the same thing with a string, you would +have to type `"\\*"'. The first backslash escapes the second one, so +that the string actually contains the two characters `\' and `*'. + + Given that you can use both regexp and string constants to describe +regular expressions, which should you use? The answer is "regexp +constants," for several reasons. + + 1. String constants are more complicated to write, and more difficult + to read. Using regexp constants makes your programs less + error-prone. Not understanding the difference between the two + kinds of constants is a common source of errors. + + 2. It is also more efficient to use regexp constants: `awk' can note + that you have supplied a regexp and store it internally in a form + that makes pattern matching more efficient. When using a string + constant, `awk' must first convert the string into this internal + form, and then perform the pattern matching. + + 3. Using regexp constants is better style; it shows clearly that you + intend a regexp match. + + +File: gawk.info, Node: Reading Files, Next: Printing, Prev: Regexp, Up: Top + +Reading Input Files +******************* + + In the typical `awk' program, all input is read either from the +standard input (by default the keyboard, but often a pipe from another +command) or from files whose names you specify on the `awk' command +line. If you specify input files, `awk' reads them in order, reading +all the data from one before going on to the next. The name of the +current input file can be found in the built-in variable `FILENAME' +(*note Built-in Variables::). + + The input is read in units called "records", and processed by the +rules of your program one record at a time. By default, each record is +one line. Each record is automatically split into chunks called +"fields". This makes it more convenient for programs to work on the +parts of a record. + + On rare occasions you will need to use the `getline' command. The +`getline' command is valuable, both because it can do explicit input +from any number of files, and because the files used with it do not +have to be named on the `awk' command line (*note Explicit Input with +`getline': Getline.). + +* Menu: + +* Records:: Controlling how data is split into records. +* Fields:: An introduction to fields. +* Non-Constant Fields:: Non-constant Field Numbers. +* Changing Fields:: Changing the Contents of a Field. +* Field Separators:: The field separator and how to change it. +* Constant Size:: Reading constant width data. +* Multiple Line:: Reading multi-line records. +* Getline:: Reading files under explicit program control + using the `getline' function. + + +File: gawk.info, Node: Records, Next: Fields, Prev: Reading Files, Up: Reading Files + +How Input is Split into Records +=============================== + + The `awk' utility divides the input for your `awk' program into +records and fields. Records are separated by a character called the +"record separator". By default, the record separator is the newline +character. This is why records are, by default, single lines. You can +use a different character for the record separator by assigning the +character to the built-in variable `RS'. + + You can change the value of `RS' in the `awk' program, like any +other variable, with the assignment operator, `=' (*note Assignment +Expressions: Assignment Ops.). The new record-separator character +should be enclosed in quotation marks, which indicate a string +constant. Often the right time to do this is at the beginning of +execution, before any input has been processed, so that the very first +record will be read with the proper separator. To do this, use the +special `BEGIN' pattern (*note The `BEGIN' and `END' Special Patterns: +BEGIN/END.). For example: + + awk 'BEGIN { RS = "/" } ; { print $0 }' BBS-list + +changes the value of `RS' to `"/"', before reading any input. This is +a string whose first character is a slash; as a result, records are +separated by slashes. Then the input file is read, and the second rule +in the `awk' program (the action with no pattern) prints each record. +Since each `print' statement adds a newline at the end of its output, +the effect of this `awk' program is to copy the input with each slash +changed to a newline. Here are the results of running the program on +`BBS-list': + + $ awk 'BEGIN { RS = "/" } ; { print $0 }' BBS-list + -| aardvark 555-5553 1200 + -| 300 B + -| alpo-net 555-3412 2400 + -| 1200 + -| 300 A + -| barfly 555-7685 1200 + -| 300 A + -| bites 555-1675 2400 + -| 1200 + -| 300 A + -| camelot 555-0542 300 C + -| core 555-2912 1200 + -| 300 C + -| fooey 555-1234 2400 + -| 1200 + -| 300 B + -| foot 555-6699 1200 + -| 300 B + -| macfoo 555-6480 1200 + -| 300 A + -| sdace 555-3430 2400 + -| 1200 + -| 300 A + -| sabafoo 555-2127 1200 + -| 300 C + -| + +Note that the entry for the `camelot' BBS is not split. In the +original data file (*note Data Files for the Examples: Sample Data +Files.), the line looks like this: + + camelot 555-0542 300 C + +It only has one baud rate; there are no slashes in the record. + + Another way to change the record separator is on the command line, +using the variable-assignment feature (*note Other Command Line +Arguments: Other Arguments.). + + awk '{ print $0 }' RS="/" BBS-list + +This sets `RS' to `/' before processing `BBS-list'. + + Using an unusual character such as `/' for the record separator +produces correct behavior in the vast majority of cases. However, the +following (extreme) pipeline prints a surprising `1'. There is one +field, consisting of a newline. The value of the built-in variable +`NF' is the number of fields in the current record. + + $ echo | awk 'BEGIN { RS = "a" } ; { print NF }' + -| 1 + +Reaching the end of an input file terminates the current input record, +even if the last character in the file is not the character in `RS' +(d.c.). + + The empty string, `""' (a string of no characters), has a special +meaning as the value of `RS': it means that records are separated by +one or more blank lines, and nothing else. *Note Multiple-Line +Records: Multiple Line, for more details. + + If you change the value of `RS' in the middle of an `awk' run, the +new value is used to delimit subsequent records, but the record +currently being processed (and records already processed) are not +affected. + + After the end of the record has been determined, `gawk' sets the +variable `RT' to the text in the input that matched `RS'. + + The value of `RS' is in fact not limited to a one-character string. +It can be any regular expression (*note Regular Expressions: Regexp.). +In general, each record ends at the next string that matches the +regular expression; the next record starts at the end of the matching +string. This general rule is actually at work in the usual case, where +`RS' contains just a newline: a record ends at the beginning of the +next matching string (the next newline in the input) and the following +record starts just after the end of this string (at the first character +of the following line). The newline, since it matches `RS', is not +part of either record. + + When `RS' is a single character, `RT' will contain the same single +character. However, when `RS' is a regular expression, then `RT' +becomes more useful; it contains the actual input text that matched the +regular expression. + + The following example illustrates both of these features. It sets +`RS' equal to a regular expression that matches either a newline, or a +series of one or more upper-case letters with optional leading and/or +trailing white space (*note Regular Expressions: Regexp.). + + $ echo record 1 AAAA record 2 BBBB record 3 | + > gawk 'BEGIN { RS = "\n|( *[[:upper:]]+ *)" } + > { print "Record =", $0, "and RT =", RT }' + -| Record = record 1 and RT = AAAA + -| Record = record 2 and RT = BBBB + -| Record = record 3 and RT = + -| + +The final line of output has an extra blank line. This is because the +value of `RT' is a newline, and then the `print' statement supplies its +own terminating newline. + + *Note A Simple Stream Editor: Simple Sed, for a more useful example +of `RS' as a regexp and `RT'. + + The use of `RS' as a regular expression and the `RT' variable are +`gawk' extensions; they are not available in compatibility mode (*note +Command Line Options: Options.). In compatibility mode, only the first +character of the value of `RS' is used to determine the end of the +record. + + The `awk' utility keeps track of the number of records that have +been read so far from the current input file. This value is stored in a +built-in variable called `FNR'. It is reset to zero when a new file is +started. Another built-in variable, `NR', is the total number of input +records read so far from all data files. It starts at zero but is +never automatically reset to zero. + + +File: gawk.info, Node: Fields, Next: Non-Constant Fields, Prev: Records, Up: Reading Files + +Examining Fields +================ + + When `awk' reads an input record, the record is automatically +separated or "parsed" by the interpreter into chunks called "fields". +By default, fields are separated by whitespace, like words in a line. +Whitespace in `awk' means any string of one or more spaces, tabs or +newlines;(1) other characters such as formfeed, and so on, that are +considered whitespace by other languages are _not_ considered +whitespace by `awk'. + + The purpose of fields is to make it more convenient for you to refer +to these pieces of the record. You don't have to use them--you can +operate on the whole record if you wish--but fields are what make +simple `awk' programs so powerful. + + To refer to a field in an `awk' program, you use a dollar-sign, `$', +followed by the number of the field you want. Thus, `$1' refers to the +first field, `$2' to the second, and so on. For example, suppose the +following is a line of input: + + This seems like a pretty nice example. + +Here the first field, or `$1', is `This'; the second field, or `$2', is +`seems'; and so on. Note that the last field, `$7', is `example.'. +Because there is no space between the `e' and the `.', the period is +considered part of the seventh field. + + `NF' is a built-in variable whose value is the number of fields in +the current record. `awk' updates the value of `NF' automatically, +each time a record is read. + + No matter how many fields there are, the last field in a record can +be represented by `$NF'. So, in the example above, `$NF' would be the +same as `$7', which is `example.'. Why this works is explained below +(*note Non-constant Field Numbers: Non-Constant Fields.). If you try +to reference a field beyond the last one, such as `$8' when the record +has only seven fields, you get the empty string. + + `$0', which looks like a reference to the "zeroth" field, is a +special case: it represents the whole input record. `$0' is used when +you are not interested in fields. + + Here are some more examples: + + $ awk '$1 ~ /foo/ { print $0 }' BBS-list + -| fooey 555-1234 2400/1200/300 B + -| foot 555-6699 1200/300 B + -| macfoo 555-6480 1200/300 A + -| sabafoo 555-2127 1200/300 C + +This example prints each record in the file `BBS-list' whose first +field contains the string `foo'. The operator `~' is called a +"matching operator" (*note How to Use Regular Expressions: Regexp +Usage.); it tests whether a string (here, the field `$1') matches a +given regular expression. + + By contrast, the following example looks for `foo' in _the entire +record_ and prints the first field and the last field for each input +record containing a match. + + $ awk '/foo/ { print $1, $NF }' BBS-list + -| fooey B + -| foot B + -| macfoo A + -| sabafoo C + + ---------- Footnotes ---------- + + (1) In POSIX `awk', newlines are not considered whitespace for +separating fields. + + +File: gawk.info, Node: Non-Constant Fields, Next: Changing Fields, Prev: Fields, Up: Reading Files + +Non-constant Field Numbers +========================== + + The number of a field does not need to be a constant. Any +expression in the `awk' language can be used after a `$' to refer to a +field. The value of the expression specifies the field number. If the +value is a string, rather than a number, it is converted to a number. +Consider this example: + + awk '{ print $NR }' + +Recall that `NR' is the number of records read so far: one in the first +record, two in the second, etc. So this example prints the first field +of the first record, the second field of the second record, and so on. +For the twentieth record, field number 20 is printed; most likely, the +record has fewer than 20 fields, so this prints a blank line. + + Here is another example of using expressions as field numbers: + + awk '{ print $(2*2) }' BBS-list + + `awk' must evaluate the expression `(2*2)' and use its value as the +number of the field to print. The `*' sign represents multiplication, +so the expression `2*2' evaluates to four. The parentheses are used so +that the multiplication is done before the `$' operation; they are +necessary whenever there is a binary operator in the field-number +expression. This example, then, prints the hours of operation (the +fourth field) for every line of the file `BBS-list'. (All of the `awk' +operators are listed, in order of decreasing precedence, in *Note +Operator Precedence (How Operators Nest): Precedence.) + + If the field number you compute is zero, you get the entire record. +Thus, `$(2-2)' has the same value as `$0'. Negative field numbers are +not allowed; trying to reference one will usually terminate your +running `awk' program. (The POSIX standard does not define what +happens when you reference a negative field number. `gawk' will notice +this and terminate your program. Other `awk' implementations may +behave differently.) + + As mentioned in *Note Examining Fields: Fields, the number of fields +in the current record is stored in the built-in variable `NF' (also +*note Built-in Variables::). The expression `$NF' is not a special +feature: it is the direct consequence of evaluating `NF' and using its +value as a field number. + + +File: gawk.info, Node: Changing Fields, Next: Field Separators, Prev: Non-Constant Fields, Up: Reading Files + +Changing the Contents of a Field +================================ + + You can change the contents of a field as seen by `awk' within an +`awk' program; this changes what `awk' perceives as the current input +record. (The actual input is untouched; `awk' _never_ modifies the +input file.) + + Consider this example and its output: + + $ awk '{ $3 = $2 - 10; print $2, $3 }' inventory-shipped + -| 13 3 + -| 15 5 + -| 15 5 + ... + +The `-' sign represents subtraction, so this program reassigns field +three, `$3', to be the value of field two minus ten, `$2 - 10'. (*Note +Arithmetic Operators: Arithmetic Ops.) Then field two, and the new +value for field three, are printed. + + In order for this to work, the text in field `$2' must make sense as +a number; the string of characters must be converted to a number in +order for the computer to do arithmetic on it. The number resulting +from the subtraction is converted back to a string of characters which +then becomes field three. *Note Conversion of Strings and Numbers: +Conversion. + + When you change the value of a field (as perceived by `awk'), the +text of the input record is recalculated to contain the new field where +the old one was. Therefore, `$0' changes to reflect the altered field. +Thus, this program prints a copy of the input file, with 10 subtracted +from the second field of each line. + + $ awk '{ $2 = $2 - 10; print $0 }' inventory-shipped + -| Jan 3 25 15 115 + -| Feb 5 32 24 226 + -| Mar 5 24 34 228 + ... + + You can also assign contents to fields that are out of range. For +example: + + $ awk '{ $6 = ($5 + $4 + $3 + $2) + > print $6 }' inventory-shipped + -| 168 + -| 297 + -| 301 + ... + +We've just created `$6', whose value is the sum of fields `$2', `$3', +`$4', and `$5'. The `+' sign represents addition. For the file +`inventory-shipped', `$6' represents the total number of parcels +shipped for a particular month. + + Creating a new field changes `awk''s internal copy of the current +input record--the value of `$0'. Thus, if you do `print $0' after +adding a field, the record printed includes the new field, with the +appropriate number of field separators between it and the previously +existing fields. + + This recomputation affects and is affected by `NF' (the number of +fields; *note Examining Fields: Fields.), and by a feature that has not +been discussed yet, the "output field separator", `OFS', which is used +to separate the fields (*note Output Separators::). For example, the +value of `NF' is set to the number of the highest field you create. + + Note, however, that merely _referencing_ an out-of-range field does +_not_ change the value of either `$0' or `NF'. Referencing an +out-of-range field only produces an empty string. For example: + + if ($(NF+1) != "") + print "can't happen" + else + print "everything is normal" + +should print `everything is normal', because `NF+1' is certain to be +out of range. (*Note The `if'-`else' Statement: If Statement, for more +information about `awk''s `if-else' statements. *Note Variable Typing +and Comparison Expressions: Typing and Comparison, for more information +about the `!=' operator.) + + It is important to note that making an assignment to an existing +field will change the value of `$0', but will not change the value of +`NF', even when you assign the empty string to a field. For example: + + $ echo a b c d | awk '{ OFS = ":"; $2 = "" + > print $0; print NF }' + -| a::c:d + -| 4 + +The field is still there; it just has an empty value. You can tell +because there are two colons in a row. + + This example shows what happens if you create a new field. + + $ echo a b c d | awk '{ OFS = ":"; $2 = ""; $6 = "new" + > print $0; print NF }' + -| a::c:d::new + -| 6 + +The intervening field, `$5' is created with an empty value (indicated +by the second pair of adjacent colons), and `NF' is updated with the +value six. + + Finally, decrementing `NF' will lose the values of the fields after +the new value of `NF', and `$0' will be recomputed. Here is an example: + + $ echo a b c d e f | ../gawk '{ print "NF =", NF; + > NF = 3; print $0 }' + -| NF = 6 + -| a b c + + +File: gawk.info, Node: Field Separators, Next: Constant Size, Prev: Changing Fields, Up: Reading Files + +Specifying How Fields are Separated +=================================== + + This section is rather long; it describes one of the most fundamental +operations in `awk'. + +* Menu: + +* Basic Field Splitting:: How fields are split with single characters + or simple strings. +* Regexp Field Splitting:: Using regexps as the field separator. +* Single Character Fields:: Making each character a separate field. +* Command Line Field Separator:: Setting `FS' from the command line. +* Field Splitting Summary:: Some final points and a summary table. + + +File: gawk.info, Node: Basic Field Splitting, Next: Regexp Field Splitting, Prev: Field Separators, Up: Field Separators + +The Basics of Field Separating +------------------------------ + + The "field separator", which is either a single character or a +regular expression, controls the way `awk' splits an input record into +fields. `awk' scans the input record for character sequences that +match the separator; the fields themselves are the text between the +matches. + + In the examples below, we use the bullet symbol "*" to represent +spaces in the output. + + If the field separator is `oo', then the following line: + + moo goo gai pan + +would be split into three fields: `m', `*g' and `*gai*pan'. Note the +leading spaces in the values of the second and third fields. + + The field separator is represented by the built-in variable `FS'. +Shell programmers take note! `awk' does _not_ use the name `IFS' which +is used by the POSIX compatible shells (such as the Bourne shell, `sh', +or the GNU Bourne-Again Shell, Bash). + + You can change the value of `FS' in the `awk' program with the +assignment operator, `=' (*note Assignment Expressions: Assignment +Ops.). Often the right time to do this is at the beginning of +execution, before any input has been processed, so that the very first +record will be read with the proper separator. To do this, use the +special `BEGIN' pattern (*note The `BEGIN' and `END' Special Patterns: +BEGIN/END.). For example, here we set the value of `FS' to the string +`","': + + awk 'BEGIN { FS = "," } ; { print $2 }' + +Given the input line, + + John Q. Smith, 29 Oak St., Walamazoo, MI 42139 + +this `awk' program extracts and prints the string `*29*Oak*St.'. + + Sometimes your input data will contain separator characters that +don't separate fields the way you thought they would. For instance, the +person's name in the example we just used might have a title or suffix +attached, such as `John Q. Smith, LXIX'. From input containing such a +name: + + John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139 + +the above program would extract `*LXIX', instead of `*29*Oak*St.'. If +you were expecting the program to print the address, you would be +surprised. The moral is: choose your data layout and separator +characters carefully to prevent such problems. + + Normally, fields are separated by whitespace sequences (spaces, tabs +and newlines), not by single spaces: two spaces in a row do not delimit +an empty field. The default value of the field separator `FS' is a +string containing a single space, `" "'. If this value were +interpreted in the usual way, each space character would separate +fields, so two spaces in a row would make an empty field between them. +The reason this does not happen is that a single space as the value of +`FS' is a special case: it is taken to specify the default manner of +delimiting fields. + + If `FS' is any other single character, such as `","', then each +occurrence of that character separates two fields. Two consecutive +occurrences delimit an empty field. If the character occurs at the +beginning or the end of the line, that too delimits an empty field. The +space character is the only single character which does not follow these +rules. + + +File: gawk.info, Node: Regexp Field Splitting, Next: Single Character Fields, Prev: Basic Field Splitting, Up: Field Separators + +Using Regular Expressions to Separate Fields +-------------------------------------------- + + The previous node discussed the use of single characters or simple +strings as the value of `FS'. More generally, the value of `FS' may be +a string containing any regular expression. In this case, each match +in the record for the regular expression separates fields. For +example, the assignment: + + FS = ", \t" + +makes every area of an input line that consists of a comma followed by a +space and a tab, into a field separator. (`\t' is an "escape sequence" +that stands for a tab; *note Escape Sequences::, for the complete list +of similar escape sequences.) + + For a less trivial example of a regular expression, suppose you want +single spaces to separate fields the way single commas were used above. +You can set `FS' to `"[ ]"' (left bracket, space, right bracket). This +regular expression matches a single space and nothing else (*note +Regular Expressions: Regexp.). + + There is an important difference between the two cases of `FS = " "' +(a single space) and `FS = "[ \t\n]+"' (left bracket, space, backslash, +"t", backslash, "n", right bracket, which is a regular expression +matching one or more spaces, tabs, or newlines). For both values of +`FS', fields are separated by runs of spaces, tabs and/or newlines. +However, when the value of `FS' is `" "', `awk' will first strip +leading and trailing whitespace from the record, and then decide where +the fields are. + + For example, the following pipeline prints `b': + + $ echo ' a b c d ' | awk '{ print $2 }' + -| b + +However, this pipeline prints `a' (note the extra spaces around each +letter): + + $ echo ' a b c d ' | awk 'BEGIN { FS = "[ \t]+" } + > { print $2 }' + -| a + +In this case, the first field is "null", or empty. + + The stripping of leading and trailing whitespace also comes into +play whenever `$0' is recomputed. For instance, study this pipeline: + + $ echo ' a b c d' | awk '{ print; $2 = $2; print }' + -| a b c d + -| a b c d + +The first `print' statement prints the record as it was read, with +leading whitespace intact. The assignment to `$2' rebuilds `$0' by +concatenating `$1' through `$NF' together, separated by the value of +`OFS'. Since the leading whitespace was ignored when finding `$1', it +is not part of the new `$0'. Finally, the last `print' statement +prints the new `$0'. + + +File: gawk.info, Node: Single Character Fields, Next: Command Line Field Separator, Prev: Regexp Field Splitting, Up: Field Separators + +Making Each Character a Separate Field +-------------------------------------- + + There are times when you may want to examine each character of a +record separately. In `gawk', this is easy to do, you simply assign +the null string (`""') to `FS'. In this case, each individual character +in the record will become a separate field. Here is an example: + + echo a b | gawk 'BEGIN { FS = "" } + { + for (i = 1; i <= NF; i = i + 1) + print "Field", i, "is", $i + }' + +The output from this is: + + Field 1 is a + Field 2 is + Field 3 is b + + Traditionally, the behavior for `FS' equal to `""' was not defined. +In this case, Unix `awk' would simply treat the entire record as only +having one field (d.c.). In compatibility mode (*note Command Line +Options: Options.), if `FS' is the null string, then `gawk' will also +behave this way. + + +File: gawk.info, Node: Command Line Field Separator, Next: Field Splitting Summary, Prev: Single Character Fields, Up: Field Separators + +Setting `FS' from the Command Line +---------------------------------- + + `FS' can be set on the command line. You use the `-F' option to do +so. For example: + + awk -F, 'PROGRAM' INPUT-FILES + +sets `FS' to be the `,' character. Notice that the option uses a +capital `F'. Contrast this with `-f', which specifies a file +containing an `awk' program. Case is significant in command line +options: the `-F' and `-f' options have nothing to do with each other. +You can use both options at the same time to set the `FS' variable +_and_ get an `awk' program from a file. + + The value used for the argument to `-F' is processed in exactly the +same way as assignments to the built-in variable `FS'. This means that +if the field separator contains special characters, they must be escaped +appropriately. For example, to use a `\' as the field separator, you +would have to type: + + # same as FS = "\\" + awk -F\\\\ '...' files ... + +Since `\' is used for quoting in the shell, `awk' will see `-F\\'. +Then `awk' processes the `\\' for escape characters (*note Escape +Sequences::), finally yielding a single `\' to be used for the field +separator. + + As a special case, in compatibility mode (*note Command Line +Options: Options.), if the argument to `-F' is `t', then `FS' is set to +the tab character. This is because if you type `-F\t' at the shell, +without any quotes, the `\' gets deleted, so `awk' figures that you +really want your fields to be separated with tabs, and not `t's. Use +`-v FS="t"' on the command line if you really do want to separate your +fields with `t's (*note Command Line Options: Options.). + + For example, let's use an `awk' program file called `baud.awk' that +contains the pattern `/300/', and the action `print $1'. Here is the +program: + + /300/ { print $1 } + + Let's also set `FS' to be the `-' character, and run the program on +the file `BBS-list'. The following command prints a list of the names +of the bulletin boards that operate at 300 baud and the first three +digits of their phone numbers: + + $ awk -F- -f baud.awk BBS-list + -| aardvark 555 + -| alpo + -| barfly 555 + ... + +Note the second line of output. In the original file (*note Data Files +for the Examples: Sample Data Files.), the second line looked like this: + + alpo-net 555-3412 2400/1200/300 A + + The `-' as part of the system's name was used as the field +separator, instead of the `-' in the phone number that was originally +intended. This demonstrates why you have to be careful in choosing +your field and record separators. + + On many Unix systems, each user has a separate entry in the system +password file, one line per user. The information in these lines is +separated by colons. The first field is the user's logon name, and the +second is the user's encrypted password. A password file entry might +look like this: + + arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh + + The following program searches the system password file, and prints +the entries for users who have no password: + + awk -F: '$2 == ""' /etc/passwd + + +File: gawk.info, Node: Field Splitting Summary, Prev: Command Line Field Separator, Up: Field Separators + +Field Splitting Summary +----------------------- + + According to the POSIX standard, `awk' is supposed to behave as if +each record is split into fields at the time that it is read. In +particular, this means that you can change the value of `FS' after a +record is read, and the value of the fields (i.e. how they were split) +should reflect the old value of `FS', not the new one. + + However, many implementations of `awk' do not work this way. +Instead, they defer splitting the fields until a field is actually +referenced. The fields will be split using the _current_ value of +`FS'! (d.c.) This behavior can be difficult to diagnose. The following +example illustrates the difference between the two methods. (The +`sed'(1) command prints just the first line of `/etc/passwd'.) + + sed 1q /etc/passwd | awk '{ FS = ":" ; print $1 }' + +will usually print + + root + +on an incorrect implementation of `awk', while `gawk' will print +something like + + root:nSijPlPhZZwgE:0:0:Root:/: + + The following table summarizes how fields are split, based on the +value of `FS'. (`==' means "is equal to.") + +`FS == " "' + Fields are separated by runs of whitespace. Leading and trailing + whitespace are ignored. This is the default. + +`FS == ANY OTHER SINGLE CHARACTER' + Fields are separated by each occurrence of the character. Multiple + successive occurrences delimit empty fields, as do leading and + trailing occurrences. The character can even be a regexp + metacharacter; it does not need to be escaped. + +`FS == REGEXP' + Fields are separated by occurrences of characters that match + REGEXP. Leading and trailing matches of REGEXP delimit empty + fields. + +`FS == ""' + Each individual character in the record becomes a separate field. + + ---------- Footnotes ---------- + + (1) The `sed' utility is a "stream editor." Its behavior is also +defined by the POSIX standard. + + +File: gawk.info, Node: Constant Size, Next: Multiple Line, Prev: Field Separators, Up: Reading Files + +Reading Fixed-width Data +======================== + + (This section discusses an advanced, experimental feature. If you +are a novice `awk' user, you may wish to skip it on the first reading.) + + `gawk' version 2.13 introduced a new facility for dealing with +fixed-width fields with no distinctive field separator. Data of this +nature arises, for example, in the input for old FORTRAN programs where +numbers are run together; or in the output of programs that did not +anticipate the use of their output as input for other programs. + + An example of the latter is a table where all the columns are lined +up by the use of a variable number of spaces and _empty fields are just +spaces_. Clearly, `awk''s normal field splitting based on `FS' will +not work well in this case. Although a portable `awk' program can use +a series of `substr' calls on `$0' (*note Built-in Functions for String +Manipulation: String Functions.), this is awkward and inefficient for a +large number of fields. + + The splitting of an input record into fixed-width fields is +specified by assigning a string containing space-separated numbers to +the built-in variable `FIELDWIDTHS'. Each number specifies the width +of the field _including_ columns between fields. If you want to ignore +the columns between fields, you can specify the width as a separate +field that is subsequently ignored. + + The following data is the output of the Unix `w' utility. It is +useful to illustrate the use of `FIELDWIDTHS'. + + 10:06pm up 21 days, 14:04, 23 users + User tty login idle JCPU PCPU what + hzuo ttyV0 8:58pm 9 5 vi p24.tex + hzang ttyV3 6:37pm 50 -csh + eklye ttyV5 9:53pm 7 1 em thes.tex + dportein ttyV6 8:17pm 1:47 -csh + gierd ttyD3 10:00pm 1 elm + dave ttyD4 9:47pm 4 4 w + brent ttyp0 26Jun91 4:46 26:46 4:41 bash + dave ttyq4 26Jun9115days 46 46 wnewmail + + The following program takes the above input, converts the idle time +to number of seconds and prints out the first two fields and the +calculated idle time. (This program uses a number of `awk' features +that haven't been introduced yet.) + + BEGIN { FIELDWIDTHS = "9 6 10 6 7 7 35" } + NR > 2 { + idle = $4 + sub(/^ */, "", idle) # strip leading spaces + if (idle == "") + idle = 0 + if (idle ~ /:/) { + split(idle, t, ":") + idle = t[1] * 60 + t[2] + } + if (idle ~ /days/) + idle *= 24 * 60 * 60 + + print $1, $2, idle + } + + Here is the result of running the program on the data: + + hzuo ttyV0 0 + hzang ttyV3 50 + eklye ttyV5 0 + dportein ttyV6 107 + gierd ttyD3 1 + dave ttyD4 0 + brent ttyp0 286 + dave ttyq4 1296000 + + Another (possibly more practical) example of fixed-width input data +would be the input from a deck of balloting cards. In some parts of +the United States, voters mark their choices by punching holes in +computer cards. These cards are then processed to count the votes for +any particular candidate or on any particular issue. Since a voter may +choose not to vote on some issue, any column on the card may be empty. +An `awk' program for processing such data could use the `FIELDWIDTHS' +feature to simplify reading the data. (Of course, getting `gawk' to +run on a system with card readers is another story!) + + Assigning a value to `FS' causes `gawk' to return to using `FS' for +field splitting. Use `FS = FS' to make this happen, without having to +know the current value of `FS'. + + This feature is still experimental, and may evolve over time. Note +that in particular, `gawk' does not attempt to verify the sanity of the +values used in the value of `FIELDWIDTHS'. + + +File: gawk.info, Node: Multiple Line, Next: Getline, Prev: Constant Size, Up: Reading Files + +Multiple-Line Records +===================== + + In some data bases, a single line cannot conveniently hold all the +information in one entry. In such cases, you can use multi-line +records. + + The first step in doing this is to choose your data format: when +records are not defined as single lines, how do you want to define them? +What should separate records? + + One technique is to use an unusual character or string to separate +records. For example, you could use the formfeed character (written +`\f' in `awk', as in C) to separate them, making each record a page of +the file. To do this, just set the variable `RS' to `"\f"' (a string +containing the formfeed character). Any other character could equally +well be used, as long as it won't be part of the data in a record. + + Another technique is to have blank lines separate records. By a +special dispensation, an empty string as the value of `RS' indicates +that records are separated by one or more blank lines. If you set `RS' +to the empty string, a record always ends at the first blank line +encountered. And the next record doesn't start until the first +non-blank line that follows--no matter how many blank lines appear in a +row, they are considered one record-separator. + + You can achieve the same effect as `RS = ""' by assigning the string +`"\n\n+"' to `RS'. This regexp matches the newline at the end of the +record, and one or more blank lines after the record. In addition, a +regular expression always matches the longest possible sequence when +there is a choice (*note How Much Text Matches?: Leftmost Longest.) So +the next record doesn't start until the first non-blank line that +follows--no matter how many blank lines appear in a row, they are +considered one record-separator. + + There is an important difference between `RS = ""' and `RS = +"\n\n+"'. In the first case, leading newlines in the input data file +are ignored, and if a file ends without extra blank lines after the +last record, the final newline is removed from the record. In the +second case, this special processing is not done (d.c.). + + Now that the input is separated into records, the second step is to +separate the fields in the record. One way to do this is to divide each +of the lines into fields in the normal manner. This happens by default +as the result of a special feature: when `RS' is set to the empty +string, the newline character _always_ acts as a field separator. This +is in addition to whatever field separations result from `FS'. + + The original motivation for this special exception was probably to +provide useful behavior in the default case (i.e. `FS' is equal to +`" "'). This feature can be a problem if you really don't want the +newline character to separate fields, since there is no way to prevent +it. However, you can work around this by using the `split' function to +break up the record manually (*note Built-in Functions for String +Manipulation: String Functions.). + + Another way to separate fields is to put each field on a separate +line: to do this, just set the variable `FS' to the string `"\n"'. +(This simple regular expression matches a single newline.) + + A practical example of a data file organized this way might be a +mailing list, where each entry is separated by blank lines. If we have +a mailing list in a file named `addresses', that looks like this: + + Jane Doe + 123 Main Street + Anywhere, SE 12345-6789 + + John Smith + 456 Tree-lined Avenue + Smallville, MW 98765-4321 + + ... + +A simple program to process this file would look like this: + + # addrs.awk --- simple mailing list program + + # Records are separated by blank lines. + # Each line is one field. + BEGIN { RS = "" ; FS = "\n" } + + { + print "Name is:", $1 + print "Address is:", $2 + print "City and State are:", $3 + print "" + } + + Running the program produces the following output: + + $ awk -f addrs.awk addresses + -| Name is: Jane Doe + -| Address is: 123 Main Street + -| City and State are: Anywhere, SE 12345-6789 + -| + -| Name is: John Smith + -| Address is: 456 Tree-lined Avenue + -| City and State are: Smallville, MW 98765-4321 + -| + ... + + *Note Printing Mailing Labels: Labels Program, for a more realistic +program that deals with address lists. + + The following table summarizes how records are split, based on the +value of `RS'. (`==' means "is equal to.") + +`RS == "\n"' + Records are separated by the newline character (`\n'). In effect, + every line in the data file is a separate record, including blank + lines. This is the default. + +`RS == ANY SINGLE CHARACTER' + Records are separated by each occurrence of the character. + Multiple successive occurrences delimit empty records. + +`RS == ""' + Records are separated by runs of blank lines. The newline + character always serves as a field separator, in addition to + whatever value `FS' may have. Leading and trailing newlines in a + file are ignored. + +`RS == REGEXP' + Records are separated by occurrences of characters that match + REGEXP. Leading and trailing matches of REGEXP delimit empty + records. + + In all cases, `gawk' sets `RT' to the input text that matched the +value specified by `RS'. + + +File: gawk.info, Node: Getline, Prev: Multiple Line, Up: Reading Files + +Explicit Input with `getline' +============================= + + So far we have been getting our input data from `awk''s main input +stream--either the standard input (usually your terminal, sometimes the +output from another program) or from the files specified on the command +line. The `awk' language has a special built-in command called +`getline' that can be used to read input under your explicit control. + +* Menu: + +* Getline Intro:: Introduction to the `getline' function. +* Plain Getline:: Using `getline' with no arguments. +* Getline/Variable:: Using `getline' into a variable. +* Getline/File:: Using `getline' from a file. +* Getline/Variable/File:: Using `getline' into a variable from a + file. +* Getline/Pipe:: Using `getline' from a pipe. +* Getline/Variable/Pipe:: Using `getline' into a variable from a + pipe. +* Getline Summary:: Summary Of `getline' Variants. + + +File: gawk.info, Node: Getline Intro, Next: Plain Getline, Prev: Getline, Up: Getline + +Introduction to `getline' +------------------------- + + This command is used in several different ways, and should _not_ be +used by beginners. It is covered here because this is the chapter on +input. The examples that follow the explanation of the `getline' +command include material that has not been covered yet. Therefore, +come back and study the `getline' command _after_ you have reviewed the +rest of this Info file and have a good knowledge of how `awk' works. + + `getline' returns one if it finds a record, and zero if the end of +the file is encountered. If there is some error in getting a record, +such as a file that cannot be opened, then `getline' returns -1. In +this case, `gawk' sets the variable `ERRNO' to a string describing the +error that occurred. + + In the following examples, COMMAND stands for a string value that +represents a shell command. + + +File: gawk.info, Node: Plain Getline, Next: Getline/Variable, Prev: Getline Intro, Up: Getline + +Using `getline' with No Arguments +--------------------------------- + + The `getline' command can be used without arguments to read input +from the current input file. All it does in this case is read the next +input record and split it up into fields. This is useful if you've +finished processing the current record, but you want to do some special +processing _right now_ on the next record. Here's an example: + + awk '{ + if ((t = index($0, "/*")) != 0) { + # value will be "" if t is 1 + tmp = substr($0, 1, t - 1) + u = index(substr($0, t + 2), "*/") + while (u == 0) { + if (getline <= 0) { + m = "unexpected EOF or error" + m = (m ": " ERRNO) + print m > "/dev/stderr" + exit + } + t = -1 + u = index($0, "*/") + } + # substr expression will be "" if */ + # occurred at end of line + $0 = tmp substr($0, t + u + 3) + } + print $0 + }' + + This `awk' program deletes all C-style comments, `/* ... */', from +the input. By replacing the `print $0' with other statements, you +could perform more complicated processing on the decommented input, +like searching for matches of a regular expression. This program has a +subtle problem--it does not work if one comment ends and another begins +on the same line. + + This form of the `getline' command sets `NF' (the number of fields; +*note Examining Fields: Fields.), `NR' (the number of records read so +far; *note How Input is Split into Records: Records.), `FNR' (the +number of records read from this input file), and the value of `$0'. + + *Note:* the new value of `$0' is used in testing the patterns of any +subsequent rules. The original value of `$0' that triggered the rule +which executed `getline' is lost (d.c.). By contrast, the `next' +statement reads a new record but immediately begins processing it +normally, starting with the first rule in the program. *Note The +`next' Statement: Next Statement. + + +File: gawk.info, Node: Getline/Variable, Next: Getline/File, Prev: Plain Getline, Up: Getline + +Using `getline' Into a Variable +------------------------------- + + You can use `getline VAR' to read the next record from `awk''s input +into the variable VAR. No other processing is done. + + For example, suppose the next line is a comment, or a special string, +and you want to read it, without triggering any rules. This form of +`getline' allows you to read that line and store it in a variable so +that the main read-a-line-and-check-each-rule loop of `awk' never sees +it. + + The following example swaps every two lines of input. For example, +given: + + wan + tew + free + phore + +it outputs: + + tew + wan + phore + free + +Here's the program: + + awk '{ + if ((getline tmp) > 0) { + print tmp + print $0 + } else + print $0 + }' + + The `getline' command used in this way sets only the variables `NR' +and `FNR' (and of course, VAR). The record is not split into fields, +so the values of the fields (including `$0') and the value of `NF' do +not change. + + +File: gawk.info, Node: Getline/File, Next: Getline/Variable/File, Prev: Getline/Variable, Up: Getline + +Using `getline' from a File +--------------------------- + + Use `getline < FILE' to read the next record from the file FILE. +Here FILE is a string-valued expression that specifies the file name. +`< FILE' is called a "redirection" since it directs input to come from +a different place. + + For example, the following program reads its input record from the +file `secondary.input' when it encounters a first field with a value +equal to 10 in the current input file. + + awk '{ + if ($1 == 10) { + getline < "secondary.input" + print + } else + print + }' + + Since the main input stream is not used, the values of `NR' and +`FNR' are not changed. But the record read is split into fields in the +normal manner, so the values of `$0' and other fields are changed. So +is the value of `NF'. + + According to POSIX, `getline < EXPRESSION' is ambiguous if +EXPRESSION contains unparenthesized operators other than `$'; for +example, `getline < dir "/" file' is ambiguous because the +concatenation operator is not parenthesized, and you should write it as +`getline < (dir "/" file)' if you want your program to be portable to +other `awk' implementations. + + +File: gawk.info, Node: Getline/Variable/File, Next: Getline/Pipe, Prev: Getline/File, Up: Getline + +Using `getline' Into a Variable from a File +------------------------------------------- + + Use `getline VAR < FILE' to read input the file FILE and put it in +the variable VAR. As above, FILE is a string-valued expression that +specifies the file from which to read. + + In this version of `getline', none of the built-in variables are +changed, and the record is not split into fields. The only variable +changed is VAR. + + According to POSIX, `getline VAR < EXPRESSION' is ambiguous if +EXPRESSION contains unparenthesized operators other than `$'; for +example, `getline < dir "/" file' is ambiguous because the +concatenation operator is not parenthesized, and you should write it as +`getline < (dir "/" file)' if you want your program to be portable to +other `awk' implementations. + + For example, the following program copies all the input files to the +output, except for records that say `@include FILENAME'. Such a record +is replaced by the contents of the file FILENAME. + + awk '{ + if (NF == 2 && $1 == "@include") { + while ((getline line < $2) > 0) + print line + close($2) + } else + print + }' + + Note here how the name of the extra input file is not built into the +program; it is taken directly from the data, from the second field on +the `@include' line. + + The `close' function is called to ensure that if two identical +`@include' lines appear in the input, the entire specified file is +included twice. *Note Closing Input and Output Files and Pipes: Close +Files And Pipes. + + One deficiency of this program is that it does not process nested +`@include' statements (`@include' statements in included files) the way +a true macro preprocessor would. *Note An Easy Way to Use Library +Functions: Igawk Program, for a program that does handle nested +`@include' statements. + + +File: gawk.info, Node: Getline/Pipe, Next: Getline/Variable/Pipe, Prev: Getline/Variable/File, Up: Getline + +Using `getline' from a Pipe +--------------------------- + + You can pipe the output of a command into `getline', using `COMMAND +| getline'. In this case, the string COMMAND is run as a shell command +and its output is piped into `awk' to be used as input. This form of +`getline' reads one record at a time from the pipe. + + For example, the following program copies its input to its output, +except for lines that begin with `@execute', which are replaced by the +output produced by running the rest of the line as a shell command: + + awk '{ + if ($1 == "@execute") { + tmp = substr($0, 10) + while ((tmp | getline) > 0) + print + close(tmp) + } else + print + }' + +The `close' function is called to ensure that if two identical +`@execute' lines appear in the input, the command is run for each one. +*Note Closing Input and Output Files and Pipes: Close Files And Pipes. + + Given the input: + + foo + bar + baz + @execute who + bletch + +the program might produce: + + foo + bar + baz + arnold ttyv0 Jul 13 14:22 + miriam ttyp0 Jul 13 14:23 (murphy:0) + bill ttyp1 Jul 13 14:23 (murphy:0) + bletch + +Notice that this program ran the command `who' and printed the result. +(If you try this program yourself, you will of course get different +results, showing you who is logged in on your system.) + + This variation of `getline' splits the record into fields, sets the +value of `NF' and recomputes the value of `$0'. The values of `NR' and +`FNR' are not changed. + + According to POSIX, `EXPRESSION | getline' is ambiguous if +EXPRESSION contains unparenthesized operators other than `$'; for +example, `"echo " "date" | getline' is ambiguous because the +concatenation operator is not parenthesized, and you should write it as +`("echo " "date") | getline' if you want your program to be portable to +other `awk' implementations. + + +File: gawk.info, Node: Getline/Variable/Pipe, Next: Getline Summary, Prev: Getline/Pipe, Up: Getline + +Using `getline' Into a Variable from a Pipe +------------------------------------------- + + When you use `COMMAND | getline VAR', the output of the command +COMMAND is sent through a pipe to `getline' and into the variable VAR. +For example, the following program reads the current date and time into +the variable `current_time', using the `date' utility, and then prints +it. + + awk 'BEGIN { + "date" | getline current_time + close("date") + print "Report printed on " current_time + }' + + In this version of `getline', none of the built-in variables are +changed, and the record is not split into fields. + + According to POSIX, `EXPRESSION | getline VAR' is ambiguous if +EXPRESSION contains unparenthesized operators other than `$'; for +example, `"echo " "date" | getline VAR' is ambiguous because the +concatenation operator is not parenthesized, and you should write it as +`("echo " "date") | getline VAR' if you want your program to be +portable to other `awk' implementations. + + +File: gawk.info, Node: Getline Summary, Prev: Getline/Variable/Pipe, Up: Getline + +Summary of `getline' Variants +----------------------------- + + With all the forms of `getline', even though `$0' and `NF', may be +updated, the record will not be tested against all the patterns in the +`awk' program, in the way that would happen if the record were read +normally by the main processing loop of `awk'. However the new record +is tested against any subsequent rules. + + Many `awk' implementations limit the number of pipelines an `awk' +program may have open to just one! In `gawk', there is no such limit. +You can open as many pipelines as the underlying operating system will +permit. + + An interesting side-effect occurs if you use `getline' (without a +redirection) inside a `BEGIN' rule. Since an unredirected `getline' +reads from the command line data files, the first `getline' command +causes `awk' to set the value of `FILENAME'. Normally, `FILENAME' does +not have a value inside `BEGIN' rules, since you have not yet started +to process the command line data files (d.c.). (*Note The `BEGIN' and +`END' Special Patterns: BEGIN/END, also *note Built-in Variables that +Convey Information: Auto-set..) + + The following table summarizes the six variants of `getline', +listing which built-in variables are set by each one. + +`getline' + sets `$0', `NF', `FNR', and `NR'. + +`getline VAR' + sets VAR, `FNR', and `NR'. + +`getline < FILE' + sets `$0', and `NF'. + +`getline VAR < FILE' + sets VAR. + +`COMMAND | getline' + sets `$0', and `NF'. + +`COMMAND | getline VAR' + sets VAR. + + +File: gawk.info, Node: Printing, Next: Expressions, Prev: Reading Files, Up: Top + +Printing Output +*************** + + One of the most common actions is to "print", or output, some or all +of the input. You use the `print' statement for simple output. You +use the `printf' statement for fancier formatting. Both are described +in this chapter. + +* Menu: + +* Print:: The `print' statement. +* Print Examples:: Simple examples of `print' statements. +* Output Separators:: The output separators and how to change them. +* OFMT:: Controlling Numeric Output With `print'. +* Printf:: The `printf' statement. +* Redirection:: How to redirect output to multiple files and + pipes. +* Special Files:: File name interpretation in `gawk'. + `gawk' allows access to inherited file + descriptors. +* Close Files And Pipes:: Closing Input and Output Files and Pipes. + + +File: gawk.info, Node: Print, Next: Print Examples, Prev: Printing, Up: Printing + +The `print' Statement +===================== + + The `print' statement does output with simple, standardized +formatting. You specify only the strings or numbers to be printed, in a +list separated by commas. They are output, separated by single spaces, +followed by a newline. The statement looks like this: + + print ITEM1, ITEM2, ... + +The entire list of items may optionally be enclosed in parentheses. The +parentheses are necessary if any of the item expressions uses the `>' +relational operator; otherwise it could be confused with a redirection +(*note Redirecting Output of `print' and `printf': Redirection.). + + The items to be printed can be constant strings or numbers, fields +of the current record (such as `$1'), variables, or any `awk' +expressions. Numeric values are converted to strings, and then printed. + + The `print' statement is completely general for computing _what_ +values to print. However, with two exceptions, you cannot specify _how_ +to print them--how many columns, whether to use exponential notation or +not, and so on. (For the exceptions, *note Output Separators::, and +*Note Controlling Numeric Output with `print': OFMT.) For that, you +need the `printf' statement (*note Using `printf' Statements for +Fancier Printing: Printf.). + + The simple statement `print' with no items is equivalent to `print +$0': it prints the entire current record. To print a blank line, use +`print ""', where `""' is the empty string. + + To print a fixed piece of text, use a string constant such as +`"Don't Panic"' as one item. If you forget to use the double-quote +characters, your text will be taken as an `awk' expression, and you +will probably get an error. Keep in mind that a space is printed +between any two items. + + Each `print' statement makes at least one line of output. But it +isn't limited to one line. If an item value is a string that contains a +newline, the newline is output along with the rest of the string. A +single `print' can make any number of lines this way. + + +File: gawk.info, Node: Print Examples, Next: Output Separators, Prev: Print, Up: Printing + +Examples of `print' Statements +============================== + + Here is an example of printing a string that contains embedded +newlines (the `\n' is an escape sequence, used to represent the newline +character; see *Note Escape Sequences::): + + $ awk 'BEGIN { print "line one\nline two\nline three" }' + -| line one + -| line two + -| line three + + Here is an example that prints the first two fields of each input +record, with a space between them: + + $ awk '{ print $1, $2 }' inventory-shipped + -| Jan 13 + -| Feb 15 + -| Mar 15 + ... + + A common mistake in using the `print' statement is to omit the comma +between two items. This often has the effect of making the items run +together in the output, with no space. The reason for this is that +juxtaposing two string expressions in `awk' means to concatenate them. +Here is the same program, without the comma: + + $ awk '{ print $1 $2 }' inventory-shipped + -| Jan13 + -| Feb15 + -| Mar15 + ... + + To someone unfamiliar with the file `inventory-shipped', neither +example's output makes much sense. A heading line at the beginning +would make it clearer. Let's add some headings to our table of months +(`$1') and green crates shipped (`$2'). We do this using the `BEGIN' +pattern (*note The `BEGIN' and `END' Special Patterns: BEGIN/END.) to +force the headings to be printed only once: + + awk 'BEGIN { print "Month Crates" + print "----- ------" } + { print $1, $2 }' inventory-shipped + +Did you already guess what happens? When run, the program prints the +following: + + Month Crates + ----- ------ + Jan 13 + Feb 15 + Mar 15 + ... + +The headings and the table data don't line up! We can fix this by +printing some spaces between the two fields: + + awk 'BEGIN { print "Month Crates" + print "----- ------" } + { print $1, " ", $2 }' inventory-shipped + + You can imagine that this way of lining up columns can get pretty +complicated when you have many columns to fix. Counting spaces for two +or three columns can be simple, but more than this and you can get lost +quite easily. This is why the `printf' statement was created (*note +Using `printf' Statements for Fancier Printing: Printf.); one of its +specialties is lining up columns of data. + + As a side point, you can continue either a `print' or `printf' +statement simply by putting a newline after any comma (*note `awk' +Statements Versus Lines: Statements/Lines.). + + +File: gawk.info, Node: Output Separators, Next: OFMT, Prev: Print Examples, Up: Printing + +Output Separators +================= + + As mentioned previously, a `print' statement contains a list of +items, separated by commas. In the output, the items are normally +separated by single spaces. This need not be the case; a single space +is only the default. You can specify any string of characters to use +as the "output field separator" by setting the built-in variable `OFS'. +The initial value of this variable is the string `" "', that is, a +single space. + + The output from an entire `print' statement is called an "output +record". Each `print' statement outputs one output record and then +outputs a string called the "output record separator". The built-in +variable `ORS' specifies this string. The initial value of `ORS' is +the string `"\n"', i.e. a newline character; thus, normally each +`print' statement makes a separate line. + + You can change how output fields and records are separated by +assigning new values to the variables `OFS' and/or `ORS'. The usual +place to do this is in the `BEGIN' rule (*note The `BEGIN' and `END' +Special Patterns: BEGIN/END.), so that it happens before any input is +processed. You may also do this with assignments on the command line, +before the names of your input files, or using the `-v' command line +option (*note Command Line Options: Options.). + + The following example prints the first and second fields of each +input record separated by a semicolon, with a blank line added after +each line: + + $ awk 'BEGIN { OFS = ";"; ORS = "\n\n" } + > { print $1, $2 }' BBS-list + -| aardvark;555-5553 + -| + -| alpo-net;555-3412 + -| + -| barfly;555-7685 + ... + + If the value of `ORS' does not contain a newline, all your output +will be run together on a single line, unless you output newlines some +other way. + + +File: gawk.info, Node: OFMT, Next: Printf, Prev: Output Separators, Up: Printing + +Controlling Numeric Output with `print' +======================================= + + When you use the `print' statement to print numeric values, `awk' +internally converts the number to a string of characters, and prints +that string. `awk' uses the `sprintf' function to do this conversion +(*note Built-in Functions for String Manipulation: String Functions.). +For now, it suffices to say that the `sprintf' function accepts a +"format specification" that tells it how to format numbers (or +strings), and that there are a number of different ways in which +numbers can be formatted. The different format specifications are +discussed more fully in *Note Format-Control Letters: Control Letters. + + The built-in variable `OFMT' contains the default format +specification that `print' uses with `sprintf' when it wants to convert +a number to a string for printing. The default value of `OFMT' is +`"%.6g"'. By supplying different format specifications as the value of +`OFMT', you can change how `print' will print your numbers. As a brief +example: + + $ awk 'BEGIN { + > OFMT = "%.0f" # print numbers as integers (rounds) + > print 17.23 }' + -| 17 + +According to the POSIX standard, `awk''s behavior will be undefined if +`OFMT' contains anything but a floating point conversion specification +(d.c.). + + +File: gawk.info, Node: Printf, Next: Redirection, Prev: OFMT, Up: Printing + +Using `printf' Statements for Fancier Printing +============================================== + + If you want more precise control over the output format than `print' +gives you, use `printf'. With `printf' you can specify the width to +use for each item, and you can specify various formatting choices for +numbers (such as what radix to use, whether to print an exponent, +whether to print a sign, and how many digits to print after the decimal +point). You do this by supplying a string, called the "format string", +which controls how and where to print the other arguments. + +* Menu: + +* Basic Printf:: Syntax of the `printf' statement. +* Control Letters:: Format-control letters. +* Format Modifiers:: Format-specification modifiers. +* Printf Examples:: Several examples. + + +File: gawk.info, Node: Basic Printf, Next: Control Letters, Prev: Printf, Up: Printf + +Introduction to the `printf' Statement +-------------------------------------- + + The `printf' statement looks like this: + + printf FORMAT, ITEM1, ITEM2, ... + +The entire list of arguments may optionally be enclosed in parentheses. +The parentheses are necessary if any of the item expressions use the +`>' relational operator; otherwise it could be confused with a +redirection (*note Redirecting Output of `print' and `printf': +Redirection.). + + The difference between `printf' and `print' is the FORMAT argument. +This is an expression whose value is taken as a string; it specifies +how to output each of the other arguments. It is called the "format +string". + + The format string is very similar to that in the ANSI C library +function `printf'. Most of FORMAT is text to be output verbatim. +Scattered among this text are "format specifiers", one per item. Each +format specifier says to output the next item in the argument list at +that place in the format. + + The `printf' statement does not automatically append a newline to its +output. It outputs only what the format string specifies. So if you +want a newline, you must include one in the format string. The output +separator variables `OFS' and `ORS' have no effect on `printf' +statements. For example: + + BEGIN { + ORS = "\nOUCH!\n"; OFS = "!" + msg = "Don't Panic!"; printf "%s\n", msg + } + + This program still prints the familiar `Don't Panic!' message. + + +File: gawk.info, Node: Control Letters, Next: Format Modifiers, Prev: Basic Printf, Up: Printf + +Format-Control Letters +---------------------- + + A format specifier starts with the character `%' and ends with a +"format-control letter"; it tells the `printf' statement how to output +one item. (If you actually want to output a `%', write `%%'.) The +format-control letter specifies what kind of value to print. The rest +of the format specifier is made up of optional "modifiers" which are +parameters to use, such as the field width. + + Here is a list of the format-control letters: + +`c' + This prints a number as an ASCII character. Thus, `printf "%c", + 65' outputs the letter `A'. The output for a string value is the + first character of the string. + +`d' +`i' + These are equivalent. They both print a decimal integer. The `%i' + specification is for compatibility with ANSI C. + +`e' +`E' + This prints a number in scientific (exponential) notation. For + example, + + printf "%4.3e\n", 1950 + + prints `1.950e+03', with a total of four significant figures of + which three follow the decimal point. The `4.3' are modifiers, + discussed below. `%E' uses `E' instead of `e' in the output. + +`f' + This prints a number in floating point notation. For example, + + printf "%4.3f", 1950 + + prints `1950.000', with a total of four significant figures of + which three follow the decimal point. The `4.3' are modifiers, + discussed below. + +`g' +`G' + This prints a number in either scientific notation or floating + point notation, whichever uses fewer characters. If the result is + printed in scientific notation, `%G' uses `E' instead of `e'. + +`o' + This prints an unsigned octal integer. (In octal, or base-eight + notation, the digits run from `0' to `7'; the decimal number eight + is represented as `10' in octal.) + +`s' + This prints a string. + +`x' +`X' + This prints an unsigned hexadecimal integer. (In hexadecimal, or + base-16 notation, the digits are `0' through `9' and `a' through + `f'. The hexadecimal digit `f' represents the decimal number 15.) + `%X' uses the letters `A' through `F' instead of `a' through `f'. + +`%' + This isn't really a format-control letter, but it does have a + meaning when used after a `%': the sequence `%%' outputs one `%'. + It does not consume an argument, and it ignores any modifiers. + + When using the integer format-control letters for values that are +outside the range of a C `long' integer, `gawk' will switch to the `%g' +format specifier. Other versions of `awk' may print invalid values, or +do something else entirely (d.c.). + + +File: gawk.info, Node: Format Modifiers, Next: Printf Examples, Prev: Control Letters, Up: Printf + +Modifiers for `printf' Formats +------------------------------ + + A format specification can also include "modifiers" that can control +how much of the item's value is printed and how much space it gets. The +modifiers come between the `%' and the format-control letter. In the +examples below, we use the bullet symbol "*" to represent spaces in the +output. Here are the possible modifiers, in the order in which they may +appear: + +`-' + The minus sign, used before the width modifier (see below), says + to left-justify the argument within its specified width. Normally + the argument is printed right-justified in the specified width. + Thus, + + printf "%-4s", "foo" + + prints `foo*'. + +`SPACE' + For numeric conversions, prefix positive values with a space, and + negative values with a minus sign. + +`+' + The plus sign, used before the width modifier (see below), says to + always supply a sign for numeric conversions, even if the data to + be formatted is positive. The `+' overrides the space modifier. + +`#' + Use an "alternate form" for certain control letters. For `%o', + supply a leading zero. For `%x', and `%X', supply a leading `0x' + or `0X' for a non-zero result. For `%e', `%E', and `%f', the + result will always contain a decimal point. For `%g', and `%G', + trailing zeros are not removed from the result. + +`0' + A leading `0' (zero) acts as a flag, that indicates output should + be padded with zeros instead of spaces. This applies even to + non-numeric output formats (d.c.). This flag only has an effect + when the field width is wider than the value to be printed. + +`WIDTH' + This is a number specifying the desired minimum width of a field. + Inserting any number between the `%' sign and the format control + character forces the field to be expanded to this width. The + default way to do this is to pad with spaces on the left. For + example, + + printf "%4s", "foo" + + prints `*foo'. + + The value of WIDTH is a minimum width, not a maximum. If the item + value requires more than WIDTH characters, it can be as wide as + necessary. Thus, + + printf "%4s", "foobar" + + prints `foobar'. + + Preceding the WIDTH with a minus sign causes the output to be + padded with spaces on the right, instead of on the left. + +`.PREC' + This is a number that specifies the precision to use when printing. + For the `e', `E', and `f' formats, this specifies the number of + digits you want printed to the right of the decimal point. For + the `g', and `G' formats, it specifies the maximum number of + significant digits. For the `d', `o', `i', `u', `x', and `X' + formats, it specifies the minimum number of digits to print. For + a string, it specifies the maximum number of characters from the + string that should be printed. Thus, + + printf "%.4s", "foobar" + + prints `foob'. + + The C library `printf''s dynamic WIDTH and PREC capability (for +example, `"%*.*s"') is supported. Instead of supplying explicit WIDTH +and/or PREC values in the format string, you pass them in the argument +list. For example: + + w = 5 + p = 3 + s = "abcdefg" + printf "%*.*s\n", w, p, s + +is exactly equivalent to + + s = "abcdefg" + printf "%5.3s\n", s + +Both programs output `**abc'. + + Earlier versions of `awk' did not support this capability. If you +must use such a version, you may simulate this feature by using +concatenation to build up the format string, like so: + + w = 5 + p = 3 + s = "abcdefg" + printf "%" w "." p "s\n", s + +This is not particularly easy to read, but it does work. + + C programmers may be used to supplying additional `l' and `h' flags +in `printf' format strings. These are not valid in `awk'. Most `awk' +implementations silently ignore these flags. If `--lint' is provided +on the command line (*note Command Line Options: Options.), `gawk' will +warn about their use. If `--posix' is supplied, their use is a fatal +error. + + +File: gawk.info, Node: Printf Examples, Prev: Format Modifiers, Up: Printf + +Examples Using `printf' +----------------------- + + Here is how to use `printf' to make an aligned table: + + awk '{ printf "%-10s %s\n", $1, $2 }' BBS-list + +prints the names of bulletin boards (`$1') of the file `BBS-list' as a +string of 10 characters, left justified. It also prints the phone +numbers (`$2') afterward on the line. This produces an aligned +two-column table of names and phone numbers: + + $ awk '{ printf "%-10s %s\n", $1, $2 }' BBS-list + -| aardvark 555-5553 + -| alpo-net 555-3412 + -| barfly 555-7685 + -| bites 555-1675 + -| camelot 555-0542 + -| core 555-2912 + -| fooey 555-1234 + -| foot 555-6699 + -| macfoo 555-6480 + -| sdace 555-3430 + -| sabafoo 555-2127 + + Did you notice that we did not specify that the phone numbers be +printed as numbers? They had to be printed as strings because the +numbers are separated by a dash. If we had tried to print the phone +numbers as numbers, all we would have gotten would have been the first +three digits, `555'. This would have been pretty confusing. + + We did not specify a width for the phone numbers because they are the +last things on their lines. We don't need to put spaces after them. + + We could make our table look even nicer by adding headings to the +tops of the columns. To do this, we use the `BEGIN' pattern (*note The +`BEGIN' and `END' Special Patterns: BEGIN/END.) to force the header to +be printed only once, at the beginning of the `awk' program: + + awk 'BEGIN { print "Name Number" + print "---- ------" } + { printf "%-10s %s\n", $1, $2 }' BBS-list + + Did you notice that we mixed `print' and `printf' statements in the +above example? We could have used just `printf' statements to get the +same results: + + awk 'BEGIN { printf "%-10s %s\n", "Name", "Number" + printf "%-10s %s\n", "----", "------" } + { printf "%-10s %s\n", $1, $2 }' BBS-list + +By printing each column heading with the same format specification used +for the elements of the column, we have made sure that the headings are +aligned just like the columns. + + The fact that the same format specification is used three times can +be emphasized by storing it in a variable, like this: + + awk 'BEGIN { format = "%-10s %s\n" + printf format, "Name", "Number" + printf format, "----", "------" } + { printf format, $1, $2 }' BBS-list + + See if you can use the `printf' statement to line up the headings and +table data for our `inventory-shipped' example covered earlier in the +section on the `print' statement (*note The `print' Statement: Print.). + + +File: gawk.info, Node: Redirection, Next: Special Files, Prev: Printf, Up: Printing + +Redirecting Output of `print' and `printf' +========================================== + + So far we have been dealing only with output that prints to the +standard output, usually your terminal. Both `print' and `printf' can +also send their output to other places. This is called "redirection". + + A redirection appears after the `print' or `printf' statement. +Redirections in `awk' are written just like redirections in shell +commands, except that they are written inside the `awk' program. + + There are three forms of output redirection: output to a file, +output appended to a file, and output through a pipe to another command. +They are all shown for the `print' statement, but they work identically +for `printf' also. + +`print ITEMS > OUTPUT-FILE' + This type of redirection prints the items into the output file + OUTPUT-FILE. The file name OUTPUT-FILE can be any expression. + Its value is changed to a string and then used as a file name + (*note Expressions::). + + When this type of redirection is used, the OUTPUT-FILE is erased + before the first output is written to it. Subsequent writes to + the same OUTPUT-FILE do not erase OUTPUT-FILE, but append to it. + If OUTPUT-FILE does not exist, then it is created. + + For example, here is how an `awk' program can write a list of BBS + names to a file `name-list' and a list of phone numbers to a file + `phone-list'. Each output file contains one name or number per + line. + + $ awk '{ print $2 > "phone-list" + > print $1 > "name-list" }' BBS-list + $ cat phone-list + -| 555-5553 + -| 555-3412 + ... + $ cat name-list + -| aardvark + -| alpo-net + ... + +`print ITEMS >> OUTPUT-FILE' + This type of redirection prints the items into the pre-existing + output file OUTPUT-FILE. The difference between this and the + single-`>' redirection is that the old contents (if any) of + OUTPUT-FILE are not erased. Instead, the `awk' output is appended + to the file. If OUTPUT-FILE does not exist, then it is created. + +`print ITEMS | COMMAND' + It is also possible to send output to another program through a + pipe instead of into a file. This type of redirection opens a + pipe to COMMAND and writes the values of ITEMS through this pipe, + to another process created to execute COMMAND. + + The redirection argument COMMAND is actually an `awk' expression. + Its value is converted to a string, whose contents give the shell + command to be run. + + For example, this produces two files, one unsorted list of BBS + names and one list sorted in reverse alphabetical order: + + awk '{ print $1 > "names.unsorted" + command = "sort -r > names.sorted" + print $1 | command }' BBS-list + + Here the unsorted list is written with an ordinary redirection + while the sorted list is written by piping through the `sort' + utility. + + This example uses redirection to mail a message to a mailing list + `bug-system'. This might be useful when trouble is encountered in + an `awk' script run periodically for system maintenance. + + report = "mail bug-system" + print "Awk script failed:", $0 | report + m = ("at record number " FNR " of " FILENAME) + print m | report + close(report) + + The message is built using string concatenation and saved in the + variable `m'. It is then sent down the pipeline to the `mail' + program. + + We call the `close' function here because it's a good idea to close + the pipe as soon as all the intended output has been sent to it. + *Note Closing Input and Output Files and Pipes: Close Files And + Pipes, for more information on this. This example also + illustrates the use of a variable to represent a FILE or COMMAND: + it is not necessary to always use a string constant. Using a + variable is generally a good idea, since `awk' requires you to + spell the string value identically every time. + + Redirecting output using `>', `>>', or `|' asks the system to open a +file or pipe only if the particular FILE or COMMAND you've specified +has not already been written to by your program, or if it has been +closed since it was last written to. + + Many `awk' implementations limit the number of pipelines an `awk' +program may have open to just one! In `gawk', there is no such limit. +You can open as many pipelines as the underlying operating system will +permit. + + +File: gawk.info, Node: Special Files, Next: Close Files And Pipes, Prev: Redirection, Up: Printing + +Special File Names in `gawk' +============================ + + Running programs conventionally have three input and output streams +already available to them for reading and writing. These are known as +the "standard input", "standard output", and "standard error output". +These streams are, by default, connected to your terminal, but they are +often redirected with the shell, via the `<', `<<', `>', `>>', `>&' and +`|' operators. Standard error is typically used for writing error +messages; the reason we have two separate streams, standard output and +standard error, is so that they can be redirected separately. + + In other implementations of `awk', the only way to write an error +message to standard error in an `awk' program is as follows: + + print "Serious error detected!" | "cat 1>&2" + +This works by opening a pipeline to a shell command which can access the +standard error stream which it inherits from the `awk' process. This +is far from elegant, and is also inefficient, since it requires a +separate process. So people writing `awk' programs often neglect to do +this. Instead, they send the error messages to the terminal, like this: + + print "Serious error detected!" > "/dev/tty" + +This usually has the same effect, but not always: although the standard +error stream is usually the terminal, it can be redirected, and when +that happens, writing to the terminal is not correct. In fact, if +`awk' is run from a background job, it may not have a terminal at all. +Then opening `/dev/tty' will fail. + + `gawk' provides special file names for accessing the three standard +streams. When you redirect input or output in `gawk', if the file name +matches one of these special names, then `gawk' directly uses the +stream it stands for. + +`/dev/stdin' + The standard input (file descriptor 0). + +`/dev/stdout' + The standard output (file descriptor 1). + +`/dev/stderr' + The standard error output (file descriptor 2). + +`/dev/fd/N' + The file associated with file descriptor N. Such a file must have + been opened by the program initiating the `awk' execution + (typically the shell). Unless you take special pains in the shell + from which you invoke `gawk', only descriptors 0, 1 and 2 are + available. + + The file names `/dev/stdin', `/dev/stdout', and `/dev/stderr' are +aliases for `/dev/fd/0', `/dev/fd/1', and `/dev/fd/2', respectively, +but they are more self-explanatory. + + The proper way to write an error message in a `gawk' program is to +use `/dev/stderr', like this: + + print "Serious error detected!" > "/dev/stderr" + + `gawk' also provides special file names that give access to +information about the running `gawk' process. Each of these "files" +provides a single record of information. To read them more than once, +you must first close them with the `close' function (*note Closing +Input and Output Files and Pipes: Close Files And Pipes.). The +filenames are: + +`/dev/pid' + Reading this file returns the process ID of the current process, + in decimal, terminated with a newline. + +`/dev/ppid' + Reading this file returns the parent process ID of the current + process, in decimal, terminated with a newline. + +`/dev/pgrpid' + Reading this file returns the process group ID of the current + process, in decimal, terminated with a newline. + +`/dev/user' + Reading this file returns a single record terminated with a + newline. The fields are separated with spaces. The fields + represent the following information: + + `$1' + The return value of the `getuid' system call (the real user + ID number). + + `$2' + The return value of the `geteuid' system call (the effective + user ID number). + + `$3' + The return value of the `getgid' system call (the real group + ID number). + + `$4' + The return value of the `getegid' system call (the effective + group ID number). + + If there are any additional fields, they are the group IDs + returned by `getgroups' system call. (Multiple groups may not be + supported on all systems.) + + These special file names may be used on the command line as data +files, as well as for I/O redirections within an `awk' program. They +may not be used as source files with the `-f' option. + + Recognition of these special file names is disabled if `gawk' is in +compatibility mode (*note Command Line Options: Options.). + + *Caution*: Unless your system actually has a `/dev/fd' directory +(or any of the other above listed special files), the interpretation of +these file names is done by `gawk' itself. For example, using +`/dev/fd/4' for output will actually write on file descriptor 4, and +not on a new file descriptor that was `dup''ed from file descriptor 4. +Most of the time this does not matter; however, it is important to +_not_ close any of the files related to file descriptors 0, 1, and 2. +If you do close one of these files, unpredictable behavior will result. + + The special files that provide process-related information may +disappear in a future version of `gawk'. *Note Probable Future +Extensions: Future Extensions. + + +File: gawk.info, Node: Close Files And Pipes, Prev: Special Files, Up: Printing + +Closing Input and Output Files and Pipes +======================================== + + If the same file name or the same shell command is used with +`getline' (*note Explicit Input with `getline': Getline.) more than +once during the execution of an `awk' program, the file is opened (or +the command is executed) only the first time. At that time, the first +record of input is read from that file or command. The next time the +same file or command is used in `getline', another record is read from +it, and so on. + + Similarly, when a file or pipe is opened for output, the file name +or command associated with it is remembered by `awk' and subsequent +writes to the same file or command are appended to the previous writes. +The file or pipe stays open until `awk' exits. + + This implies that if you want to start reading the same file again +from the beginning, or if you want to rerun a shell command (rather than +reading more output from the command), you must take special steps. +What you must do is use the `close' function, as follows: + + close(FILENAME) + +or + + close(COMMAND) + + The argument FILENAME or COMMAND can be any expression. Its value +must _exactly_ match the string that was used to open the file or start +the command (spaces and other "irrelevant" characters included). For +example, if you open a pipe with this: + + "sort -r names" | getline foo + +then you must close it with this: + + close("sort -r names") + + Once this function call is executed, the next `getline' from that +file or command, or the next `print' or `printf' to that file or +command, will reopen the file or rerun the command. + + Because the expression that you use to close a file or pipeline must +exactly match the expression used to open the file or run the command, +it is good practice to use a variable to store the file name or command. +The previous example would become + + sortcom = "sort -r names" + sortcom | getline foo + ... + close(sortcom) + +This helps avoid hard-to-find typographical errors in your `awk' +programs. + + Here are some reasons why you might need to close an output file: + + * To write a file and read it back later on in the same `awk' + program. Close the file when you are finished writing it; then + you can start reading it with `getline'. + + * To write numerous files, successively, in the same `awk' program. + If you don't close the files, eventually you may exceed a system + limit on the number of open files in one process. So close each + one when you are finished writing it. + + * To make a command finish. When you redirect output through a pipe, + the command reading the pipe normally continues to try to read + input as long as the pipe is open. Often this means the command + cannot really do its work until the pipe is closed. For example, + if you redirect output to the `mail' program, the message is not + actually sent until the pipe is closed. + + * To run the same program a second time, with the same arguments. + This is not the same thing as giving more input to the first run! + + For example, suppose you pipe output to the `mail' program. If you + output several lines redirected to this pipe without closing it, + they make a single message of several lines. By contrast, if you + close the pipe after each line of output, then each line makes a + separate message. + + `close' returns a value of zero if the close succeeded. Otherwise, +the value will be non-zero. In this case, `gawk' sets the variable +`ERRNO' to a string describing the error that occurred. + + If you use more files than the system allows you to have open, +`gawk' will attempt to multiplex the available open files among your +data files. `gawk''s ability to do this depends upon the facilities of +your operating system: it may not always work. It is therefore both +good practice and good portability advice to always use `close' on your +files when you are done with them. + + +File: gawk.info, Node: Expressions, Next: Patterns and Actions, Prev: Printing, Up: Top + +Expressions +*********** + + Expressions are the basic building blocks of `awk' patterns and +actions. An expression evaluates to a value, which you can print, test, +store in a variable or pass to a function. Additionally, an expression +can assign a new value to a variable or a field, with an assignment +operator. + + An expression can serve as a pattern or action statement on its own. +Most other kinds of statements contain one or more expressions which +specify data on which to operate. As in other languages, expressions +in `awk' include variables, array references, constants, and function +calls, as well as combinations of these with various operators. + +* Menu: + +* Constants:: String, numeric, and regexp constants. +* Using Constant Regexps:: When and how to use a regexp constant. +* Variables:: Variables give names to values for later use. +* Conversion:: The conversion of strings to numbers and vice + versa. +* Arithmetic Ops:: Arithmetic operations (`+', `-', + etc.) +* Concatenation:: Concatenating strings. +* Assignment Ops:: Changing the value of a variable or a field. +* Increment Ops:: Incrementing the numeric value of a variable. +* Truth Values:: What is ``true'' and what is ``false''. +* Typing and Comparison:: How variables acquire types, and how this + affects comparison of numbers and strings with + `<', etc. +* Boolean Ops:: Combining comparison expressions using boolean + operators `||' (``or''), `&&' + (``and'') and `!' (``not''). +* Conditional Exp:: Conditional expressions select between two + subexpressions under control of a third + subexpression. +* Function Calls:: A function call is an expression. +* Precedence:: How various operators nest. + + +File: gawk.info, Node: Constants, Next: Using Constant Regexps, Prev: Expressions, Up: Expressions + +Constant Expressions +==================== + + The simplest type of expression is the "constant", which always has +the same value. There are three types of constants: numeric constants, +string constants, and regular expression constants. + +* Menu: + +* Scalar Constants:: Numeric and string constants. +* Regexp Constants:: Regular Expression constants. + + +File: gawk.info, Node: Scalar Constants, Next: Regexp Constants, Prev: Constants, Up: Constants + +Numeric and String Constants +---------------------------- + + A "numeric constant" stands for a number. This number can be an +integer, a decimal fraction, or a number in scientific (exponential) +notation.(1) Here are some examples of numeric constants, which all +have the same value: + + 105 + 1.05e+2 + 1050e-1 + + A string constant consists of a sequence of characters enclosed in +double-quote marks. For example: + + "parrot" + +represents the string whose contents are `parrot'. Strings in `gawk' +can be of any length and they can contain any of the possible eight-bit +ASCII characters including ASCII NUL (character code zero). Other `awk' +implementations may have difficulty with some character codes. + + ---------- Footnotes ---------- + + (1) The internal representation uses double-precision floating point +numbers. If you don't know what that means, then don't worry about it. + + +File: gawk.info, Node: Regexp Constants, Prev: Scalar Constants, Up: Constants + +Regular Expression Constants +---------------------------- + + A regexp constant is a regular expression description enclosed in +slashes, such as `/^beginning and end$/'. Most regexps used in `awk' +programs are constant, but the `~' and `!~' matching operators can also +match computed or "dynamic" regexps (which are just ordinary strings or +variables that contain a regexp). + + +File: gawk.info, Node: Using Constant Regexps, Next: Variables, Prev: Constants, Up: Expressions + +Using Regular Expression Constants +================================== + + When used on the right hand side of the `~' or `!~' operators, a +regexp constant merely stands for the regexp that is to be matched. + + Regexp constants (such as `/foo/') may be used like simple +expressions. When a regexp constant appears by itself, it has the same +meaning as if it appeared in a pattern, i.e. `($0 ~ /foo/)' (d.c.) +(*note Expressions as Patterns: Expression Patterns.). This means that +the two code segments, + + if ($0 ~ /barfly/ || $0 ~ /camelot/) + print "found" + +and + + if (/barfly/ || /camelot/) + print "found" + +are exactly equivalent. + + One rather bizarre consequence of this rule is that the following +boolean expression is valid, but does not do what the user probably +intended: + + # note that /foo/ is on the left of the ~ + if (/foo/ ~ $1) print "found foo" + +This code is "obviously" testing `$1' for a match against the regexp +`/foo/'. But in fact, the expression `/foo/ ~ $1' actually means `($0 +~ /foo/) ~ $1'. In other words, first match the input record against +the regexp `/foo/'. The result will be either zero or one, depending +upon the success or failure of the match. Then match that result +against the first field in the record. + + Since it is unlikely that you would ever really wish to make this +kind of test, `gawk' will issue a warning when it sees this construct in +a program. + + Another consequence of this rule is that the assignment statement + + matches = /foo/ + +will assign either zero or one to the variable `matches', depending +upon the contents of the current input record. + + This feature of the language was never well documented until the +POSIX specification. + + Constant regular expressions are also used as the first argument for +the `gensub', `sub' and `gsub' functions, and as the second argument of +the `match' function (*note Built-in Functions for String Manipulation: +String Functions.). Modern implementations of `awk', including `gawk', +allow the third argument of `split' to be a regexp constant, while some +older implementations do not (d.c.). + + This can lead to confusion when attempting to use regexp constants +as arguments to user defined functions (*note User-defined Functions: +User-defined.). For example: + + function mysub(pat, repl, str, global) + { + if (global) + gsub(pat, repl, str) + else + sub(pat, repl, str) + return str + } + + { + ... + text = "hi! hi yourself!" + mysub(/hi/, "howdy", text, 1) + ... + } + + In this example, the programmer wishes to pass a regexp constant to +the user-defined function `mysub', which will in turn pass it on to +either `sub' or `gsub'. However, what really happens is that the `pat' +parameter will be either one or zero, depending upon whether or not +`$0' matches `/hi/'. + + As it is unlikely that you would ever really wish to pass a truth +value in this way, `gawk' will issue a warning when it sees a regexp +constant used as a parameter to a user-defined function. + + +File: gawk.info, Node: Variables, Next: Conversion, Prev: Using Constant Regexps, Up: Expressions + +Variables +========= + + Variables are ways of storing values at one point in your program for +use later in another part of your program. You can manipulate them +entirely within your program text, and you can also assign values to +them on the `awk' command line. + +* Menu: + +* Using Variables:: Using variables in your programs. +* Assignment Options:: Setting variables on the command line and a + summary of command line syntax. This is an + advanced method of input. + + +File: gawk.info, Node: Using Variables, Next: Assignment Options, Prev: Variables, Up: Variables + +Using Variables in a Program +---------------------------- + + Variables let you give names to values and refer to them later. You +have already seen variables in many of the examples. The name of a +variable must be a sequence of letters, digits and underscores, but it +may not begin with a digit. Case is significant in variable names; `a' +and `A' are distinct variables. + + A variable name is a valid expression by itself; it represents the +variable's current value. Variables are given new values with +"assignment operators", "increment operators" and "decrement operators". +*Note Assignment Expressions: Assignment Ops. + + A few variables have special built-in meanings, such as `FS', the +field separator, and `NF', the number of fields in the current input +record. *Note Built-in Variables::, for a list of them. These +built-in variables can be used and assigned just like all other +variables, but their values are also used or changed automatically by +`awk'. All built-in variables names are entirely upper-case. + + Variables in `awk' can be assigned either numeric or string values. +By default, variables are initialized to the empty string, which is +zero if converted to a number. There is no need to "initialize" each +variable explicitly in `awk', the way you would in C and in most other +traditional languages. + + +File: gawk.info, Node: Assignment Options, Prev: Using Variables, Up: Variables + +Assigning Variables on the Command Line +--------------------------------------- + + You can set any `awk' variable by including a "variable assignment" +among the arguments on the command line when you invoke `awk' (*note +Other Command Line Arguments: Other Arguments.). Such an assignment has +this form: + + VARIABLE=TEXT + +With it, you can set a variable either at the beginning of the `awk' +run or in between input files. + + If you precede the assignment with the `-v' option, like this: + + -v VARIABLE=TEXT + +then the variable is set at the very beginning, before even the `BEGIN' +rules are run. The `-v' option and its assignment must precede all the +file name arguments, as well as the program text. (*Note Command Line +Options: Options, for more information about the `-v' option.) + + Otherwise, the variable assignment is performed at a time determined +by its position among the input file arguments: after the processing of +the preceding input file argument. For example: + + awk '{ print $n }' n=4 inventory-shipped n=2 BBS-list + +prints the value of field number `n' for all input records. Before the +first file is read, the command line sets the variable `n' equal to +four. This causes the fourth field to be printed in lines from the +file `inventory-shipped'. After the first file has finished, but +before the second file is started, `n' is set to two, so that the +second field is printed in lines from `BBS-list'. + + $ awk '{ print $n }' n=4 inventory-shipped n=2 BBS-list + -| 15 + -| 24 + ... + -| 555-5553 + -| 555-3412 + ... + + Command line arguments are made available for explicit examination by +the `awk' program in an array named `ARGV' (*note Using `ARGC' and +`ARGV': ARGC and ARGV.). + + `awk' processes the values of command line assignments for escape +sequences (d.c.) (*note Escape Sequences::). + + +File: gawk.info, Node: Conversion, Next: Arithmetic Ops, Prev: Variables, Up: Expressions + +Conversion of Strings and Numbers +================================= + + Strings are converted to numbers, and numbers to strings, if the +context of the `awk' program demands it. For example, if the value of +either `foo' or `bar' in the expression `foo + bar' happens to be a +string, it is converted to a number before the addition is performed. +If numeric values appear in string concatenation, they are converted to +strings. Consider this: + + two = 2; three = 3 + print (two three) + 4 + +This prints the (numeric) value 27. The numeric values of the +variables `two' and `three' are converted to strings and concatenated +together, and the resulting string is converted back to the number 23, +to which four is then added. + + If, for some reason, you need to force a number to be converted to a +string, concatenate the empty string, `""', with that number. To force +a string to be converted to a number, add zero to that string. + + A string is converted to a number by interpreting any numeric prefix +of the string as numerals: `"2.5"' converts to 2.5, `"1e3"' converts to +1000, and `"25fix"' has a numeric value of 25. Strings that can't be +interpreted as valid numbers are converted to zero. + + The exact manner in which numbers are converted into strings is +controlled by the `awk' built-in variable `CONVFMT' (*note Built-in +Variables::). Numbers are converted using the `sprintf' function +(*note Built-in Functions for String Manipulation: String Functions.) +with `CONVFMT' as the format specifier. + + `CONVFMT''s default value is `"%.6g"', which prints a value with at +least six significant digits. For some applications you will want to +change it to specify more precision. Double precision on most modern +machines gives you 16 or 17 decimal digits of precision. + + Strange results can happen if you set `CONVFMT' to a string that +doesn't tell `sprintf' how to format floating point numbers in a useful +way. For example, if you forget the `%' in the format, all numbers +will be converted to the same constant string. + + As a special case, if a number is an integer, then the result of +converting it to a string is _always_ an integer, no matter what the +value of `CONVFMT' may be. Given the following code fragment: + + CONVFMT = "%2.2f" + a = 12 + b = a "" + +`b' has the value `"12"', not `"12.00"' (d.c.). + + Prior to the POSIX standard, `awk' specified that the value of +`OFMT' was used for converting numbers to strings. `OFMT' specifies +the output format to use when printing numbers with `print'. `CONVFMT' +was introduced in order to separate the semantics of conversion from +the semantics of printing. Both `CONVFMT' and `OFMT' have the same +default value: `"%.6g"'. In the vast majority of cases, old `awk' +programs will not change their behavior. However, this use of `OFMT' +is something to keep in mind if you must port your program to other +implementations of `awk'; we recommend that instead of changing your +programs, you just port `gawk' itself! *Note The `print' Statement: +Print, for more information on the `print' statement. + + +File: gawk.info, Node: Arithmetic Ops, Next: Concatenation, Prev: Conversion, Up: Expressions + +Arithmetic Operators +==================== + + The `awk' language uses the common arithmetic operators when +evaluating expressions. All of these arithmetic operators follow normal +precedence rules, and work as you would expect them to. + + Here is a file `grades' containing a list of student names and three +test scores per student (it's a small class): + + Pat 100 97 58 + Sandy 84 72 93 + Chris 72 92 89 + +This programs takes the file `grades', and prints the average of the +scores. + + $ awk '{ sum = $2 + $3 + $4 ; avg = sum / 3 + > print $1, avg }' grades + -| Pat 85 + -| Sandy 83 + -| Chris 84.3333 + + This table lists the arithmetic operators in `awk', in order from +highest precedence to lowest: + +`- X' + Negation. + +`+ X' + Unary plus. The expression is converted to a number. + +`X ^ Y' +`X ** Y' + Exponentiation: X raised to the Y power. `2 ^ 3' has the value + eight. The character sequence `**' is equivalent to `^'. (The + POSIX standard only specifies the use of `^' for exponentiation.) + +`X * Y' + Multiplication. + +`X / Y' + Division. Since all numbers in `awk' are real numbers, the result + is not rounded to an integer: `3 / 4' has the value 0.75. + +`X % Y' + Remainder. The quotient is rounded toward zero to an integer, + multiplied by Y and this result is subtracted from X. This + operation is sometimes known as "trunc-mod." The following + relation always holds: + + b * int(a / b) + (a % b) == a + + One possibly undesirable effect of this definition of remainder is + that `X % Y' is negative if X is negative. Thus, + + -17 % 8 = -1 + + In other `awk' implementations, the signedness of the remainder + may be machine dependent. + +`X + Y' + Addition. + +`X - Y' + Subtraction. + + For maximum portability, do not use the `**' operator. + + Unary plus and minus have the same precedence, the multiplication +operators all have the same precedence, and addition and subtraction +have the same precedence. + + +File: gawk.info, Node: Concatenation, Next: Assignment Ops, Prev: Arithmetic Ops, Up: Expressions + +String Concatenation +==================== + + There is only one string operation: concatenation. It does not have +a specific operator to represent it. Instead, concatenation is +performed by writing expressions next to one another, with no operator. +For example: + + $ awk '{ print "Field number one: " $1 }' BBS-list + -| Field number one: aardvark + -| Field number one: alpo-net + ... + + Without the space in the string constant after the `:', the line +would run together. For example: + + $ awk '{ print "Field number one:" $1 }' BBS-list + -| Field number one:aardvark + -| Field number one:alpo-net + ... + + Since string concatenation does not have an explicit operator, it is +often necessary to insure that it happens where you want it to by using +parentheses to enclose the items to be concatenated. For example, the +following code fragment does not concatenate `file' and `name' as you +might expect: + + file = "file" + name = "name" + print "something meaningful" > file name + +It is necessary to use the following: + + print "something meaningful" > (file name) + + We recommend that you use parentheses around concatenation in all +but the most common contexts (such as on the right-hand side of `='). + + +File: gawk.info, Node: Assignment Ops, Next: Increment Ops, Prev: Concatenation, Up: Expressions + +Assignment Expressions +====================== + + An "assignment" is an expression that stores a new value into a +variable. For example, let's assign the value one to the variable `z': + + z = 1 + + After this expression is executed, the variable `z' has the value +one. Whatever old value `z' had before the assignment is forgotten. + + Assignments can store string values also. For example, this would +store the value `"this food is good"' in the variable `message': + + thing = "food" + predicate = "good" + message = "this " thing " is " predicate + +(This also illustrates string concatenation.) + + The `=' sign is called an "assignment operator". It is the simplest +assignment operator because the value of the right-hand operand is +stored unchanged. + + Most operators (addition, concatenation, and so on) have no effect +except to compute a value. If you ignore the value, you might as well +not use the operator. An assignment operator is different; it does +produce a value, but even if you ignore the value, the assignment still +makes itself felt through the alteration of the variable. We call this +a "side effect". + + The left-hand operand of an assignment need not be a variable (*note +Variables::); it can also be a field (*note Changing the Contents of a +Field: Changing Fields.) or an array element (*note Arrays in `awk': +Arrays.). These are all called "lvalues", which means they can appear +on the left-hand side of an assignment operator. The right-hand +operand may be any expression; it produces the new value which the +assignment stores in the specified variable, field or array element. +(Such values are called "rvalues"). + + It is important to note that variables do _not_ have permanent types. +The type of a variable is simply the type of whatever value it happens +to hold at the moment. In the following program fragment, the variable +`foo' has a numeric value at first, and a string value later on: + + foo = 1 + print foo + foo = "bar" + print foo + +When the second assignment gives `foo' a string value, the fact that it +previously had a numeric value is forgotten. + + String values that do not begin with a digit have a numeric value of +zero. After executing this code, the value of `foo' is five: + + foo = "a string" + foo = foo + 5 + +(Note that using a variable as a number and then later as a string can +be confusing and is poor programming style. The above examples +illustrate how `awk' works, _not_ how you should write your own +programs!) + + An assignment is an expression, so it has a value: the same value +that is assigned. Thus, `z = 1' as an expression has the value one. +One consequence of this is that you can write multiple assignments +together: + + x = y = z = 0 + +stores the value zero in all three variables. It does this because the +value of `z = 0', which is zero, is stored into `y', and then the value +of `y = z = 0', which is zero, is stored into `x'. + + You can use an assignment anywhere an expression is called for. For +example, it is valid to write `x != (y = 1)' to set `y' to one and then +test whether `x' equals one. But this style tends to make programs +hard to read; except in a one-shot program, you should not use such +nesting of assignments. + + Aside from `=', there are several other assignment operators that do +arithmetic with the old value of the variable. For example, the +operator `+=' computes a new value by adding the right-hand value to +the old value of the variable. Thus, the following assignment adds +five to the value of `foo': + + foo += 5 + +This is equivalent to the following: + + foo = foo + 5 + +Use whichever one makes the meaning of your program clearer. + + There are situations where using `+=' (or any assignment operator) +is _not_ the same as simply repeating the left-hand operand in the +right-hand expression. For example: + + # Thanks to Pat Rankin for this example + BEGIN { + foo[rand()] += 5 + for (x in foo) + print x, foo[x] + + bar[rand()] = bar[rand()] + 5 + for (x in bar) + print x, bar[x] + } + +The indices of `bar' are guaranteed to be different, because `rand' +will return different values each time it is called. (Arrays and the +`rand' function haven't been covered yet. *Note Arrays in `awk': +Arrays, and see *Note Numeric Built-in Functions: Numeric Functions, +for more information). This example illustrates an important fact +about the assignment operators: the left-hand expression is only +evaluated _once_. + + It is also up to the implementation as to which expression is +evaluated first, the left-hand one or the right-hand one. Consider +this example: + + i = 1 + a[i += 2] = i + 1 + +The value of `a[3]' could be either two or four. + + Here is a table of the arithmetic assignment operators. In each +case, the right-hand operand is an expression whose value is converted +to a number. + +`LVALUE += INCREMENT' + Adds INCREMENT to the value of LVALUE to make the new value of + LVALUE. + +`LVALUE -= DECREMENT' + Subtracts DECREMENT from the value of LVALUE. + +`LVALUE *= COEFFICIENT' + Multiplies the value of LVALUE by COEFFICIENT. + +`LVALUE /= DIVISOR' + Divides the value of LVALUE by DIVISOR. + +`LVALUE %= MODULUS' + Sets LVALUE to its remainder by MODULUS. + +`LVALUE ^= POWER' +`LVALUE **= POWER' + Raises LVALUE to the power POWER. (Only the `^=' operator is + specified by POSIX.) + + For maximum portability, do not use the `**=' operator. + + +File: gawk.info, Node: Increment Ops, Next: Truth Values, Prev: Assignment Ops, Up: Expressions + +Increment and Decrement Operators +================================= + + "Increment" and "decrement operators" increase or decrease the value +of a variable by one. You could do the same thing with an assignment +operator, so the increment operators add no power to the `awk' +language; but they are convenient abbreviations for very common +operations. + + The operator to add one is written `++'. It can be used to increment +a variable either before or after taking its value. + + To pre-increment a variable V, write `++V'. This adds one to the +value of V and that new value is also the value of this expression. +The assignment expression `V += 1' is completely equivalent. + + Writing the `++' after the variable specifies post-increment. This +increments the variable value just the same; the difference is that the +value of the increment expression itself is the variable's _old_ value. +Thus, if `foo' has the value four, then the expression `foo++' has the +value four, but it changes the value of `foo' to five. + + The post-increment `foo++' is nearly equivalent to writing `(foo += +1) - 1'. It is not perfectly equivalent because all numbers in `awk' +are floating point: in floating point, `foo + 1 - 1' does not +necessarily equal `foo'. But the difference is minute as long as you +stick to numbers that are fairly small (less than 10e12). + + Any lvalue can be incremented. Fields and array elements are +incremented just like variables. (Use `$(i++)' when you wish to do a +field reference and a variable increment at the same time. The +parentheses are necessary because of the precedence of the field +reference operator, `$'.) + + The decrement operator `--' works just like `++' except that it +subtracts one instead of adding. Like `++', it can be used before the +lvalue to pre-decrement or after it to post-decrement. + + Here is a summary of increment and decrement expressions. + +`++LVALUE' + This expression increments LVALUE and the new value becomes the + value of the expression. + +`LVALUE++' + This expression increments LVALUE, but the value of the expression + is the _old_ value of LVALUE. + +`--LVALUE' + Like `++LVALUE', but instead of adding, it subtracts. It + decrements LVALUE and delivers the value that results. + +`LVALUE--' + Like `LVALUE++', but instead of adding, it subtracts. It + decrements LVALUE. The value of the expression is the _old_ value + of LVALUE. + + +File: gawk.info, Node: Truth Values, Next: Typing and Comparison, Prev: Increment Ops, Up: Expressions + +True and False in `awk' +======================= + + Many programming languages have a special representation for the +concepts of "true" and "false." Such languages usually use the special +constants `true' and `false', or perhaps their upper-case equivalents. + + `awk' is different. It borrows a very simple concept of true and +false from C. In `awk', any non-zero numeric value, _or_ any non-empty +string value is true. Any other value (zero or the null string, `""') +is false. The following program will print `A strange truth value' +three times: + + BEGIN { + if (3.1415927) + print "A strange truth value" + if ("Four Score And Seven Years Ago") + print "A strange truth value" + if (j = 57) + print "A strange truth value" + } + + There is a surprising consequence of the "non-zero or non-null" rule: +The string constant `"0"' is actually true, since it is non-null (d.c.). + + +File: gawk.info, Node: Typing and Comparison, Next: Boolean Ops, Prev: Truth Values, Up: Expressions + +Variable Typing and Comparison Expressions +========================================== + + Unlike other programming languages, `awk' variables do not have a +fixed type. Instead, they can be either a number or a string, depending +upon the value that is assigned to them. + + The 1992 POSIX standard introduced the concept of a "numeric +string", which is simply a string that looks like a number, for +example, `" +2"'. This concept is used for determining the type of a +variable. + + The type of the variable is important, since the types of two +variables determine how they are compared. + + In `gawk', variable typing follows these rules. + + 1. A numeric literal or the result of a numeric operation has the + NUMERIC attribute. + + 2. A string literal or the result of a string operation has the STRING + attribute. + + 3. Fields, `getline' input, `FILENAME', `ARGV' elements, `ENVIRON' + elements and the elements of an array created by `split' that are + numeric strings have the STRNUM attribute. Otherwise, they have + the STRING attribute. Uninitialized variables also have the + STRNUM attribute. + + 4. Attributes propagate across assignments, but are not changed by + any use. + + The last rule is particularly important. In the following program, +`a' has numeric type, even though it is later used in a string +operation. + + BEGIN { + a = 12.345 + b = a " is a cute number" + print b + } + + When two operands are compared, either string comparison or numeric +comparison may be used, depending on the attributes of the operands, +according to the following, symmetric, matrix: + + +---------------------------------------------- + | STRING NUMERIC STRNUM + --------+---------------------------------------------- + | + STRING | string string string + | + NUMERIC | string numeric numeric + | + STRNUM | string numeric numeric + --------+---------------------------------------------- + + The basic idea is that user input that looks numeric, and _only_ +user input, should be treated as numeric, even though it is actually +made of characters, and is therefore also a string. + + "Comparison expressions" compare strings or numbers for +relationships such as equality. They are written using "relational +operators", which are a superset of those in C. Here is a table of +them: + +`X < Y' + True if X is less than Y. + +`X <= Y' + True if X is less than or equal to Y. + +`X > Y' + True if X is greater than Y. + +`X >= Y' + True if X is greater than or equal to Y. + +`X == Y' + True if X is equal to Y. + +`X != Y' + True if X is not equal to Y. + +`X ~ Y' + True if the string X matches the regexp denoted by Y. + +`X !~ Y' + True if the string X does not match the regexp denoted by Y. + +`SUBSCRIPT in ARRAY' + True if the array ARRAY has an element with the subscript + SUBSCRIPT. + + Comparison expressions have the value one if true and zero if false. + + When comparing operands of mixed types, numeric operands are +converted to strings using the value of `CONVFMT' (*note Conversion of +Strings and Numbers: Conversion.). + + Strings are compared by comparing the first character of each, then +the second character of each, and so on. Thus `"10"' is less than +`"9"'. If there are two strings where one is a prefix of the other, +the shorter string is less than the longer one. Thus `"abc"' is less +than `"abcd"'. + + It is very easy to accidentally mistype the `==' operator, and leave +off one of the `='s. The result is still valid `awk' code, but the +program will not do what you mean: + + if (a = b) # oops! should be a == b + ... + else + ... + +Unless `b' happens to be zero or the null string, the `if' part of the +test will always succeed. Because the operators are so similar, this +kind of error is very difficult to spot when scanning the source code. + + Here are some sample expressions, how `gawk' compares them, and what +the result of the comparison is. + +`1.5 <= 2.0' + numeric comparison (true) + +`"abc" >= "xyz"' + string comparison (false) + +`1.5 != " +2"' + string comparison (true) + +`"1e2" < "3"' + string comparison (true) + +`a = 2; b = "2"' +`a == b' + string comparison (true) + +`a = 2; b = " +2"' +`a == b' + string comparison (false) + + In this example, + + $ echo 1e2 3 | awk '{ print ($1 < $2) ? "true" : "false" }' + -| false + +the result is `false' since both `$1' and `$2' are numeric strings and +thus both have the STRNUM attribute, dictating a numeric comparison. + + The purpose of the comparison rules and the use of numeric strings is +to attempt to produce the behavior that is "least surprising," while +still "doing the right thing." + + String comparisons and regular expression comparisons are very +different. For example, + + x == "foo" + +has the value of one, or is true, if the variable `x' is precisely +`foo'. By contrast, + + x ~ /foo/ + +has the value one if `x' contains `foo', such as `"Oh, what a fool am +I!"'. + + The right hand operand of the `~' and `!~' operators may be either a +regexp constant (`/.../'), or an ordinary expression, in which case the +value of the expression as a string is used as a dynamic regexp (*note +How to Use Regular Expressions: Regexp Usage.; also *note Using Dynamic +Regexps: Computed Regexps.). + + In recent implementations of `awk', a constant regular expression in +slashes by itself is also an expression. The regexp `/REGEXP/' is an +abbreviation for this comparison expression: + + $0 ~ /REGEXP/ + + One special place where `/foo/' is _not_ an abbreviation for `$0 ~ +/foo/' is when it is the right-hand operand of `~' or `!~'! *Note +Using Regular Expression Constants: Using Constant Regexps, where this +is discussed in more detail. + + +File: gawk.info, Node: Boolean Ops, Next: Conditional Exp, Prev: Typing and Comparison, Up: Expressions + +Boolean Expressions +=================== + + A "boolean expression" is a combination of comparison expressions or +matching expressions, using the boolean operators "or" (`||'), "and" +(`&&'), and "not" (`!'), along with parentheses to control nesting. +The truth value of the boolean expression is computed by combining the +truth values of the component expressions. Boolean expressions are +also referred to as "logical expressions". The terms are equivalent. + + Boolean expressions can be used wherever comparison and matching +expressions can be used. They can be used in `if', `while', `do' and +`for' statements (*note Control Statements in Actions: Statements.). +They have numeric values (one if true, zero if false), which come into +play if the result of the boolean expression is stored in a variable, or +used in arithmetic. + + In addition, every boolean expression is also a valid pattern, so +you can use one as a pattern to control the execution of rules. + + Here are descriptions of the three boolean operators, with examples. + +`BOOLEAN1 && BOOLEAN2' + True if both BOOLEAN1 and BOOLEAN2 are true. For example, the + following statement prints the current input record if it contains + both `2400' and `foo'. + + if ($0 ~ /2400/ && $0 ~ /foo/) print + + The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is true. + This can make a difference when BOOLEAN2 contains expressions that + have side effects: in the case of `$0 ~ /foo/ && ($2 == bar++)', + the variable `bar' is not incremented if there is no `foo' in the + record. + +`BOOLEAN1 || BOOLEAN2' + True if at least one of BOOLEAN1 or BOOLEAN2 is true. For + example, the following statement prints all records in the input + that contain _either_ `2400' or `foo', or both. + + if ($0 ~ /2400/ || $0 ~ /foo/) print + + The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is false. + This can make a difference when BOOLEAN2 contains expressions + that have side effects. + +`! BOOLEAN' + True if BOOLEAN is false. For example, the following program + prints all records in the input file `BBS-list' that do _not_ + contain the string `foo'. + + awk '{ if (! ($0 ~ /foo/)) print }' BBS-list + + The `&&' and `||' operators are called "short-circuit" operators +because of the way they work. Evaluation of the full expression is +"short-circuited" if the result can be determined part way through its +evaluation. + + You can continue a statement that uses `&&' or `||' simply by +putting a newline after them. But you cannot put a newline in front of +either of these operators without using backslash continuation (*note +`awk' Statements Versus Lines: Statements/Lines.). + + The actual value of an expression using the `!' operator will be +either one or zero, depending upon the truth value of the expression it +is applied to. + + The `!' operator is often useful for changing the sense of a flag +variable from false to true and back again. For example, the following +program is one way to print lines in between special bracketing lines: + + $1 == "START" { interested = ! interested } + interested == 1 { print } + $1 == "END" { interested = ! interested } + +The variable `interested', like all `awk' variables, starts out +initialized to zero, which is also false. When a line is seen whose +first field is `START', the value of `interested' is toggled to true, +using `!'. The next rule prints lines as long as `interested' is true. +When a line is seen whose first field is `END', `interested' is toggled +back to false. + + +File: gawk.info, Node: Conditional Exp, Next: Function Calls, Prev: Boolean Ops, Up: Expressions + +Conditional Expressions +======================= + + A "conditional expression" is a special kind of expression with +three operands. It allows you to use one expression's value to select +one of two other expressions. + + The conditional expression is the same as in the C language: + + SELECTOR ? IF-TRUE-EXP : IF-FALSE-EXP + +There are three subexpressions. The first, SELECTOR, is always +computed first. If it is "true" (not zero and not null) then +IF-TRUE-EXP is computed next and its value becomes the value of the +whole expression. Otherwise, IF-FALSE-EXP is computed next and its +value becomes the value of the whole expression. + + For example, this expression produces the absolute value of `x': + + x > 0 ? x : -x + + Each time the conditional expression is computed, exactly one of +IF-TRUE-EXP and IF-FALSE-EXP is computed; the other is ignored. This +is important when the expressions contain side effects. For example, +this conditional expression examines element `i' of either array `a' or +array `b', and increments `i'. + + x == y ? a[i++] : b[i++] + +This is guaranteed to increment `i' exactly once, because each time +only one of the two increment expressions is executed, and the other is +not. *Note Arrays in `awk': Arrays, for more information about arrays. + + As a minor `gawk' extension, you can continue a statement that uses +`?:' simply by putting a newline after either character. However, you +cannot put a newline in front of either character without using +backslash continuation (*note `awk' Statements Versus Lines: +Statements/Lines.). + + +File: gawk.info, Node: Function Calls, Next: Precedence, Prev: Conditional Exp, Up: Expressions + +Function Calls +============== + + A "function" is a name for a particular calculation. Because it has +a name, you can ask for it by name at any point in the program. For +example, the function `sqrt' computes the square root of a number. + + A fixed set of functions are "built-in", which means they are +available in every `awk' program. The `sqrt' function is one of these. +*Note Built-in Functions: Built-in, for a list of built-in functions +and their descriptions. In addition, you can define your own functions +for use in your program. *Note User-defined Functions: User-defined, +for how to do this. + + The way to use a function is with a "function call" expression, +which consists of the function name followed immediately by a list of +"arguments" in parentheses. The arguments are expressions which +provide the raw materials for the function's calculations. When there +is more than one argument, they are separated by commas. If there are +no arguments, write just `()' after the function name. Here are some +examples: + + sqrt(x^2 + y^2) one argument + atan2(y, x) two arguments + rand() no arguments + + *Do not put any space between the function name and the +open-parenthesis!* A user-defined function name looks just like the +name of a variable, and space would make the expression look like +concatenation of a variable with an expression inside parentheses. +Space before the parenthesis is harmless with built-in functions, but +it is best not to get into the habit of using space to avoid mistakes +with user-defined functions. + + Each function expects a particular number of arguments. For +example, the `sqrt' function must be called with a single argument, the +number to take the square root of: + + sqrt(ARGUMENT) + + Some of the built-in functions allow you to omit the final argument. +If you do so, they use a reasonable default. *Note Built-in Functions: +Built-in, for full details. If arguments are omitted in calls to +user-defined functions, then those arguments are treated as local +variables, initialized to the empty string (*note User-defined +Functions: User-defined.). + + Like every other expression, the function call has a value, which is +computed by the function based on the arguments you give it. In this +example, the value of `sqrt(ARGUMENT)' is the square root of ARGUMENT. +A function can also have side effects, such as assigning values to +certain variables or doing I/O. + + Here is a command to read numbers, one number per line, and print the +square root of each one: + + $ awk '{ print "The square root of", $1, "is", sqrt($1) }' + 1 + -| The square root of 1 is 1 + 3 + -| The square root of 3 is 1.73205 + 5 + -| The square root of 5 is 2.23607 + Control-d + + +File: gawk.info, Node: Precedence, Prev: Function Calls, Up: Expressions + +Operator Precedence (How Operators Nest) +======================================== + + "Operator precedence" determines how operators are grouped, when +different operators appear close by in one expression. For example, +`*' has higher precedence than `+'; thus, `a + b * c' means to multiply +`b' and `c', and then add `a' to the product (i.e. `a + (b * c)'). + + You can overrule the precedence of the operators by using +parentheses. You can think of the precedence rules as saying where the +parentheses are assumed to be if you do not write parentheses yourself. +In fact, it is wise to always use parentheses whenever you have an +unusual combination of operators, because other people who read the +program may not remember what the precedence is in this case. You +might forget, too; then you could make a mistake. Explicit parentheses +will help prevent any such mistake. + + When operators of equal precedence are used together, the leftmost +operator groups first, except for the assignment, conditional and +exponentiation operators, which group in the opposite order. Thus, `a +- b + c' groups as `(a - b) + c', and `a = b = c' groups as `a = (b = +c)'. + + The precedence of prefix unary operators does not matter as long as +only unary operators are involved, because there is only one way to +interpret them--innermost first. Thus, `$++i' means `$(++i)' and +`++$x' means `++($x)'. However, when another operator follows the +operand, then the precedence of the unary operators can matter. Thus, +`$x^2' means `($x)^2', but `-x^2' means `-(x^2)', because `-' has lower +precedence than `^' while `$' has higher precedence. + + Here is a table of `awk''s operators, in order from highest +precedence to lowest: + +`(...)' + Grouping. + +`$' + Field. + +`++ --' + Increment, decrement. + +`^ **' + Exponentiation. These operators group right-to-left. (The `**' + operator is not specified by POSIX.) + +`+ - !' + Unary plus, minus, logical "not". + +`* / %' + Multiplication, division, modulus. + +`+ -' + Addition, subtraction. + +`Concatenation' + No special token is used to indicate concatenation. The operands + are simply written side by side. + +`< <= == !=' +`> >= >> |' + Relational, and redirection. The relational operators and the + redirections have the same precedence level. Characters such as + `>' serve both as relationals and as redirections; the context + distinguishes between the two meanings. + + Note that the I/O redirection operators in `print' and `printf' + statements belong to the statement level, not to expressions. The + redirection does not produce an expression which could be the + operand of another operator. As a result, it does not make sense + to use a redirection operator near another operator of lower + precedence, without parentheses. Such combinations, for example + `print foo > a ? b : c', result in syntax errors. The correct way + to write this statement is `print foo > (a ? b : c)'. + +`~ !~' + Matching, non-matching. + +`in' + Array membership. + +`&&' + Logical "and". + +`||' + Logical "or". + +`?:' + Conditional. This operator groups right-to-left. + +`= += -= *=' +`/= %= ^= **=' + Assignment. These operators group right-to-left. (The `**=' + operator is not specified by POSIX.) + + +File: gawk.info, Node: Patterns and Actions, Next: Statements, Prev: Expressions, Up: Top + +Patterns and Actions +******************** + + As you have already seen, each `awk' statement consists of a pattern +with an associated action. This chapter describes how you build +patterns and actions. + +* Menu: + +* Pattern Overview:: What goes into a pattern. +* Action Overview:: What goes into an action. + + +File: gawk.info, Node: Pattern Overview, Next: Action Overview, Prev: Patterns and Actions, Up: Patterns and Actions + +Pattern Elements +================ + + Patterns in `awk' control the execution of rules: a rule is executed +when its pattern matches the current input record. This section +explains all about how to write patterns. + +* Menu: + +* Kinds of Patterns:: A list of all kinds of patterns. +* Regexp Patterns:: Using regexps as patterns. +* Expression Patterns:: Any expression can be used as a pattern. +* Ranges:: Pairs of patterns specify record ranges. +* BEGIN/END:: Specifying initialization and cleanup rules. +* Empty:: The empty pattern, which matches every record. + + +File: gawk.info, Node: Kinds of Patterns, Next: Regexp Patterns, Prev: Pattern Overview, Up: Pattern Overview + +Kinds of Patterns +----------------- + + Here is a summary of the types of patterns supported in `awk'. + +`/REGULAR EXPRESSION/' + A regular expression as a pattern. It matches when the text of the + input record fits the regular expression. (*Note Regular + Expressions: Regexp.) + +`EXPRESSION' + A single expression. It matches when its value is non-zero (if a + number) or non-null (if a string). (*Note Expressions as + Patterns: Expression Patterns.) + +`PAT1, PAT2' + A pair of patterns separated by a comma, specifying a range of + records. The range includes both the initial record that matches + PAT1, and the final record that matches PAT2. (*Note Specifying + Record Ranges with Patterns: Ranges.) + +`BEGIN' +`END' + Special patterns for you to supply start-up or clean-up actions + for your `awk' program. (*Note The `BEGIN' and `END' Special + Patterns: BEGIN/END.) + +`EMPTY' + The empty pattern matches every input record. (*Note The Empty + Pattern: Empty.) + + +File: gawk.info, Node: Regexp Patterns, Next: Expression Patterns, Prev: Kinds of Patterns, Up: Pattern Overview + +Regular Expressions as Patterns +------------------------------- + + We have been using regular expressions as patterns since our early +examples. This kind of pattern is simply a regexp constant in the +pattern part of a rule. Its meaning is `$0 ~ /PATTERN/'. The pattern +matches when the input record matches the regexp. For example: + + /foo|bar|baz/ { buzzwords++ } + END { print buzzwords, "buzzwords seen" } + + +File: gawk.info, Node: Expression Patterns, Next: Ranges, Prev: Regexp Patterns, Up: Pattern Overview + +Expressions as Patterns +----------------------- + + Any `awk' expression is valid as an `awk' pattern. Then the pattern +matches if the expression's value is non-zero (if a number) or non-null +(if a string). + + The expression is reevaluated each time the rule is tested against a +new input record. If the expression uses fields such as `$1', the +value depends directly on the new input record's text; otherwise, it +depends only on what has happened so far in the execution of the `awk' +program, but that may still be useful. + + A very common kind of expression used as a pattern is the comparison +expression, using the comparison operators described in *Note Variable +Typing and Comparison Expressions: Typing and Comparison. + + Regexp matching and non-matching are also very common expressions. +The left operand of the `~' and `!~' operators is a string. The right +operand is either a constant regular expression enclosed in slashes +(`/REGEXP/'), or any expression, whose string value is used as a +dynamic regular expression (*note Using Dynamic Regexps: Computed +Regexps.). + + The following example prints the second field of each input record +whose first field is precisely `foo'. + + $ awk '$1 == "foo" { print $2 }' BBS-list + +(There is no output, since there is no BBS site named "foo".) Contrast +this with the following regular expression match, which would accept +any record with a first field that contains `foo': + + $ awk '$1 ~ /foo/ { print $2 }' BBS-list + -| 555-1234 + -| 555-6699 + -| 555-6480 + -| 555-2127 + + Boolean expressions are also commonly used as patterns. Whether the +pattern matches an input record depends on whether its subexpressions +match. + + For example, the following command prints all records in `BBS-list' +that contain both `2400' and `foo'. + + $ awk '/2400/ && /foo/' BBS-list + -| fooey 555-1234 2400/1200/300 B + + The following command prints all records in `BBS-list' that contain +_either_ `2400' or `foo', or both. + + $ awk '/2400/ || /foo/' BBS-list + -| alpo-net 555-3412 2400/1200/300 A + -| bites 555-1675 2400/1200/300 A + -| fooey 555-1234 2400/1200/300 B + -| foot 555-6699 1200/300 B + -| macfoo 555-6480 1200/300 A + -| sdace 555-3430 2400/1200/300 A + -| sabafoo 555-2127 1200/300 C + + The following command prints all records in `BBS-list' that do _not_ +contain the string `foo'. + + $ awk '! /foo/' BBS-list + -| aardvark 555-5553 1200/300 B + -| alpo-net 555-3412 2400/1200/300 A + -| barfly 555-7685 1200/300 A + -| bites 555-1675 2400/1200/300 A + -| camelot 555-0542 300 C + -| core 555-2912 1200/300 C + -| sdace 555-3430 2400/1200/300 A + + The subexpressions of a boolean operator in a pattern can be +constant regular expressions, comparisons, or any other `awk' +expressions. Range patterns are not expressions, so they cannot appear +inside boolean patterns. Likewise, the special patterns `BEGIN' and +`END', which never match any input record, are not expressions and +cannot appear inside boolean patterns. + + A regexp constant as a pattern is also a special case of an +expression pattern. `/foo/' as an expression has the value one if `foo' +appears in the current input record; thus, as a pattern, `/foo/' +matches any record containing `foo'. + + +File: gawk.info, Node: Ranges, Next: BEGIN/END, Prev: Expression Patterns, Up: Pattern Overview + +Specifying Record Ranges with Patterns +-------------------------------------- + + A "range pattern" is made of two patterns separated by a comma, of +the form `BEGPAT, ENDPAT'. It matches ranges of consecutive input +records. The first pattern, BEGPAT, controls where the range begins, +and the second one, ENDPAT, controls where it ends. For example, + + awk '$1 == "on", $1 == "off"' + +prints every record between `on'/`off' pairs, inclusive. + + A range pattern starts out by matching BEGPAT against every input +record; when a record matches BEGPAT, the range pattern becomes "turned +on". The range pattern matches this record. As long as it stays +turned on, it automatically matches every input record read. It also +matches ENDPAT against every input record; when that succeeds, the +range pattern is turned off again for the following record. Then it +goes back to checking BEGPAT against each record. + + The record that turns on the range pattern and the one that turns it +off both match the range pattern. If you don't want to operate on +these records, you can write `if' statements in the rule's action to +distinguish them from the records you are interested in. + + It is possible for a pattern to be turned both on and off by the same +record, if the record satisfies both conditions. Then the action is +executed for just that record. + + For example, suppose you have text between two identical markers (say +the `%' symbol) that you wish to ignore. You might try to combine a +range pattern that describes the delimited text with the `next' +statement (not discussed yet, *note The `next' Statement: Next +Statement.), which causes `awk' to skip any further processing of the +current record and start over again with the next input record. Such a +program would like this: + + /^%$/,/^%$/ { next } + { print } + +This program fails because the range pattern is both turned on and +turned off by the first line with just a `%' on it. To accomplish this +task, you must write the program this way, using a flag: + + /^%$/ { skip = ! skip; next } + skip == 1 { next } # skip lines with `skip' set + + Note that in a range pattern, the `,' has the lowest precedence (is +evaluated last) of all the operators. Thus, for example, the following +program attempts to combine a range pattern with another, simpler test. + + echo Yes | awk '/1/,/2/ || /Yes/' + + The author of this program intended it to mean `(/1/,/2/) || /Yes/'. +However, `awk' interprets this as `/1/, (/2/ || /Yes/)'. This cannot +be changed or worked around; range patterns do not combine with other +patterns. + + +File: gawk.info, Node: BEGIN/END, Next: Empty, Prev: Ranges, Up: Pattern Overview + +The `BEGIN' and `END' Special Patterns +-------------------------------------- + + `BEGIN' and `END' are special patterns. They are not used to match +input records. Rather, they supply start-up or clean-up actions for +your `awk' script. + +* Menu: + +* Using BEGIN/END:: How and why to use BEGIN/END rules. +* I/O And BEGIN/END:: I/O issues in BEGIN/END rules. + + +File: gawk.info, Node: Using BEGIN/END, Next: I/O And BEGIN/END, Prev: BEGIN/END, Up: BEGIN/END + +Startup and Cleanup Actions +........................... + + A `BEGIN' rule is executed, once, before the first input record has +been read. An `END' rule is executed, once, after all the input has +been read. For example: + + $ awk ' + > BEGIN { print "Analysis of \"foo\"" } + > /foo/ { ++n } + > END { print "\"foo\" appears " n " times." }' BBS-list + -| Analysis of "foo" + -| "foo" appears 4 times. + + This program finds the number of records in the input file `BBS-list' +that contain the string `foo'. The `BEGIN' rule prints a title for the +report. There is no need to use the `BEGIN' rule to initialize the +counter `n' to zero, as `awk' does this automatically (*note +Variables::). + + The second rule increments the variable `n' every time a record +containing the pattern `foo' is read. The `END' rule prints the value +of `n' at the end of the run. + + The special patterns `BEGIN' and `END' cannot be used in ranges or +with boolean operators (indeed, they cannot be used with any operators). + + An `awk' program may have multiple `BEGIN' and/or `END' rules. They +are executed in the order they appear, all the `BEGIN' rules at +start-up and all the `END' rules at termination. `BEGIN' and `END' +rules may be intermixed with other rules. This feature was added in +the 1987 version of `awk', and is included in the POSIX standard. The +original (1978) version of `awk' required you to put the `BEGIN' rule +at the beginning of the program, and the `END' rule at the end, and +only allowed one of each. This is no longer required, but it is a good +idea in terms of program organization and readability. + + Multiple `BEGIN' and `END' rules are useful for writing library +functions, since each library file can have its own `BEGIN' and/or +`END' rule to do its own initialization and/or cleanup. Note that the +order in which library functions are named on the command line controls +the order in which their `BEGIN' and `END' rules are executed. +Therefore you have to be careful to write such rules in library files +so that the order in which they are executed doesn't matter. *Note +Command Line Options: Options, for more information on using library +functions. *Note A Library of `awk' Functions: Library Functions, for +a number of useful library functions. + + If an `awk' program only has a `BEGIN' rule, and no other rules, +then the program exits after the `BEGIN' rule has been run. (The +original version of `awk' used to keep reading and ignoring input until +end of file was seen.) However, if an `END' rule exists, then the +input will be read, even if there are no other rules in the program. +This is necessary in case the `END' rule checks the `FNR' and `NR' +variables (d.c.). + + `BEGIN' and `END' rules must have actions; there is no default +action for these rules since there is no current record when they run. + + +File: gawk.info, Node: I/O And BEGIN/END, Prev: Using BEGIN/END, Up: BEGIN/END + +Input/Output from `BEGIN' and `END' Rules +......................................... + + There are several (sometimes subtle) issues involved when doing I/O +from a `BEGIN' or `END' rule. + + The first has to do with the value of `$0' in a `BEGIN' rule. Since +`BEGIN' rules are executed before any input is read, there simply is no +input record, and therefore no fields, when executing `BEGIN' rules. +References to `$0' and the fields yield a null string or zero, +depending upon the context. One way to give `$0' a real value is to +execute a `getline' command without a variable (*note Explicit Input +with `getline': Getline.). Another way is to simply assign a value to +it. + + The second point is similar to the first, but from the other +direction. Inside an `END' rule, what is the value of `$0' and `NF'? +Traditionally, due largely to implementation issues, `$0' and `NF' were +_undefined_ inside an `END' rule. The POSIX standard specified that +`NF' was available in an `END' rule, containing the number of fields +from the last input record. Due most probably to an oversight, the +standard does not say that `$0' is also preserved, although logically +one would think that it should be. In fact, `gawk' does preserve the +value of `$0' for use in `END' rules. Be aware, however, that Unix +`awk', and possibly other implementations, do not. + + The third point follows from the first two. What is the meaning of +`print' inside a `BEGIN' or `END' rule? The meaning is the same as +always, `print $0'. If `$0' is the null string, then this prints an +empty line. Many long time `awk' programmers use `print' in `BEGIN' +and `END' rules, to mean `print ""', relying on `$0' being null. While +you might generally get away with this in `BEGIN' rules, in `gawk' at +least, it is a very bad idea in `END' rules. It is also poor style, +since if you want an empty line in the output, you should say so +explicitly in your program. + + +File: gawk.info, Node: Empty, Prev: BEGIN/END, Up: Pattern Overview + +The Empty Pattern +----------------- + + An empty (i.e. non-existent) pattern is considered to match _every_ +input record. For example, the program: + + awk '{ print $1 }' BBS-list + +prints the first field of every record. + + +File: gawk.info, Node: Action Overview, Prev: Pattern Overview, Up: Patterns and Actions + +Overview of Actions +=================== + + An `awk' program or script consists of a series of rules and +function definitions, interspersed. (Functions are described later. +*Note User-defined Functions: User-defined.) + + A rule contains a pattern and an action, either of which (but not +both) may be omitted. The purpose of the "action" is to tell `awk' +what to do once a match for the pattern is found. Thus, in outline, an +`awk' program generally looks like this: + + [PATTERN] [{ ACTION }] + [PATTERN] [{ ACTION }] + ... + function NAME(ARGS) { ... } + ... + + An action consists of one or more `awk' "statements", enclosed in +curly braces (`{' and `}'). Each statement specifies one thing to be +done. The statements are separated by newlines or semicolons. + + The curly braces around an action must be used even if the action +contains only one statement, or even if it contains no statements at +all. However, if you omit the action entirely, omit the curly braces as +well. An omitted action is equivalent to `{ print $0 }'. + + /foo/ { } # match foo, do nothing - empty action + /foo/ # match foo, print the record - omitted action + + Here are the kinds of statements supported in `awk': + + * Expressions, which can call functions or assign values to variables + (*note Expressions::). Executing this kind of statement simply + computes the value of the expression. This is useful when the + expression has side effects (*note Assignment Expressions: + Assignment Ops.). + + * Control statements, which specify the control flow of `awk' + programs. The `awk' language gives you C-like constructs (`if', + `for', `while', and `do') as well as a few special ones (*note + Control Statements in Actions: Statements.). + + * Compound statements, which consist of one or more statements + enclosed in curly braces. A compound statement is used in order + to put several statements together in the body of an `if', + `while', `do' or `for' statement. + + * Input statements, using the `getline' command (*note Explicit + Input with `getline': Getline.), the `next' statement (*note The + `next' Statement: Next Statement.), and the `nextfile' statement + (*note The `nextfile' Statement: Nextfile Statement.). + + * Output statements, `print' and `printf'. *Note Printing Output: + Printing. + + * Deletion statements, for deleting array elements. *Note The + `delete' Statement: Delete. + + +File: gawk.info, Node: Statements, Next: Built-in Variables, Prev: Patterns and Actions, Up: Top + +Control Statements in Actions +***************************** + + "Control statements" such as `if', `while', and so on control the +flow of execution in `awk' programs. Most of the control statements in +`awk' are patterned on similar statements in C. + + All the control statements start with special keywords such as `if' +and `while', to distinguish them from simple expressions. + + Many control statements contain other statements; for example, the +`if' statement contains another statement which may or may not be +executed. The contained statement is called the "body". If you want +to include more than one statement in the body, group them into a +single "compound statement" with curly braces, separating them with +newlines or semicolons. + +* Menu: + +* If Statement:: Conditionally execute some `awk' + statements. +* While Statement:: Loop until some condition is satisfied. +* Do Statement:: Do specified action while looping until some + condition is satisfied. +* For Statement:: Another looping statement, that provides + initialization and increment clauses. +* Break Statement:: Immediately exit the innermost enclosing loop. +* Continue Statement:: Skip to the end of the innermost enclosing + loop. +* Next Statement:: Stop processing the current input record. +* Nextfile Statement:: Stop processing the current file. +* Exit Statement:: Stop execution of `awk'. + + +File: gawk.info, Node: If Statement, Next: While Statement, Prev: Statements, Up: Statements + +The `if'-`else' Statement +========================= + + The `if'-`else' statement is `awk''s decision-making statement. It +looks like this: + + if (CONDITION) THEN-BODY [else ELSE-BODY] + +The CONDITION is an expression that controls what the rest of the +statement will do. If CONDITION is true, THEN-BODY is executed; +otherwise, ELSE-BODY is executed. The `else' part of the statement is +optional. The condition is considered false if its value is zero or +the null string, and true otherwise. + + Here is an example: + + if (x % 2 == 0) + print "x is even" + else + print "x is odd" + + In this example, if the expression `x % 2 == 0' is true (that is, +the value of `x' is evenly divisible by two), then the first `print' +statement is executed, otherwise the second `print' statement is +executed. + + If the `else' appears on the same line as THEN-BODY, and THEN-BODY +is not a compound statement (i.e. not surrounded by curly braces), then +a semicolon must separate THEN-BODY from `else'. To illustrate this, +let's rewrite the previous example: + + if (x % 2 == 0) print "x is even"; else + print "x is odd" + +If you forget the `;', `awk' won't be able to interpret the statement, +and you will get a syntax error. + + We would not actually write this example this way, because a human +reader might fail to see the `else' if it were not the first thing on +its line. + + +File: gawk.info, Node: While Statement, Next: Do Statement, Prev: If Statement, Up: Statements + +The `while' Statement +===================== + + In programming, a "loop" means a part of a program that can be +executed two or more times in succession. + + The `while' statement is the simplest looping statement in `awk'. +It repeatedly executes a statement as long as a condition is true. It +looks like this: + + while (CONDITION) + BODY + +Here BODY is a statement that we call the "body" of the loop, and +CONDITION is an expression that controls how long the loop keeps +running. + + The first thing the `while' statement does is test CONDITION. If +CONDITION is true, it executes the statement BODY. (The CONDITION is +true when the value is not zero and not a null string.) After BODY has +been executed, CONDITION is tested again, and if it is still true, BODY +is executed again. This process repeats until CONDITION is no longer +true. If CONDITION is initially false, the body of the loop is never +executed, and `awk' continues with the statement following the loop. + + This example prints the first three fields of each record, one per +line. + + awk '{ i = 1 + while (i <= 3) { + print $i + i++ + } + }' inventory-shipped + +Here the body of the loop is a compound statement enclosed in braces, +containing two statements. + + The loop works like this: first, the value of `i' is set to one. +Then, the `while' tests whether `i' is less than or equal to three. +This is true when `i' equals one, so the `i'-th field is printed. Then +the `i++' increments the value of `i' and the loop repeats. The loop +terminates when `i' reaches four. + + As you can see, a newline is not required between the condition and +the body; but using one makes the program clearer unless the body is a +compound statement or is very simple. The newline after the open-brace +that begins the compound statement is not required either, but the +program would be harder to read without it. + + +File: gawk.info, Node: Do Statement, Next: For Statement, Prev: While Statement, Up: Statements + +The `do'-`while' Statement +========================== + + The `do' loop is a variation of the `while' looping statement. The +`do' loop executes the BODY once, and then repeats BODY as long as +CONDITION is true. It looks like this: + + do + BODY + while (CONDITION) + + Even if CONDITION is false at the start, BODY is executed at least +once (and only once, unless executing BODY makes CONDITION true). +Contrast this with the corresponding `while' statement: + + while (CONDITION) + BODY + +This statement does not execute BODY even once if CONDITION is false to +begin with. + + Here is an example of a `do' statement: + + awk '{ i = 1 + do { + print $0 + i++ + } while (i <= 10) + }' + +This program prints each input record ten times. It isn't a very +realistic example, since in this case an ordinary `while' would do just +as well. But this reflects actual experience; there is only +occasionally a real use for a `do' statement. + + +File: gawk.info, Node: For Statement, Next: Break Statement, Prev: Do Statement, Up: Statements + +The `for' Statement +=================== + + The `for' statement makes it more convenient to count iterations of a +loop. The general form of the `for' statement looks like this: + + for (INITIALIZATION; CONDITION; INCREMENT) + BODY + +The INITIALIZATION, CONDITION and INCREMENT parts are arbitrary `awk' +expressions, and BODY stands for any `awk' statement. + + The `for' statement starts by executing INITIALIZATION. Then, as +long as CONDITION is true, it repeatedly executes BODY and then +INCREMENT. Typically INITIALIZATION sets a variable to either zero or +one, INCREMENT adds one to it, and CONDITION compares it against the +desired number of iterations. + + Here is an example of a `for' statement: + + awk '{ for (i = 1; i <= 3; i++) + print $i + }' inventory-shipped + +This prints the first three fields of each input record, one field per +line. + + You cannot set more than one variable in the INITIALIZATION part +unless you use a multiple assignment statement such as `x = y = 0', +which is possible only if all the initial values are equal. (But you +can initialize additional variables by writing their assignments as +separate statements preceding the `for' loop.) + + The same is true of the INCREMENT part; to increment additional +variables, you must write separate statements at the end of the loop. +The C compound expression, using C's comma operator, would be useful in +this context, but it is not supported in `awk'. + + Most often, INCREMENT is an increment expression, as in the example +above. But this is not required; it can be any expression whatever. +For example, this statement prints all the powers of two between one +and 100: + + for (i = 1; i <= 100; i *= 2) + print i + + Any of the three expressions in the parentheses following the `for' +may be omitted if there is nothing to be done there. Thus, +`for (; x > 0;)' is equivalent to `while (x > 0)'. If the CONDITION is +omitted, it is treated as TRUE, effectively yielding an "infinite loop" +(i.e. a loop that will never terminate). + + In most cases, a `for' loop is an abbreviation for a `while' loop, +as shown here: + + INITIALIZATION + while (CONDITION) { + BODY + INCREMENT + } + +The only exception is when the `continue' statement (*note The +`continue' Statement: Continue Statement.) is used inside the loop; +changing a `for' statement to a `while' statement in this way can +change the effect of the `continue' statement inside the loop. + + There is an alternate version of the `for' loop, for iterating over +all the indices of an array: + + for (i in array) + DO SOMETHING WITH array[i] + +*Note Scanning All Elements of an Array: Scanning an Array, for more +information on this version of the `for' loop. + + The `awk' language has a `for' statement in addition to a `while' +statement because often a `for' loop is both less work to type and more +natural to think of. Counting the number of iterations is very common +in loops. It can be easier to think of this counting as part of +looping rather than as something to do inside the loop. + + The next section has more complicated examples of `for' loops. + + +File: gawk.info, Node: Break Statement, Next: Continue Statement, Prev: For Statement, Up: Statements + +The `break' Statement +===================== + + The `break' statement jumps out of the innermost `for', `while', or +`do' loop that encloses it. The following example finds the smallest +divisor of any integer, and also identifies prime numbers: + + awk '# find smallest divisor of num + { num = $1 + for (div = 2; div*div <= num; div++) + if (num % div == 0) + break + if (num % div == 0) + printf "Smallest divisor of %d is %d\n", num, div + else + printf "%d is prime\n", num + }' + + When the remainder is zero in the first `if' statement, `awk' +immediately "breaks out" of the containing `for' loop. This means that +`awk' proceeds immediately to the statement following the loop and +continues processing. (This is very different from the `exit' +statement which stops the entire `awk' program. *Note The `exit' +Statement: Exit Statement.) + + Here is another program equivalent to the previous one. It +illustrates how the CONDITION of a `for' or `while' could just as well +be replaced with a `break' inside an `if': + + awk '# find smallest divisor of num + { num = $1 + for (div = 2; ; div++) { + if (num % div == 0) { + printf "Smallest divisor of %d is %d\n", num, div + break + } + if (div*div > num) { + printf "%d is prime\n", num + break + } + } + }' + + As described above, the `break' statement has no meaning when used +outside the body of a loop. However, although it was never documented, +historical implementations of `awk' have treated the `break' statement +outside of a loop as if it were a `next' statement (*note The `next' +Statement: Next Statement.). Recent versions of Unix `awk' no longer +allow this usage. `gawk' will support this use of `break' only if +`--traditional' has been specified on the command line (*note Command +Line Options: Options.). Otherwise, it will be treated as an error, +since the POSIX standard specifies that `break' should only be used +inside the body of a loop (d.c.). + + +File: gawk.info, Node: Continue Statement, Next: Next Statement, Prev: Break Statement, Up: Statements + +The `continue' Statement +======================== + + The `continue' statement, like `break', is used only inside `for', +`while', and `do' loops. It skips over the rest of the loop body, +causing the next cycle around the loop to begin immediately. Contrast +this with `break', which jumps out of the loop altogether. + + The `continue' statement in a `for' loop directs `awk' to skip the +rest of the body of the loop, and resume execution with the +increment-expression of the `for' statement. The following program +illustrates this fact: + + awk 'BEGIN { + for (x = 0; x <= 20; x++) { + if (x == 5) + continue + printf "%d ", x + } + print "" + }' + +This program prints all the numbers from zero to 20, except for five, +for which the `printf' is skipped. Since the increment `x++' is not +skipped, `x' does not remain stuck at five. Contrast the `for' loop +above with this `while' loop: + + awk 'BEGIN { + x = 0 + while (x <= 20) { + if (x == 5) + continue + printf "%d ", x + x++ + } + print "" + }' + +This program loops forever once `x' gets to five. + + As described above, the `continue' statement has no meaning when +used outside the body of a loop. However, although it was never +documented, historical implementations of `awk' have treated the +`continue' statement outside of a loop as if it were a `next' statement +(*note The `next' Statement: Next Statement.). Recent versions of Unix +`awk' no longer allow this usage. `gawk' will support this use of +`continue' only if `--traditional' has been specified on the command +line (*note Command Line Options: Options.). Otherwise, it will be +treated as an error, since the POSIX standard specifies that `continue' +should only be used inside the body of a loop (d.c.). + + +File: gawk.info, Node: Next Statement, Next: Nextfile Statement, Prev: Continue Statement, Up: Statements + +The `next' Statement +==================== + + The `next' statement forces `awk' to immediately stop processing the +current record and go on to the next record. This means that no +further rules are executed for the current record. The rest of the +current rule's action is not executed either. + + Contrast this with the effect of the `getline' function (*note +Explicit Input with `getline': Getline.). That too causes `awk' to +read the next record immediately, but it does not alter the flow of +control in any way. So the rest of the current action executes with a +new input record. + + At the highest level, `awk' program execution is a loop that reads +an input record and then tests each rule's pattern against it. If you +think of this loop as a `for' statement whose body contains the rules, +then the `next' statement is analogous to a `continue' statement: it +skips to the end of the body of this implicit loop, and executes the +increment (which reads another record). + + For example, if your `awk' program works only on records with four +fields, and you don't want it to fail when given bad input, you might +use this rule near the beginning of the program: + + NF != 4 { + err = sprintf("%s:%d: skipped: NF != 4\n", FILENAME, FNR) + print err > "/dev/stderr" + next + } + +so that the following rules will not see the bad record. The error +message is redirected to the standard error output stream, as error +messages should be. *Note Special File Names in `gawk': Special Files. + + According to the POSIX standard, the behavior is undefined if the +`next' statement is used in a `BEGIN' or `END' rule. `gawk' will treat +it as a syntax error. Although POSIX permits it, some other `awk' +implementations don't allow the `next' statement inside function bodies +(*note User-defined Functions: User-defined.). Just as any other +`next' statement, a `next' inside a function body reads the next record +and starts processing it with the first rule in the program. + + If the `next' statement causes the end of the input to be reached, +then the code in any `END' rules will be executed. *Note The `BEGIN' +and `END' Special Patterns: BEGIN/END. + + *Caution:* Some `awk' implementations generate a run-time error if +you use the `next' statement inside a user-defined function (*note +User-defined Functions: User-defined.). `gawk' does not have this +problem. + + +File: gawk.info, Node: Nextfile Statement, Next: Exit Statement, Prev: Next Statement, Up: Statements + +The `nextfile' Statement +======================== + + `gawk' provides the `nextfile' statement, which is similar to the +`next' statement. However, instead of abandoning processing of the +current record, the `nextfile' statement instructs `gawk' to stop +processing the current data file. + + Upon execution of the `nextfile' statement, `FILENAME' is updated to +the name of the next data file listed on the command line, `FNR' is +reset to one, `ARGIND' is incremented, and processing starts over with +the first rule in the progam. *Note Built-in Variables::. + + If the `nextfile' statement causes the end of the input to be +reached, then the code in any `END' rules will be executed. *Note The +`BEGIN' and `END' Special Patterns: BEGIN/END. + + The `nextfile' statement is a `gawk' extension; it is not +(currently) available in any other `awk' implementation. *Note +Implementing `nextfile' as a Function: Nextfile Function, for a +user-defined function you can use to simulate the `nextfile' statement. + + The `nextfile' statement would be useful if you have many data files +to process, and you expect that you would not want to process every +record in every file. Normally, in order to move on to the next data +file, you would have to continue scanning the unwanted records. The +`nextfile' statement accomplishes this much more efficiently. + + *Caution:* Versions of `gawk' prior to 3.0 used two words (`next +file') for the `nextfile' statement. This was changed in 3.0 to one +word, since the treatment of `file' was inconsistent. When it appeared +after `next', it was a keyword. Otherwise, it was a regular +identifier. The old usage is still accepted. However, `gawk' will +generate a warning message, and support for `next file' will eventually +be discontinued in a future version of `gawk'. + + +File: gawk.info, Node: Exit Statement, Prev: Nextfile Statement, Up: Statements + +The `exit' Statement +==================== + + The `exit' statement causes `awk' to immediately stop executing the +current rule and to stop processing input; any remaining input is +ignored. It looks like this: + + exit [RETURN CODE] + + If an `exit' statement is executed from a `BEGIN' rule the program +stops processing everything immediately. No input records are read. +However, if an `END' rule is present, it is executed (*note The `BEGIN' +and `END' Special Patterns: BEGIN/END.). + + If `exit' is used as part of an `END' rule, it causes the program to +stop immediately. + + An `exit' statement that is not part of a `BEGIN' or `END' rule +stops the execution of any further automatic rules for the current +record, skips reading any remaining input records, and executes the +`END' rule if there is one. + + If you do not want the `END' rule to do its job in this case, you +can set a variable to non-zero before the `exit' statement, and check +that variable in the `END' rule. *Note Assertions: Assert Function, +for an example that does this. + + If an argument is supplied to `exit', its value is used as the exit +status code for the `awk' process. If no argument is supplied, `exit' +returns status zero (success). In the case where an argument is +supplied to a first `exit' statement, and then `exit' is called a +second time with no argument, the previously supplied exit value is +used (d.c.). + + For example, let's say you've discovered an error condition you +really don't know how to handle. Conventionally, programs report this +by exiting with a non-zero status. Your `awk' program can do this +using an `exit' statement with a non-zero argument. Here is an example: + + BEGIN { + if (("date" | getline date_now) < 0) { + print "Can't get system date" > "/dev/stderr" + exit 1 + } + print "current date is", date_now + close("date") + } + + +File: gawk.info, Node: Built-in Variables, Next: Arrays, Prev: Statements, Up: Top + +Built-in Variables +****************** + + Most `awk' variables are available for you to use for your own +purposes; they never change except when your program assigns values to +them, and never affect anything except when your program examines them. +However, a few variables in `awk' have special built-in meanings. Some +of them `awk' examines automatically, so that they enable you to tell +`awk' how to do certain things. Others are set automatically by `awk', +so that they carry information from the internal workings of `awk' to +your program. + + This chapter documents all the built-in variables of `gawk'. Most +of them are also documented in the chapters describing their areas of +activity. + +* Menu: + +* User-modified:: Built-in variables that you change to control + `awk'. +* Auto-set:: Built-in variables where `awk' gives you + information. +* ARGC and ARGV:: Ways to use `ARGC' and `ARGV'. + + +File: gawk.info, Node: User-modified, Next: Auto-set, Prev: Built-in Variables, Up: Built-in Variables + +Built-in Variables that Control `awk' +===================================== + + This is an alphabetical list of the variables which you can change to +control how `awk' does certain things. Those variables that are +specific to `gawk' are marked with an asterisk, `*'. + +`CONVFMT' + This string controls conversion of numbers to strings (*note + Conversion of Strings and Numbers: Conversion.). It works by + being passed, in effect, as the first argument to the `sprintf' + function (*note Built-in Functions for String Manipulation: String + Functions.). Its default value is `"%.6g"'. `CONVFMT' was + introduced by the POSIX standard. + +`FIELDWIDTHS *' + This is a space separated list of columns that tells `gawk' how to + split input with fixed, columnar boundaries. It is an + experimental feature. Assigning to `FIELDWIDTHS' overrides the + use of `FS' for field splitting. *Note Reading Fixed-width Data: + Constant Size, for more information. + + If `gawk' is in compatibility mode (*note Command Line Options: + Options.), then `FIELDWIDTHS' has no special meaning, and field + splitting operations are done based exclusively on the value of + `FS'. + +`FS' + `FS' is the input field separator (*note Specifying How Fields are + Separated: Field Separators.). The value is a single-character + string or a multi-character regular expression that matches the + separations between fields in an input record. If the value is + the null string (`""'), then each character in the record becomes + a separate field. + + The default value is `" "', a string consisting of a single space. + As a special exception, this value means that any sequence of + spaces, tabs, and/or newlines is a single separator.(1) It also + causes spaces, tabs, and newlines at the beginning and end of a + record to be ignored. + + You can set the value of `FS' on the command line using the `-F' + option: + + awk -F, 'PROGRAM' INPUT-FILES + + If `gawk' is using `FIELDWIDTHS' for field-splitting, assigning a + value to `FS' will cause `gawk' to return to the normal, + `FS'-based, field splitting. An easy way to do this is to simply + say `FS = FS', perhaps with an explanatory comment. + +`IGNORECASE *' + If `IGNORECASE' is non-zero or non-null, then all string + comparisons, and all regular expression matching are + case-independent. Thus, regexp matching with `~' and `!~', and + the `gensub', `gsub', `index', `match', `split' and `sub' + functions, record termination with `RS', and field splitting with + `FS' all ignore case when doing their particular regexp operations. + *Note Case-sensitivity in Matching: Case-sensitivity. + + If `gawk' is in compatibility mode (*note Command Line Options: + Options.), then `IGNORECASE' has no special meaning, and string + and regexp operations are always case-sensitive. + +`OFMT' + This string controls conversion of numbers to strings (*note + Conversion of Strings and Numbers: Conversion.) for printing with + the `print' statement. It works by being passed, in effect, as + the first argument to the `sprintf' function (*note Built-in + Functions for String Manipulation: String Functions.). Its + default value is `"%.6g"'. Earlier versions of `awk' also used + `OFMT' to specify the format for converting numbers to strings in + general expressions; this is now done by `CONVFMT'. + +`OFS' + This is the output field separator (*note Output Separators::). + It is output between the fields output by a `print' statement. Its + default value is `" "', a string consisting of a single space. + +`ORS' + This is the output record separator. It is output at the end of + every `print' statement. Its default value is `"\n"'. (*Note + Output Separators::.) + +`RS' + This is `awk''s input record separator. Its default value is a + string containing a single newline character, which means that an + input record consists of a single line of text. It can also be + the null string, in which case records are separated by runs of + blank lines, or a regexp, in which case records are separated by + matches of the regexp in the input text. (*Note How Input is + Split into Records: Records.) + +`SUBSEP' + `SUBSEP' is the subscript separator. It has the default value of + `"\034"', and is used to separate the parts of the indices of a + multi-dimensional array. Thus, the expression `foo["A", "B"]' + really accesses `foo["A\034B"]' (*note Multi-dimensional Arrays: + Multi-dimensional.). + + ---------- Footnotes ---------- + + (1) In POSIX `awk', newline does not count as whitespace. + + +File: gawk.info, Node: Auto-set, Next: ARGC and ARGV, Prev: User-modified, Up: Built-in Variables + +Built-in Variables that Convey Information +========================================== + + This is an alphabetical list of the variables that are set +automatically by `awk' on certain occasions in order to provide +information to your program. Those variables that are specific to +`gawk' are marked with an asterisk, `*'. + +`ARGC' +`ARGV' + The command-line arguments available to `awk' programs are stored + in an array called `ARGV'. `ARGC' is the number of command-line + arguments present. *Note Other Command Line Arguments: Other + Arguments. Unlike most `awk' arrays, `ARGV' is indexed from zero + to `ARGC' - 1. For example: + + $ awk 'BEGIN { + > for (i = 0; i < ARGC; i++) + > print ARGV[i] + > }' inventory-shipped BBS-list + -| awk + -| inventory-shipped + -| BBS-list + + In this example, `ARGV[0]' contains `"awk"', `ARGV[1]' contains + `"inventory-shipped"', and `ARGV[2]' contains `"BBS-list"'. The + value of `ARGC' is three, one more than the index of the last + element in `ARGV', since the elements are numbered from zero. + + The names `ARGC' and `ARGV', as well as the convention of indexing + the array from zero to `ARGC' - 1, are derived from the C + language's method of accessing command line arguments. *Note + Using `ARGC' and `ARGV': ARGC and ARGV, for information about how + `awk' uses these variables. + +`ARGIND *' + The index in `ARGV' of the current file being processed. Every + time `gawk' opens a new data file for processing, it sets `ARGIND' + to the index in `ARGV' of the file name. When `gawk' is + processing the input files, it is always true that `FILENAME == + ARGV[ARGIND]'. + + This variable is useful in file processing; it allows you to tell + how far along you are in the list of data files, and to + distinguish between successive instances of the same filename on + the command line. + + While you can change the value of `ARGIND' within your `awk' + program, `gawk' will automatically set it to a new value when the + next file is opened. + + This variable is a `gawk' extension. In other `awk' + implementations, or if `gawk' is in compatibility mode (*note + Command Line Options: Options.), it is not special. + +`ENVIRON' + An associative array that contains the values of the environment. + The array indices are the environment variable names; the values + are the values of the particular environment variables. For + example, `ENVIRON["HOME"]' might be `/home/arnold'. Changing this + array does not affect the environment passed on to any programs + that `awk' may spawn via redirection or the `system' function. + (In a future version of `gawk', it may do so.) + + Some operating systems may not have environment variables. On + such systems, the `ENVIRON' array is empty (except for + `ENVIRON["AWKPATH"]'). + +`ERRNO *' + If a system error occurs either doing a redirection for `getline', + during a read for `getline', or during a `close' operation, then + `ERRNO' will contain a string describing the error. + + This variable is a `gawk' extension. In other `awk' + implementations, or if `gawk' is in compatibility mode (*note + Command Line Options: Options.), it is not special. + +`FILENAME' + This is the name of the file that `awk' is currently reading. + When no data files are listed on the command line, `awk' reads + from the standard input, and `FILENAME' is set to `"-"'. + `FILENAME' is changed each time a new file is read (*note Reading + Input Files: Reading Files.). Inside a `BEGIN' rule, the value of + `FILENAME' is `""', since there are no input files being processed + yet.(1) (d.c.) + +`FNR' + `FNR' is the current record number in the current file. `FNR' is + incremented each time a new record is read (*note Explicit Input + with `getline': Getline.). It is reinitialized to zero each time + a new input file is started. + +`NF' + `NF' is the number of fields in the current input record. `NF' is + set each time a new record is read, when a new field is created, + or when `$0' changes (*note Examining Fields: Fields.). + +`NR' + This is the number of input records `awk' has processed since the + beginning of the program's execution (*note How Input is Split + into Records: Records.). `NR' is set each time a new record is + read. + +`RLENGTH' + `RLENGTH' is the length of the substring matched by the `match' + function (*note Built-in Functions for String Manipulation: String + Functions.). `RLENGTH' is set by invoking the `match' function. + Its value is the length of the matched string, or -1 if no match + was found. + +`RSTART' + `RSTART' is the start-index in characters of the substring matched + by the `match' function (*note Built-in Functions for String + Manipulation: String Functions.). `RSTART' is set by invoking the + `match' function. Its value is the position of the string where + the matched substring starts, or zero if no match was found. + +`RT *' + `RT' is set each time a record is read. It contains the input text + that matched the text denoted by `RS', the record separator. + + This variable is a `gawk' extension. In other `awk' + implementations, or if `gawk' is in compatibility mode (*note + Command Line Options: Options.), it is not special. + + A side note about `NR' and `FNR'. `awk' simply increments both of +these variables each time it reads a record, instead of setting them to +the absolute value of the number of records read. This means that your +program can change these variables, and their new values will be +incremented for each record (d.c.). For example: + + $ echo '1 + > 2 + > 3 + > 4' | awk 'NR == 2 { NR = 17 } + > { print NR }' + -| 1 + -| 17 + -| 18 + -| 19 + +Before `FNR' was added to the `awk' language (*note Major Changes +between V7 and SVR3.1: V7/SVR3.1.), many `awk' programs used this +feature to track the number of records in a file by resetting `NR' to +zero when `FILENAME' changed. + + ---------- Footnotes ---------- + + (1) Some early implementations of Unix `awk' initialized `FILENAME' +to `"-"', even if there were data files to be processed. This behavior +was incorrect, and should not be relied upon in your programs. + + +File: gawk.info, Node: ARGC and ARGV, Prev: Auto-set, Up: Built-in Variables + +Using `ARGC' and `ARGV' +======================= + + In *Note Built-in Variables that Convey Information: Auto-set, you +saw this program describing the information contained in `ARGC' and +`ARGV': + + $ awk 'BEGIN { + > for (i = 0; i < ARGC; i++) + > print ARGV[i] + > }' inventory-shipped BBS-list + -| awk + -| inventory-shipped + -| BBS-list + +In this example, `ARGV[0]' contains `"awk"', `ARGV[1]' contains +`"inventory-shipped"', and `ARGV[2]' contains `"BBS-list"'. + + Notice that the `awk' program is not entered in `ARGV'. The other +special command line options, with their arguments, are also not +entered. But variable assignments on the command line _are_ treated as +arguments, and do show up in the `ARGV' array. + + Your program can alter `ARGC' and the elements of `ARGV'. Each time +`awk' reaches the end of an input file, it uses the next element of +`ARGV' as the name of the next input file. By storing a different +string there, your program can change which files are read. You can +use `"-"' to represent the standard input. By storing additional +elements and incrementing `ARGC' you can cause additional files to be +read. + + If you decrease the value of `ARGC', that eliminates input files +from the end of the list. By recording the old value of `ARGC' +elsewhere, your program can treat the eliminated arguments as something +other than file names. + + To eliminate a file from the middle of the list, store the null +string (`""') into `ARGV' in place of the file's name. As a special +feature, `awk' ignores file names that have been replaced with the null +string. You may also use the `delete' statement to remove elements from +`ARGV' (*note The `delete' Statement: Delete.). + + All of these actions are typically done from the `BEGIN' rule, +before actual processing of the input begins. *Note Splitting a Large +File Into Pieces: Split Program, and see *Note Duplicating Output Into +Multiple Files: Tee Program, for an example of each way of removing +elements from `ARGV'. + + The following fragment processes `ARGV' in order to examine, and +then remove, command line options. + + BEGIN { + for (i = 1; i < ARGC; i++) { + if (ARGV[i] == "-v") + verbose = 1 + else if (ARGV[i] == "-d") + debug = 1 + else if (ARGV[i] ~ /^-?/) { + e = sprintf("%s: unrecognized option -- %c", + ARGV[0], substr(ARGV[i], 1, ,1)) + print e > "/dev/stderr" + } else + break + delete ARGV[i] + } + } + + +File: gawk.info, Node: Arrays, Next: Built-in, Prev: Built-in Variables, Up: Top + +Arrays in `awk' +*************** + + An "array" is a table of values, called "elements". The elements of +an array are distinguished by their indices. "Indices" may be either +numbers or strings. `awk' maintains a single set of names that may be +used for naming variables, arrays and functions (*note User-defined +Functions: User-defined.). Thus, you cannot have a variable and an +array with the same name in the same `awk' program. + +* Menu: + +* Array Intro:: Introduction to Arrays +* Reference to Elements:: How to examine one element of an array. +* Assigning Elements:: How to change an element of an array. +* Array Example:: Basic Example of an Array +* Scanning an Array:: A variation of the `for' statement. It + loops through the indices of an array's + existing elements. +* Delete:: The `delete' statement removes an element + from an array. +* Numeric Array Subscripts:: How to use numbers as subscripts in + `awk'. +* Uninitialized Subscripts:: Using Uninitialized variables as subscripts. +* Multi-dimensional:: Emulating multi-dimensional arrays in + `awk'. +* Multi-scanning:: Scanning multi-dimensional arrays. + + +File: gawk.info, Node: Array Intro, Next: Reference to Elements, Prev: Arrays, Up: Arrays + +Introduction to Arrays +====================== + + The `awk' language provides one-dimensional "arrays" for storing +groups of related strings or numbers. + + Every `awk' array must have a name. Array names have the same +syntax as variable names; any valid variable name would also be a valid +array name. But you cannot use one name in both ways (as an array and +as a variable) in one `awk' program. + + Arrays in `awk' superficially resemble arrays in other programming +languages; but there are fundamental differences. In `awk', you don't +need to specify the size of an array before you start to use it. +Additionally, any number or string in `awk' may be used as an array +index, not just consecutive integers. + + In most other languages, you have to "declare" an array and specify +how many elements or components it contains. In such languages, the +declaration causes a contiguous block of memory to be allocated for that +many elements. An index in the array usually must be a positive +integer; for example, the index zero specifies the first element in the +array, which is actually stored at the beginning of the block of +memory. Index one specifies the second element, which is stored in +memory right after the first element, and so on. It is impossible to +add more elements to the array, because it has room for only as many +elements as you declared. (Some languages allow arbitrary starting and +ending indices, e.g., `15 .. 27', but the size of the array is still +fixed when the array is declared.) + + A contiguous array of four elements might look like this, +conceptually, if the element values are eight, `"foo"', `""' and 30: + + +---------+---------+--------+---------+ + | 8 | "foo" | "" | 30 | value + +---------+---------+--------+---------+ + 0 1 2 3 index + +Only the values are stored; the indices are implicit from the order of +the values. Eight is the value at index zero, because eight appears in +the position with zero elements before it. + + Arrays in `awk' are different: they are "associative". This means +that each array is a collection of pairs: an index, and its +corresponding array element value: + + Element 4 Value 30 + Element 2 Value "foo" + Element 1 Value 8 + Element 3 Value "" + +We have shown the pairs in jumbled order because their order is +irrelevant. + + One advantage of associative arrays is that new pairs can be added +at any time. For example, suppose we add to the above array a tenth +element whose value is `"number ten"'. The result is this: + + Element 10 Value "number ten" + Element 4 Value 30 + Element 2 Value "foo" + Element 1 Value 8 + Element 3 Value "" + +Now the array is "sparse", which just means some indices are missing: +it has elements 1-4 and 10, but doesn't have elements 5, 6, 7, 8, or 9. + + Another consequence of associative arrays is that the indices don't +have to be positive integers. Any number, or even a string, can be an +index. For example, here is an array which translates words from +English into French: + + Element "dog" Value "chien" + Element "cat" Value "chat" + Element "one" Value "un" + Element 1 Value "un" + +Here we decided to translate the number one in both spelled-out and +numeric form--thus illustrating that a single array can have both +numbers and strings as indices. (In fact, array subscripts are always +strings; this is discussed in more detail in *Note Using Numbers to +Subscript Arrays: Numeric Array Subscripts.) + + When `awk' creates an array for you, e.g., with the `split' built-in +function, that array's indices are consecutive integers starting at one. +(*Note Built-in Functions for String Manipulation: String Functions.) + + +File: gawk.info, Node: Reference to Elements, Next: Assigning Elements, Prev: Array Intro, Up: Arrays + +Referring to an Array Element +============================= + + The principal way of using an array is to refer to one of its +elements. An array reference is an expression which looks like this: + + ARRAY[INDEX] + +Here, ARRAY is the name of an array. The expression INDEX is the index +of the element of the array that you want. + + The value of the array reference is the current value of that array +element. For example, `foo[4.3]' is an expression for the element of +array `foo' at index `4.3'. + + If you refer to an array element that has no recorded value, the +value of the reference is `""', the null string. This includes elements +to which you have not assigned any value, and elements that have been +deleted (*note The `delete' Statement: Delete.). Such a reference +automatically creates that array element, with the null string as its +value. (In some cases, this is unfortunate, because it might waste +memory inside `awk'.) + + You can find out if an element exists in an array at a certain index +with the expression: + + INDEX in ARRAY + +This expression tests whether or not the particular index exists, +without the side effect of creating that element if it is not present. +The expression has the value one (true) if `ARRAY[INDEX]' exists, and +zero (false) if it does not exist. + + For example, to test whether the array `frequencies' contains the +index `2', you could write this statement: + + if (2 in frequencies) + print "Subscript 2 is present." + + Note that this is _not_ a test of whether or not the array +`frequencies' contains an element whose _value_ is two. (There is no +way to do that except to scan all the elements.) Also, this _does not_ +create `frequencies[2]', while the following (incorrect) alternative +would do so: + + if (frequencies[2] != "") + print "Subscript 2 is present." + + +File: gawk.info, Node: Assigning Elements, Next: Array Example, Prev: Reference to Elements, Up: Arrays + +Assigning Array Elements +======================== + + Array elements are lvalues: they can be assigned values just like +`awk' variables: + + ARRAY[SUBSCRIPT] = VALUE + +Here ARRAY is the name of your array. The expression SUBSCRIPT is the +index of the element of the array that you want to assign a value. The +expression VALUE is the value you are assigning to that element of the +array. + + +File: gawk.info, Node: Array Example, Next: Scanning an Array, Prev: Assigning Elements, Up: Arrays + +Basic Array Example +=================== + + The following program takes a list of lines, each beginning with a +line number, and prints them out in order of line number. The line +numbers are not in order, however, when they are first read: they are +scrambled. This program sorts the lines by making an array using the +line numbers as subscripts. It then prints out the lines in sorted +order of their numbers. It is a very simple program, and gets confused +if it encounters repeated numbers, gaps, or lines that don't begin with +a number. + + { + if ($1 > max) + max = $1 + arr[$1] = $0 + } + + END { + for (x = 1; x <= max; x++) + print arr[x] + } + + The first rule keeps track of the largest line number seen so far; +it also stores each line into the array `arr', at an index that is the +line's number. + + The second rule runs after all the input has been read, to print out +all the lines. + + When this program is run with the following input: + + 5 I am the Five man + 2 Who are you? The new number two! + 4 . . . And four on the floor + 1 Who is number one? + 3 I three you. + +its output is this: + + 1 Who is number one? + 2 Who are you? The new number two! + 3 I three you. + 4 . . . And four on the floor + 5 I am the Five man + + If a line number is repeated, the last line with a given number +overrides the others. + + Gaps in the line numbers can be handled with an easy improvement to +the program's `END' rule: + + END { + for (x = 1; x <= max; x++) + if (x in arr) + print arr[x] + } + + +File: gawk.info, Node: Scanning an Array, Next: Delete, Prev: Array Example, Up: Arrays + +Scanning All Elements of an Array +================================= + + In programs that use arrays, you often need a loop that executes +once for each element of an array. In other languages, where arrays are +contiguous and indices are limited to positive integers, this is easy: +you can find all the valid indices by counting from the lowest index up +to the highest. This technique won't do the job in `awk', since any +number or string can be an array index. So `awk' has a special kind of +`for' statement for scanning an array: + + for (VAR in ARRAY) + BODY + +This loop executes BODY once for each index in ARRAY that your program +has previously used, with the variable VAR set to that index. + + Here is a program that uses this form of the `for' statement. The +first rule scans the input records and notes which words appear (at +least once) in the input, by storing a one into the array `used' with +the word as index. The second rule scans the elements of `used' to +find all the distinct words that appear in the input. It prints each +word that is more than 10 characters long, and also prints the number of +such words. *Note Built-in Functions for String Manipulation: String +Functions, for more information on the built-in function `length'. + + # Record a 1 for each word that is used at least once. + { + for (i = 1; i <= NF; i++) + used[$i] = 1 + } + + # Find number of distinct words more than 10 characters long. + END { + for (x in used) + if (length(x) > 10) { + ++num_long_words + print x + } + print num_long_words, "words longer than 10 characters" + } + +*Note Generating Word Usage Counts: Word Sorting, for a more detailed +example of this type. + + The order in which elements of the array are accessed by this +statement is determined by the internal arrangement of the array +elements within `awk' and cannot be controlled or changed. This can +lead to problems if new elements are added to ARRAY by statements in +the loop body; you cannot predict whether or not the `for' loop will +reach them. Similarly, changing VAR inside the loop may produce +strange results. It is best to avoid such things. + + +File: gawk.info, Node: Delete, Next: Numeric Array Subscripts, Prev: Scanning an Array, Up: Arrays + +The `delete' Statement +====================== + + You can remove an individual element of an array using the `delete' +statement: + + delete ARRAY[INDEX] + + Once you have deleted an array element, you can no longer obtain any +value the element once had. It is as if you had never referred to it +and had never given it any value. + + Here is an example of deleting elements in an array: + + for (i in frequencies) + delete frequencies[i] + +This example removes all the elements from the array `frequencies'. + + If you delete an element, a subsequent `for' statement to scan the +array will not report that element, and the `in' operator to check for +the presence of that element will return zero (i.e. false): + + delete foo[4] + if (4 in foo) + print "This will never be printed" + + It is important to note that deleting an element is _not_ the same +as assigning it a null value (the empty string, `""'). + + foo[4] = "" + if (4 in foo) + print "This is printed, even though foo[4] is empty" + + It is not an error to delete an element that does not exist. + + You can delete all the elements of an array with a single statement, +by leaving off the subscript in the `delete' statement. + + delete ARRAY + + This ability is a `gawk' extension; it is not available in +compatibility mode (*note Command Line Options: Options.). + + Using this version of the `delete' statement is about three times +more efficient than the equivalent loop that deletes each element one +at a time. + + The following statement provides a portable, but non-obvious way to +clear out an array. + + # thanks to Michael Brennan for pointing this out + split("", array) + + The `split' function (*note Built-in Functions for String +Manipulation: String Functions.) clears out the target array first. +This call asks it to split apart the null string. Since there is no +data to split out, the function simply clears the array and then +returns. + + +File: gawk.info, Node: Numeric Array Subscripts, Next: Uninitialized Subscripts, Prev: Delete, Up: Arrays + +Using Numbers to Subscript Arrays +================================= + + An important aspect of arrays to remember is that _array subscripts +are always strings_. If you use a numeric value as a subscript, it +will be converted to a string value before it is used for subscripting +(*note Conversion of Strings and Numbers: Conversion.). + + This means that the value of the built-in variable `CONVFMT' can +potentially affect how your program accesses elements of an array. For +example: + + xyz = 12.153 + data[xyz] = 1 + CONVFMT = "%2.2f" + if (xyz in data) + printf "%s is in data\n", xyz + else + printf "%s is not in data\n", xyz + +This prints `12.15 is not in data'. The first statement gives `xyz' a +numeric value. Assigning to `data[xyz]' subscripts `data' with the +string value `"12.153"' (using the default conversion value of +`CONVFMT', `"%.6g"'), and assigns one to `data["12.153"]'. The program +then changes the value of `CONVFMT'. The test `(xyz in data)' +generates a new string value from `xyz', this time `"12.15"', since the +value of `CONVFMT' only allows two significant digits. This test fails, +since `"12.15"' is a different string from `"12.153"'. + + According to the rules for conversions (*note Conversion of Strings +and Numbers: Conversion.), integer values are always converted to +strings as integers, no matter what the value of `CONVFMT' may happen +to be. So the usual case of: + + for (i = 1; i <= maxsub; i++) + do something with array[i] + +will work, no matter what the value of `CONVFMT'. + + Like many things in `awk', the majority of the time things work as +you would expect them to work. But it is useful to have a precise +knowledge of the actual rules, since sometimes they can have a subtle +effect on your programs. + + +File: gawk.info, Node: Uninitialized Subscripts, Next: Multi-dimensional, Prev: Numeric Array Subscripts, Up: Arrays + +Using Uninitialized Variables as Subscripts +=========================================== + + Suppose you want to print your input data in reverse order. A +reasonable attempt at a program to do so (with some test data) might +look like this: + + $ echo 'line 1 + > line 2 + > line 3' | awk '{ l[lines] = $0; ++lines } + > END { + > for (i = lines-1; i >= 0; --i) + > print l[i] + > }' + -| line 3 + -| line 2 + + Unfortunately, the very first line of input data did not come out in +the output! + + At first glance, this program should have worked. The variable +`lines' is uninitialized, and uninitialized variables have the numeric +value zero. So, the value of `l[0]' should have been printed. + + The issue here is that subscripts for `awk' arrays are *always* +strings. And uninitialized variables, when used as strings, have the +value `""', not zero. Thus, `line 1' ended up stored in `l[""]'. + + The following version of the program works correctly: + + { l[lines++] = $0 } + END { + for (i = lines - 1; i >= 0; --i) + print l[i] + } + + Here, the `++' forces `l' to be numeric, thus making the "old value" +numeric zero, which is then converted to `"0"' as the array subscript. + + As we have just seen, even though it is somewhat unusual, the null +string (`""') is a valid array subscript (d.c.). If `--lint' is provided +on the command line (*note Command Line Options: Options.), `gawk' will +warn about the use of the null string as a subscript. + + +File: gawk.info, Node: Multi-dimensional, Next: Multi-scanning, Prev: Uninitialized Subscripts, Up: Arrays + +Multi-dimensional Arrays +======================== + + A multi-dimensional array is an array in which an element is +identified by a sequence of indices, instead of a single index. For +example, a two-dimensional array requires two indices. The usual way +(in most languages, including `awk') to refer to an element of a +two-dimensional array named `grid' is with `grid[X,Y]'. + + Multi-dimensional arrays are supported in `awk' through +concatenation of indices into one string. What happens is that `awk' +converts the indices into strings (*note Conversion of Strings and +Numbers: Conversion.) and concatenates them together, with a separator +between them. This creates a single string that describes the values +of the separate indices. The combined string is used as a single index +into an ordinary, one-dimensional array. The separator used is the +value of the built-in variable `SUBSEP'. + + For example, suppose we evaluate the expression `foo[5,12] = "value"' +when the value of `SUBSEP' is `"@"'. The numbers five and 12 are +converted to strings and concatenated with an `@' between them, +yielding `"5@12"'; thus, the array element `foo["5@12"]' is set to +`"value"'. + + Once the element's value is stored, `awk' has no record of whether +it was stored with a single index or a sequence of indices. The two +expressions `foo[5,12]' and `foo[5 SUBSEP 12]' are always equivalent. + + The default value of `SUBSEP' is the string `"\034"', which contains +a non-printing character that is unlikely to appear in an `awk' program +or in most input data. + + The usefulness of choosing an unlikely character comes from the fact +that index values that contain a string matching `SUBSEP' lead to +combined strings that are ambiguous. Suppose that `SUBSEP' were `"@"'; +then `foo["a@b", "c"]' and `foo["a", "b@c"]' would be indistinguishable +because both would actually be stored as `foo["a@b@c"]'. + + You can test whether a particular index-sequence exists in a +"multi-dimensional" array with the same operator `in' used for single +dimensional arrays. Instead of a single index as the left-hand operand, +write the whole sequence of indices, separated by commas, in +parentheses: + + (SUBSCRIPT1, SUBSCRIPT2, ...) in ARRAY + + The following example treats its input as a two-dimensional array of +fields; it rotates this array 90 degrees clockwise and prints the +result. It assumes that all lines have the same number of elements. + + awk '{ + if (max_nf < NF) + max_nf = NF + max_nr = NR + for (x = 1; x <= NF; x++) + vector[x, NR] = $x + } + + END { + for (x = 1; x <= max_nf; x++) { + for (y = max_nr; y >= 1; --y) + printf("%s ", vector[x, y]) + printf("\n") + } + }' + +When given the input: + + 1 2 3 4 5 6 + 2 3 4 5 6 1 + 3 4 5 6 1 2 + 4 5 6 1 2 3 + +it produces: + + 4 3 2 1 + 5 4 3 2 + 6 5 4 3 + 1 6 5 4 + 2 1 6 5 + 3 2 1 6 + + +File: gawk.info, Node: Multi-scanning, Prev: Multi-dimensional, Up: Arrays + +Scanning Multi-dimensional Arrays +================================= + + There is no special `for' statement for scanning a +"multi-dimensional" array; there cannot be one, because in truth there +are no multi-dimensional arrays or elements; there is only a +multi-dimensional _way of accessing_ an array. + + However, if your program has an array that is always accessed as +multi-dimensional, you can get the effect of scanning it by combining +the scanning `for' statement (*note Scanning All Elements of an Array: +Scanning an Array.) with the `split' built-in function (*note Built-in +Functions for String Manipulation: String Functions.). It works like +this: + + for (combined in array) { + split(combined, separate, SUBSEP) + ... + } + +This sets `combined' to each concatenated, combined index in the array, +and splits it into the individual indices by breaking it apart where +the value of `SUBSEP' appears. The split-out indices become the +elements of the array `separate'. + + Thus, suppose you have previously stored a value in `array[1, +"foo"]'; then an element with index `"1\034foo"' exists in `array'. +(Recall that the default value of `SUBSEP' is the character with code +034.) Sooner or later the `for' statement will find that index and do +an iteration with `combined' set to `"1\034foo"'. Then the `split' +function is called as follows: + + split("1\034foo", separate, "\034") + +The result of this is to set `separate[1]' to `"1"' and `separate[2]' +to `"foo"'. Presto, the original sequence of separate indices has been +recovered. + + +File: gawk.info, Node: Built-in, Next: User-defined, Prev: Arrays, Up: Top + +Built-in Functions +****************** + + "Built-in" functions are functions that are always available for +your `awk' program to call. This chapter defines all the built-in +functions in `awk'; some of them are mentioned in other sections, but +they are summarized here for your convenience. (You can also define +new functions yourself. *Note User-defined Functions: User-defined.) + +* Menu: + +* Calling Built-in:: How to call built-in functions. +* Numeric Functions:: Functions that work with numbers, including + `int', `sin' and `rand'. +* String Functions:: Functions for string manipulation, such as + `split', `match', and + `sprintf'. +* I/O Functions:: Functions for files and shell commands. +* Time Functions:: Functions for dealing with time stamps. + + +File: gawk.info, Node: Calling Built-in, Next: Numeric Functions, Prev: Built-in, Up: Built-in + +Calling Built-in Functions +========================== + + To call a built-in function, write the name of the function followed +by arguments in parentheses. For example, `atan2(y + z, 1)' is a call +to the function `atan2', with two arguments. + + Whitespace is ignored between the built-in function name and the +open-parenthesis, but we recommend that you avoid using whitespace +there. User-defined functions do not permit whitespace in this way, and +you will find it easier to avoid mistakes by following a simple +convention which always works: no whitespace after a function name. + + Each built-in function accepts a certain number of arguments. In +some cases, arguments can be omitted. The defaults for omitted +arguments vary from function to function and are described under the +individual functions. In some `awk' implementations, extra arguments +given to built-in functions are ignored. However, in `gawk', it is a +fatal error to give extra arguments to a built-in function. + + When a function is called, expressions that create the function's +actual parameters are evaluated completely before the function call is +performed. For example, in the code fragment: + + i = 4 + j = sqrt(i++) + +the variable `i' is set to five before `sqrt' is called with a value of +four for its actual parameter. + + The order of evaluation of the expressions used for the function's +parameters is undefined. Thus, you should not write programs that +assume that parameters are evaluated from left to right or from right +to left. For example, + + i = 5 + j = atan2(i++, i *= 2) + + If the order of evaluation is left to right, then `i' first becomes +six, and then 12, and `atan2' is called with the two arguments six and +12. But if the order of evaluation is right to left, `i' first becomes +10, and then 11, and `atan2' is called with the two arguments 11 and 10. + + +File: gawk.info, Node: Numeric Functions, Next: String Functions, Prev: Calling Built-in, Up: Built-in + +Numeric Built-in Functions +========================== + + Here is a full list of built-in functions that work with numbers. +Optional parameters are enclosed in square brackets ("[" and "]"). + +`int(X)' + This produces the nearest integer to X, located between X and zero, + truncated toward zero. + + For example, `int(3)' is three, `int(3.9)' is three, `int(-3.9)' + is -3, and `int(-3)' is -3 as well. + +`sqrt(X)' + This gives you the positive square root of X. It reports an error + if X is negative. Thus, `sqrt(4)' is two. + +`exp(X)' + This gives you the exponential of X (`e ^ X'), or reports an error + if X is out of range. The range of values X can have depends on + your machine's floating point representation. + +`log(X)' + This gives you the natural logarithm of X, if X is positive; + otherwise, it reports an error. + +`sin(X)' + This gives you the sine of X, with X in radians. + +`cos(X)' + This gives you the cosine of X, with X in radians. + +`atan2(Y, X)' + This gives you the arctangent of `Y / X' in radians. + +`rand()' + This gives you a random number. The values of `rand' are + uniformly-distributed between zero and one. The value is never + zero and never one. + + Often you want random integers instead. Here is a user-defined + function you can use to obtain a random non-negative integer less + than N: + + function randint(n) { + return int(n * rand()) + } + + The multiplication produces a random real number greater than zero + and less than `n'. We then make it an integer (using `int') + between zero and `n' - 1, inclusive. + + Here is an example where a similar function is used to produce + random integers between one and N. This program prints a new + random number for each input record. + + awk ' + # Function to roll a simulated die. + function roll(n) { return 1 + int(rand() * n) } + + # Roll 3 six-sided dice and + # print total number of points. + { + printf("%d points\n", + roll(6)+roll(6)+roll(6)) + }' + + *Caution:* In most `awk' implementations, including `gawk', `rand' + starts generating numbers from the same starting number, or + "seed", each time you run `awk'. Thus, a program will generate + the same results each time you run it. The numbers are random + within one `awk' run, but predictable from run to run. This is + convenient for debugging, but if you want a program to do + different things each time it is used, you must change the seed to + a value that will be different in each run. To do this, use + `srand'. + +`srand([X])' + The function `srand' sets the starting point, or seed, for + generating random numbers to the value X. + + Each seed value leads to a particular sequence of random + numbers.(1) Thus, if you set the seed to the same value a second + time, you will get the same sequence of random numbers again. + + If you omit the argument X, as in `srand()', then the current date + and time of day are used for a seed. This is the way to get random + numbers that are truly unpredictable. + + The return value of `srand' is the previous seed. This makes it + easy to keep track of the seeds for use in consistently reproducing + sequences of random numbers. + + ---------- Footnotes ---------- + + (1) Computer generated random numbers really are not truly random. +They are technically known as "pseudo-random." This means that while +the numbers in a sequence appear to be random, you can in fact generate +the same sequence of random numbers over and over again. + + +File: gawk.info, Node: String Functions, Next: I/O Functions, Prev: Numeric Functions, Up: Built-in + +Built-in Functions for String Manipulation +========================================== + + The functions in this section look at or change the text of one or +more strings. Optional parameters are enclosed in square brackets ("[" +and "]"). + +`index(IN, FIND)' + This searches the string IN for the first occurrence of the string + FIND, and returns the position in characters where that occurrence + begins in the string IN. For example: + + $ awk 'BEGIN { print index("peanut", "an") }' + -| 3 + + If FIND is not found, `index' returns zero. (Remember that string + indices in `awk' start at one.) + +`length([STRING])' + This gives you the number of characters in STRING. If STRING is a + number, the length of the digit string representing that number is + returned. For example, `length("abcde")' is five. By contrast, + `length(15 * 35)' works out to three. How? Well, 15 * 35 = 525, + and 525 is then converted to the string `"525"', which has three + characters. + + If no argument is supplied, `length' returns the length of `$0'. + + In older versions of `awk', you could call the `length' function + without any parentheses. Doing so is marked as "deprecated" in the + POSIX standard. This means that while you can do this in your + programs, it is a feature that can eventually be removed from a + future version of the standard. Therefore, for maximal + portability of your `awk' programs, you should always supply the + parentheses. + +`match(STRING, REGEXP)' + The `match' function searches the string, STRING, for the longest, + leftmost substring matched by the regular expression, REGEXP. It + returns the character position, or "index", of where that + substring begins (one, if it starts at the beginning of STRING). + If no match is found, it returns zero. + + The `match' function sets the built-in variable `RSTART' to the + index. It also sets the built-in variable `RLENGTH' to the length + in characters of the matched substring. If no match is found, + `RSTART' is set to zero, and `RLENGTH' to -1. + + For example: + + awk '{ + if ($1 == "FIND") + regex = $2 + else { + where = match($0, regex) + if (where != 0) + print "Match of", regex, "found at", \ + where, "in", $0 + } + }' + + This program looks for lines that match the regular expression + stored in the variable `regex'. This regular expression can be + changed. If the first word on a line is `FIND', `regex' is + changed to be the second word on that line. Therefore, given: + + FIND ru+n + My program runs + but not very quickly + FIND Melvin + JF+KM + This line is property of Reality Engineering Co. + Melvin was here. + + `awk' prints: + + Match of ru+n found at 12 in My program runs + Match of Melvin found at 1 in Melvin was here. + +`split(STRING, ARRAY [, FIELDSEP])' + This divides STRING into pieces separated by FIELDSEP, and stores + the pieces in ARRAY. The first piece is stored in `ARRAY[1]', the + second piece in `ARRAY[2]', and so forth. The string value of the + third argument, FIELDSEP, is a regexp describing where to split + STRING (much as `FS' can be a regexp describing where to split + input records). If the FIELDSEP is omitted, the value of `FS' is + used. `split' returns the number of elements created. + + The `split' function splits strings into pieces in a manner + similar to the way input lines are split into fields. For example: + + split("cul-de-sac", a, "-") + + splits the string `cul-de-sac' into three fields using `-' as the + separator. It sets the contents of the array `a' as follows: + + a[1] = "cul" + a[2] = "de" + a[3] = "sac" + + The value returned by this call to `split' is three. + + As with input field-splitting, when the value of FIELDSEP is + `" "', leading and trailing whitespace is ignored, and the elements + are separated by runs of whitespace. + + Also as with input field-splitting, if FIELDSEP is the null + string, each individual character in the string is split into its + own array element. (This is a `gawk'-specific extension.) + + Recent implementations of `awk', including `gawk', allow the third + argument to be a regexp constant (`/abc/'), as well as a string + (d.c.). The POSIX standard allows this as well. + + Before splitting the string, `split' deletes any previously + existing elements in the array ARRAY (d.c.). + +`sprintf(FORMAT, EXPRESSION1,...)' + This returns (without printing) the string that `printf' would + have printed out with the same arguments (*note Using `printf' + Statements for Fancier Printing: Printf.). For example: + + sprintf("pi = %.2f (approx.)", 22/7) + + returns the string `"pi = 3.14 (approx.)"'. + +`sub(REGEXP, REPLACEMENT [, TARGET])' + The `sub' function alters the value of TARGET. It searches this + value, which is treated as a string, for the leftmost longest + substring matched by the regular expression, REGEXP, extending + this match as far as possible. Then the entire string is changed + by replacing the matched text with REPLACEMENT. The modified + string becomes the new value of TARGET. + + This function is peculiar because TARGET is not simply used to + compute a value, and not just any expression will do: it must be a + variable, field or array element, so that `sub' can store a + modified value there. If this argument is omitted, then the + default is to use and alter `$0'. + + For example: + + str = "water, water, everywhere" + sub(/at/, "ith", str) + + sets `str' to `"wither, water, everywhere"', by replacing the + leftmost, longest occurrence of `at' with `ith'. + + The `sub' function returns the number of substitutions made (either + one or zero). + + If the special character `&' appears in REPLACEMENT, it stands for + the precise substring that was matched by REGEXP. (If the regexp + can match more than one string, then this precise substring may + vary.) For example: + + awk '{ sub(/candidate/, "& and his wife"); print }' + + changes the first occurrence of `candidate' to `candidate and his + wife' on each input line. + + Here is another example: + + awk 'BEGIN { + str = "daabaaa" + sub(/a*/, "c&c", str) + print str + }' + -| dcaacbaaa + + This shows how `&' can represent a non-constant string, and also + illustrates the "leftmost, longest" rule in regexp matching (*note + How Much Text Matches?: Leftmost Longest.). + + The effect of this special character (`&') can be turned off by + putting a backslash before it in the string. As usual, to insert + one backslash in the string, you must write two backslashes. + Therefore, write `\\&' in a string constant to include a literal + `&' in the replacement. For example, here is how to replace the + first `|' on each line with an `&': + + awk '{ sub(/\|/, "\\&"); print }' + + *Note:* As mentioned above, the third argument to `sub' must be a + variable, field or array reference. Some versions of `awk' allow + the third argument to be an expression which is not an lvalue. In + such a case, `sub' would still search for the pattern and return + zero or one, but the result of the substitution (if any) would be + thrown away because there is no place to put it. Such versions of + `awk' accept expressions like this: + + sub(/USA/, "United States", "the USA and Canada") + + For historical compatibility, `gawk' will accept erroneous code, + such as in the above example. However, using any other + non-changeable object as the third parameter will cause a fatal + error, and your program will not run. + +`gsub(REGEXP, REPLACEMENT [, TARGET])' + This is similar to the `sub' function, except `gsub' replaces + _all_ of the longest, leftmost, _non-overlapping_ matching + substrings it can find. The `g' in `gsub' stands for "global," + which means replace everywhere. For example: + + awk '{ gsub(/Britain/, "United Kingdom"); print }' + + replaces all occurrences of the string `Britain' with `United + Kingdom' for all input records. + + The `gsub' function returns the number of substitutions made. If + the variable to be searched and altered, TARGET, is omitted, then + the entire input record, `$0', is used. + + As in `sub', the characters `&' and `\' are special, and the third + argument must be an lvalue. + +`gensub(REGEXP, REPLACEMENT, HOW [, TARGET])' + `gensub' is a general substitution function. Like `sub' and + `gsub', it searches the target string TARGET for matches of the + regular expression REGEXP. Unlike `sub' and `gsub', the modified + string is returned as the result of the function, and the original + target string is _not_ changed. If HOW is a string beginning with + `g' or `G', then it replaces all matches of REGEXP with + REPLACEMENT. Otherwise, HOW is a number indicating which match of + REGEXP to replace. If no TARGET is supplied, `$0' is used instead. + + `gensub' provides an additional feature that is not available in + `sub' or `gsub': the ability to specify components of a regexp in + the replacement text. This is done by using parentheses in the + regexp to mark the components, and then specifying `\N' in the + replacement text, where N is a digit from one to nine. For + example: + + $ gawk ' + > BEGIN { + > a = "abc def" + > b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a) + > print b + > }' + -| def abc + + As described above for `sub', you must type two backslashes in + order to get one into the string. + + In the replacement text, the sequence `\0' represents the entire + matched text, as does the character `&'. + + This example shows how you can use the third argument to control + which match of the regexp should be changed. + + $ echo a b c a b c | + > gawk '{ print gensub(/a/, "AA", 2) }' + -| a b c AA b c + + In this case, `$0' is used as the default target string. `gensub' + returns the new string as its result, which is passed directly to + `print' for printing. + + If the HOW argument is a string that does not begin with `g' or + `G', or if it is a number that is less than zero, only one + substitution is performed. + + `gensub' is a `gawk' extension; it is not available in + compatibility mode (*note Command Line Options: Options.). + +`substr(STRING, START [, LENGTH])' + This returns a LENGTH-character-long substring of STRING, starting + at character number START. The first character of a string is + character number one. For example, `substr("washington", 5, 3)' + returns `"ing"'. + + If LENGTH is not present, this function returns the whole suffix of + STRING that begins at character number START. For example, + `substr("washington", 5)' returns `"ington"'. The whole suffix is + also returned if LENGTH is greater than the number of characters + remaining in the string, counting from character number START. + + *Note:* The string returned by `substr' _cannot_ be assigned to. + Thus, it is a mistake to attempt to change a portion of a string, + like this: + + string = "abcdef" + # try to get "abCDEf", won't work + substr(string, 3, 3) = "CDE" + + or to use `substr' as the third agument of `sub' or `gsub': + + gsub(/xyz/, "pdq", substr($0, 5, 20)) # WRONG + +`tolower(STRING)' + This returns a copy of STRING, with each upper-case character in + the string replaced with its corresponding lower-case character. + Non-alphabetic characters are left unchanged. For example, + `tolower("MiXeD cAsE 123")' returns `"mixed case 123"'. + +`toupper(STRING)' + This returns a copy of STRING, with each lower-case character in + the string replaced with its corresponding upper-case character. + Non-alphabetic characters are left unchanged. For example, + `toupper("MiXeD cAsE 123")' returns `"MIXED CASE 123"'. + +More About `\' and `&' with `sub', `gsub' and `gensub' +------------------------------------------------------ + + When using `sub', `gsub' or `gensub', and trying to get literal +backslashes and ampersands into the replacement text, you need to +remember that there are several levels of "escape processing" going on. + + First, there is the "lexical" level, which is when `awk' reads your +program, and builds an internal copy of your program that can be +executed. + + Then there is the run-time level, when `awk' actually scans the +replacement string to determine what to generate. + + At both levels, `awk' looks for a defined set of characters that can +come after a backslash. At the lexical level, it looks for the escape +sequences listed in *Note Escape Sequences::. Thus, for every `\' that +`awk' will process at the run-time level, you type two `\'s at the +lexical level. When a character that is not valid for an escape +sequence follows the `\', Unix `awk' and `gawk' both simply remove the +initial `\', and put the following character into the string. Thus, for +example, `"a\qb"' is treated as `"aqb"'. + + At the run-time level, the various functions handle sequences of `\' +and `&' differently. The situation is (sadly) somewhat complex. + + Historically, the `sub' and `gsub' functions treated the two +character sequence `\&' specially; this sequence was replaced in the +generated text with a single `&'. Any other `\' within the REPLACEMENT +string that did not precede an `&' was passed through unchanged. To +illustrate with a table: + + You type `sub' sees `sub' generates + -------- ---------- --------------- + `\&' `&' the matched text + `\\&' `\&' a literal `&' + `\\\&' `\&' a literal `&' + `\\\\&' `\\&' a literal `\&' + `\\\\\&' `\\&' a literal `\&' + `\\\\\\&' `\\\&' a literal `\\&' + `\\q' `\q' a literal `\q' + +This table shows both the lexical level processing, where an odd number +of backslashes becomes an even number at the run time level, and the +run-time processing done by `sub'. (For the sake of simplicity, the +rest of the tables below only show the case of even numbers of `\'s +entered at the lexical level.) + + The problem with the historical approach is that there is no way to +get a literal `\' followed by the matched text. + + The 1992 POSIX standard attempted to fix this problem. The standard +says that `sub' and `gsub' look for either a `\' or an `&' after the +`\'. If either one follows a `\', that character is output literally. +The interpretation of `\' and `&' then becomes like this: + + You type `sub' sees `sub' generates + -------- ---------- --------------- + `&' `&' the matched text + `\\&' `\&' a literal `&' + `\\\\&' `\\&' a literal `\', then the matched text + `\\\\\\&' `\\\&' a literal `\&' + +This would appear to solve the problem. Unfortunately, the phrasing of +the standard is unusual. It says, in effect, that `\' turns off the +special meaning of any following character, but that for anything other +than `\' and `&', such special meaning is undefined. This wording +leads to two problems. + + 1. Backslashes must now be doubled in the REPLACEMENT string, breaking + historical `awk' programs. + + 2. To make sure that an `awk' program is portable, _every_ character + in the REPLACEMENT string must be preceded with a backslash.(1) + + The POSIX standard is under revision.(2) Because of the above +problems, proposed text for the revised standard reverts to rules that +correspond more closely to the original existing practice. The proposed +rules have special cases that make it possible to produce a `\' +preceding the matched text. + + You type `sub' sees `sub' generates + -------- ---------- --------------- + `\\\\\\&' `\\\&' a literal `\&' + `\\\\&' `\\&' a literal `\', followed by the matched text + `\\&' `\&' a literal `&' + `\\q' `\q' a literal `\q' + + In a nutshell, at the run-time level, there are now three special +sequences of characters, `\\\&', `\\&' and `\&', whereas historically, +there was only one. However, as in the historical case, any `\' that +is not part of one of these three sequences is not special, and appears +in the output literally. + + `gawk' 3.0 follows these proposed POSIX rules for `sub' and `gsub'. +Whether these proposed rules will actually become codified into the +standard is unknown at this point. Subsequent `gawk' releases will +track the standard and implement whatever the final version specifies; +this Info file will be updated as well. + + The rules for `gensub' are considerably simpler. At the run-time +level, whenever `gawk' sees a `\', if the following character is a +digit, then the text that matched the corresponding parenthesized +subexpression is placed in the generated output. Otherwise, no matter +what the character after the `\' is, that character will appear in the +generated text, and the `\' will not. + + You type `gensub' sees `gensub' generates + -------- ------------- ------------------ + `&' `&' the matched text + `\\&' `\&' a literal `&' + `\\\\' `\\' a literal `\' + `\\\\&' `\\&' a literal `\', then the matched text + `\\\\\\&' `\\\&' a literal `\&' + `\\q' `\q' a literal `q' + + Because of the complexity of the lexical and run-time level +processing, and the special cases for `sub' and `gsub', we recommend +the use of `gawk' and `gensub' for when you have to do substitutions. + + ---------- Footnotes ---------- + + (1) This consequence was certainly unintended. + + (2) As of December 1995, with final approval and publication +hopefully sometime in 1996. + + +File: gawk.info, Node: I/O Functions, Next: Time Functions, Prev: String Functions, Up: Built-in + +Built-in Functions for Input/Output +=================================== + + The following functions are related to Input/Output (I/O). Optional +parameters are enclosed in square brackets ("[" and "]"). + +`close(FILENAME)' + Close the file FILENAME, for input or output. The argument may + alternatively be a shell command that was used for redirecting to + or from a pipe; then the pipe is closed. *Note Closing Input and + Output Files and Pipes: Close Files And Pipes, for more + information. + +`fflush([FILENAME])' + Flush any buffered output associated FILENAME, which is either a + file opened for writing, or a shell command for redirecting output + to a pipe. + + Many utility programs will "buffer" their output; they save + information to be written to a disk file or terminal in memory, + until there is enough for it to be worthwhile to send the data to + the ouput device. This is often more efficient than writing every + little bit of information as soon as it is ready. However, + sometimes it is necessary to force a program to "flush" its + buffers; that is, write the information to its destination, even + if a buffer is not full. This is the purpose of the `fflush' + function; `gawk' too buffers its output, and the `fflush' function + can be used to force `gawk' to flush its buffers. + + `fflush' is a recent (1994) addition to the Bell Labs research + version of `awk'; it is not part of the POSIX standard, and will + not be available if `--posix' has been specified on the command + line (*note Command Line Options: Options.). + + `gawk' extends the `fflush' function in two ways. The first is to + allow no argument at all. In this case, the buffer for the + standard output is flushed. The second way is to allow the null + string (`""') as the argument. In this case, the buffers for _all_ + open output files and pipes are flushed. + + `fflush' returns zero if the buffer was successfully flushed, and + nonzero otherwise. + +`system(COMMAND)' + The system function allows the user to execute operating system + commands and then return to the `awk' program. The `system' + function executes the command given by the string COMMAND. It + returns, as its value, the status returned by the command that was + executed. + + For example, if the following fragment of code is put in your `awk' + program: + + END { + system("date | mail -s 'awk run done' root") + } + + the system administrator will be sent mail when the `awk' program + finishes processing input and begins its end-of-input processing. + + Note that redirecting `print' or `printf' into a pipe is often + enough to accomplish your task. However, if your `awk' program is + interactive, `system' is useful for cranking up large + self-contained programs, such as a shell or an editor. + + Some operating systems cannot implement the `system' function. + `system' causes a fatal error if it is not supported. + +Interactive vs. Non-Interactive Buffering +----------------------------------------- + + As a side point, buffering issues can be even more confusing +depending upon whether or not your program is "interactive", i.e., +communicating with a user sitting at a keyboard.(1) + + Interactive programs generally "line buffer" their output; they +write out every line. Non-interactive programs wait until they have a +full buffer, which may be many lines of output. + + Here is an example of the difference. + + $ awk '{ print $1 + $2 }' + 1 1 + -| 2 + 2 3 + -| 5 + Control-d + +Each line of output is printed immediately. Compare that behavior with +this example. + + $ awk '{ print $1 + $2 }' | cat + 1 1 + 2 3 + Control-d + -| 2 + -| 5 + +Here, no output is printed until after the `Control-D' is typed, since +it is all buffered, and sent down the pipe to `cat' in one shot. + +Controlling Output Buffering with `system' +------------------------------------------ + + The `fflush' function provides explicit control over output +buffering for individual files and pipes. However, its use is not +portable to many other `awk' implementations. An alternative method to +flush output buffers is by calling `system' with a null string as its +argument: + + system("") # flush output + +`gawk' treats this use of the `system' function as a special case, and +is smart enough not to run a shell (or other command interpreter) with +the empty command. Therefore, with `gawk', this idiom is not only +useful, it is efficient. While this method should work with other +`awk' implementations, it will not necessarily avoid starting an +unnecessary shell. (Other implementations may only flush the buffer +associated with the standard output, and not necessarily all buffered +output.) + + If you think about what a programmer expects, it makes sense that +`system' should flush any pending output. The following program: + + BEGIN { + print "first print" + system("echo system echo") + print "second print" + } + +must print + + first print + system echo + second print + +and not + + system echo + first print + second print + + If `awk' did not flush its buffers before calling `system', the +latter (undesirable) output is what you would see. + + ---------- Footnotes ---------- + + (1) A program is interactive if the standard output is connected to +a terminal device. + + +File: gawk.info, Node: Time Functions, Prev: I/O Functions, Up: Built-in + +Functions for Dealing with Time Stamps +====================================== + + A common use for `awk' programs is the processing of log files +containing time stamp information, indicating when a particular log +record was written. Many programs log their time stamp in the form +returned by the `time' system call, which is the number of seconds +since a particular epoch. On POSIX systems, it is the number of +seconds since Midnight, January 1, 1970, UTC. + + In order to make it easier to process such log files, and to produce +useful reports, `gawk' provides two functions for working with time +stamps. Both of these are `gawk' extensions; they are not specified in +the POSIX standard, nor are they in any other known version of `awk'. + + Optional parameters are enclosed in square brackets ("[" and "]"). + +`systime()' + This function returns the current time as the number of seconds + since the system epoch. On POSIX systems, this is the number of + seconds since Midnight, January 1, 1970, UTC. It may be a + different number on other systems. + +`strftime([FORMAT [, TIMESTAMP]])' + This function returns a string. It is similar to the function of + the same name in ANSI C. The time specified by TIMESTAMP is used + to produce a string, based on the contents of the FORMAT string. + The TIMESTAMP is in the same format as the value returned by the + `systime' function. If no TIMESTAMP argument is supplied, `gawk' + will use the current time of day as the time stamp. If no FORMAT + argument is supplied, `strftime' uses `"%a %b %d %H:%M:%S %Z %Y"'. + This format string produces output (almost) equivalent to that of + the `date' utility. (Versions of `gawk' prior to 3.0 require the + FORMAT argument.) + + The `systime' function allows you to compare a time stamp from a log +file with the current time of day. In particular, it is easy to +determine how long ago a particular record was logged. It also allows +you to produce log records using the "seconds since the epoch" format. + + The `strftime' function allows you to easily turn a time stamp into +human-readable information. It is similar in nature to the `sprintf' +function (*note Built-in Functions for String Manipulation: String +Functions.), in that it copies non-format specification characters +verbatim to the returned string, while substituting date and time +values for format specifications in the FORMAT string. + + `strftime' is guaranteed by the ANSI C standard to support the +following date format specifications: + +`%a' + The locale's abbreviated weekday name. + +`%A' + The locale's full weekday name. + +`%b' + The locale's abbreviated month name. + +`%B' + The locale's full month name. + +`%c' + The locale's "appropriate" date and time representation. + +`%d' + The day of the month as a decimal number (01-31). + +`%H' + The hour (24-hour clock) as a decimal number (00-23). + +`%I' + The hour (12-hour clock) as a decimal number (01-12). + +`%j' + The day of the year as a decimal number (001-366). + +`%m' + The month as a decimal number (01-12). + +`%M' + The minute as a decimal number (00-59). + +`%p' + The locale's equivalent of the AM/PM designations associated with + a 12-hour clock. + +`%S' + The second as a decimal number (00-60).(1) + +`%U' + The week number of the year (the first Sunday as the first day of + week one) as a decimal number (00-53). + +`%w' + The weekday as a decimal number (0-6). Sunday is day zero. + +`%W' + The week number of the year (the first Monday as the first day of + week one) as a decimal number (00-53). + +`%x' + The locale's "appropriate" date representation. + +`%X' + The locale's "appropriate" time representation. + +`%y' + The year without century as a decimal number (00-99). + +`%Y' + The year with century as a decimal number (e.g., 1995). + +`%Z' + The time zone name or abbreviation, or no characters if no time + zone is determinable. + +`%%' + A literal `%'. + + If a conversion specifier is not one of the above, the behavior is +undefined.(2) + + Informally, a "locale" is the geographic place in which a program is +meant to run. For example, a common way to abbreviate the date +September 4, 1991 in the United States would be "9/4/91". In many +countries in Europe, however, it would be abbreviated "4.9.91". Thus, +the `%x' specification in a `"US"' locale might produce `9/4/91', while +in a `"EUROPE"' locale, it might produce `4.9.91'. The ANSI C standard +defines a default `"C"' locale, which is an environment that is typical +of what most C programmers are used to. + + A public-domain C version of `strftime' is supplied with `gawk' for +systems that are not yet fully ANSI-compliant. If that version is used +to compile `gawk' (*note Installing `gawk': Installation.), then the +following additional format specifications are available: + +`%D' + Equivalent to specifying `%m/%d/%y'. + +`%e' + The day of the month, padded with a space if it is only one digit. + +`%h' + Equivalent to `%b', above. + +`%n' + A newline character (ASCII LF). + +`%r' + Equivalent to specifying `%I:%M:%S %p'. + +`%R' + Equivalent to specifying `%H:%M'. + +`%T' + Equivalent to specifying `%H:%M:%S'. + +`%t' + A tab character. + +`%k' + The hour (24-hour clock) as a decimal number (0-23). Single digit + numbers are padded with a space. + +`%l' + The hour (12-hour clock) as a decimal number (1-12). Single digit + numbers are padded with a space. + +`%C' + The century, as a number between 00 and 99. + +`%u' + The weekday as a decimal number [1 (Monday)-7]. + +`%V' + The week number of the year (the first Monday as the first day of + week one) as a decimal number (01-53). The method for determining + the week number is as specified by ISO 8601 (to wit: if the week + containing January 1 has four or more days in the new year, then + it is week one, otherwise it is week 53 of the previous year and + the next week is week one). + +`%G' + The year with century of the ISO week number, as a decimal number. + + For example, January 1, 1993, is in week 53 of 1992. Thus, the year + of its ISO week number is 1992, even though its year is 1993. + Similarly, December 31, 1973, is in week 1 of 1974. Thus, the year + of its ISO week number is 1974, even though its year is 1973. + +`%g' + The year without century of the ISO week number, as a decimal + number (00-99). + +`%Ec %EC %Ex %Ey %EY %Od %Oe %OH %OI' +`%Om %OM %OS %Ou %OU %OV %Ow %OW %Oy' + These are "alternate representations" for the specifications that + use only the second letter (`%c', `%C', and so on). They are + recognized, but their normal representations are used.(3) (These + facilitate compliance with the POSIX `date' utility.) + +`%v' + The date in VMS format (e.g., 20-JUN-1991). + +`%z' + The timezone offset in a +HHMM format (e.g., the format necessary + to produce RFC-822/RFC-1036 date headers). + + This example is an `awk' implementation of the POSIX `date' utility. +Normally, the `date' utility prints the current date and time of day +in a well known format. However, if you provide an argument to it that +begins with a `+', `date' will copy non-format specifier characters to +the standard output, and will interpret the current time according to +the format specifiers in the string. For example: + + $ date '+Today is %A, %B %d, %Y.' + -| Today is Thursday, July 11, 1991. + + Here is the `gawk' version of the `date' utility. It has a shell +"wrapper", to handle the `-u' option, which requires that `date' run as +if the time zone was set to UTC. + + #! /bin/sh + # + # date --- approximate the P1003.2 'date' command + + case $1 in + -u) TZ=GMT0 # use UTC + export TZ + shift ;; + esac + + gawk 'BEGIN { + format = "%a %b %d %H:%M:%S %Z %Y" + exitval = 0 + + if (ARGC > 2) + exitval = 1 + else if (ARGC == 2) { + format = ARGV[1] + if (format ~ /^\+/) + format = substr(format, 2) # remove leading + + } + print strftime(format) + exit exitval + }' "$@" + + ---------- Footnotes ---------- + + (1) Occasionally there are minutes in a year with a leap second, +which is why the seconds can go up to 60. + + (2) This is because ANSI C leaves the behavior of the C version of +`strftime' undefined, and `gawk' will use the system's version of +`strftime' if it's there. Typically, the conversion specifier will +either not appear in the returned string, or it will appear literally. + + (3) If you don't understand any of this, don't worry about it; these +facilities are meant to make it easier to "internationalize" programs. + + +File: gawk.info, Node: User-defined, Next: Invoking Gawk, Prev: Built-in, Up: Top + +User-defined Functions +********************** + + Complicated `awk' programs can often be simplified by defining your +own functions. User-defined functions can be called just like built-in +ones (*note Function Calls::), but it is up to you to define them--to +tell `awk' what they should do. + +* Menu: + +* Definition Syntax:: How to write definitions and what they mean. +* Function Example:: An example function definition and what it + does. +* Function Caveats:: Things to watch out for. +* Return Statement:: Specifying the value a function returns. + + +File: gawk.info, Node: Definition Syntax, Next: Function Example, Prev: User-defined, Up: User-defined + +Function Definition Syntax +========================== + + Definitions of functions can appear anywhere between the rules of an +`awk' program. Thus, the general form of an `awk' program is extended +to include sequences of rules _and_ user-defined function definitions. +There is no need in `awk' to put the definition of a function before +all uses of the function. This is because `awk' reads the entire +program before starting to execute any of it. + + The definition of a function named NAME looks like this: + + function NAME(PARAMETER-LIST) + { + BODY-OF-FUNCTION + } + +NAME is the name of the function to be defined. A valid function name +is like a valid variable name: a sequence of letters, digits and +underscores, not starting with a digit. Within a single `awk' program, +any particular name can only be used as a variable, array or function. + + PARAMETER-LIST is a list of the function's arguments and local +variable names, separated by commas. When the function is called, the +argument names are used to hold the argument values given in the call. +The local variables are initialized to the empty string. A function +cannot have two parameters with the same name. + + The BODY-OF-FUNCTION consists of `awk' statements. It is the most +important part of the definition, because it says what the function +should actually _do_. The argument names exist to give the body a way +to talk about the arguments; local variables, to give the body places +to keep temporary values. + + Argument names are not distinguished syntactically from local +variable names; instead, the number of arguments supplied when the +function is called determines how many argument variables there are. +Thus, if three argument values are given, the first three names in +PARAMETER-LIST are arguments, and the rest are local variables. + + It follows that if the number of arguments is not the same in all +calls to the function, some of the names in PARAMETER-LIST may be +arguments on some occasions and local variables on others. Another way +to think of this is that omitted arguments default to the null string. + + Usually when you write a function you know how many names you intend +to use for arguments and how many you intend to use as local variables. +It is conventional to place some extra space between the arguments and +the local variables, to document how your function is supposed to be +used. + + During execution of the function body, the arguments and local +variable values hide or "shadow" any variables of the same names used +in the rest of the program. The shadowed variables are not accessible +in the function definition, because there is no way to name them while +their names have been taken away for the local variables. All other +variables used in the `awk' program can be referenced or set normally +in the function's body. + + The arguments and local variables last only as long as the function +body is executing. Once the body finishes, you can once again access +the variables that were shadowed while the function was running. + + The function body can contain expressions which call functions. They +can even call this function, either directly or by way of another +function. When this happens, we say the function is "recursive". + + In many `awk' implementations, including `gawk', the keyword +`function' may be abbreviated `func'. However, POSIX only specifies +the use of the keyword `function'. This actually has some practical +implications. If `gawk' is in POSIX-compatibility mode (*note Command +Line Options: Options.), then the following statement will _not_ define +a function: + + func foo() { a = sqrt($1) ; print a } + +Instead it defines a rule that, for each record, concatenates the value +of the variable `func' with the return value of the function `foo'. If +the resulting string is non-null, the action is executed. This is +probably not what was desired. (`awk' accepts this input as +syntactically valid, since functions may be used before they are defined +in `awk' programs.) + + To ensure that your `awk' programs are portable, always use the +keyword `function' when defining a function. + + +File: gawk.info, Node: Function Example, Next: Function Caveats, Prev: Definition Syntax, Up: User-defined + +Function Definition Examples +============================ + + Here is an example of a user-defined function, called `myprint', that +takes a number and prints it in a specific format. + + function myprint(num) + { + printf "%6.3g\n", num + } + +To illustrate, here is an `awk' rule which uses our `myprint' function: + + $3 > 0 { myprint($3) } + +This program prints, in our special format, all the third fields that +contain a positive number in our input. Therefore, when given: + + 1.2 3.4 5.6 7.8 + 9.10 11.12 -13.14 15.16 + 17.18 19.20 21.22 23.24 + +this program, using our function to format the results, prints: + + 5.6 + 21.2 + + This function deletes all the elements in an array. + + function delarray(a, i) + { + for (i in a) + delete a[i] + } + + When working with arrays, it is often necessary to delete all the +elements in an array and start over with a new list of elements (*note +The `delete' Statement: Delete.). Instead of having to repeat this +loop everywhere in your program that you need to clear out an array, +your program can just call `delarray'. + + Here is an example of a recursive function. It takes a string as an +input parameter, and returns the string in backwards order. + + function rev(str, start) + { + if (start == 0) + return "" + + return (substr(str, start, 1) rev(str, start - 1)) + } + + If this function is in a file named `rev.awk', we can test it this +way: + + $ echo "Don't Panic!" | + > gawk --source '{ print rev($0, length($0)) }' -f rev.awk + -| !cinaP t'noD + + Here is an example that uses the built-in function `strftime'. +(*Note Functions for Dealing with Time Stamps: Time Functions, for more +information on `strftime'.) The C `ctime' function takes a timestamp +and returns it in a string, formatted in a well known fashion. Here is +an `awk' version: + + # ctime.awk + # + # awk version of C ctime(3) function + + function ctime(ts, format) + { + format = "%a %b %d %H:%M:%S %Z %Y" + if (ts == 0) + ts = systime() # use current time as default + return strftime(format, ts) + } + + +File: gawk.info, Node: Function Caveats, Next: Return Statement, Prev: Function Example, Up: User-defined + +Calling User-defined Functions +============================== + + "Calling a function" means causing the function to run and do its +job. A function call is an expression, and its value is the value +returned by the function. + + A function call consists of the function name followed by the +arguments in parentheses. What you write in the call for the arguments +are `awk' expressions; each time the call is executed, these +expressions are evaluated, and the values are the actual arguments. For +example, here is a call to `foo' with three arguments (the first being +a string concatenation): + + foo(x y, "lose", 4 * z) + + *Caution:* whitespace characters (spaces and tabs) are not allowed +between the function name and the open-parenthesis of the argument list. +If you write whitespace by mistake, `awk' might think that you mean to +concatenate a variable with an expression in parentheses. However, it +notices that you used a function name and not a variable name, and +reports an error. + + When a function is called, it is given a _copy_ of the values of its +arguments. This is known as "call by value". The caller may use a +variable as the expression for the argument, but the called function +does not know this: it only knows what value the argument had. For +example, if you write this code: + + foo = "bar" + z = myfunc(foo) + +then you should not think of the argument to `myfunc' as being "the +variable `foo'." Instead, think of the argument as the string value, +`"bar"'. + + If the function `myfunc' alters the values of its local variables, +this has no effect on any other variables. Thus, if `myfunc' does this: + + function myfunc(str) + { + print str + str = "zzz" + print str + } + +to change its first argument variable `str', this _does not_ change the +value of `foo' in the caller. The role of `foo' in calling `myfunc' +ended when its value, `"bar"', was computed. If `str' also exists +outside of `myfunc', the function body cannot alter this outer value, +because it is shadowed during the execution of `myfunc' and cannot be +seen or changed from there. + + However, when arrays are the parameters to functions, they are _not_ +copied. Instead, the array itself is made available for direct +manipulation by the function. This is usually called "call by +reference". Changes made to an array parameter inside the body of a +function _are_ visible outside that function. This can be *very* +dangerous if you do not watch what you are doing. For example: + + function changeit(array, ind, nvalue) + { + array[ind] = nvalue + } + + BEGIN { + a[1] = 1; a[2] = 2; a[3] = 3 + changeit(a, 2, "two") + printf "a[1] = %s, a[2] = %s, a[3] = %s\n", + a[1], a[2], a[3] + } + +This program prints `a[1] = 1, a[2] = two, a[3] = 3', because +`changeit' stores `"two"' in the second element of `a'. + + Some `awk' implementations allow you to call a function that has not +been defined, and only report a problem at run-time when the program +actually tries to call the function. For example: + + BEGIN { + if (0) + foo() + else + bar() + } + function bar() { ... } + # note that `foo' is not defined + +Since the `if' statement will never be true, it is not really a problem +that `foo' has not been defined. Usually though, it is a problem if a +program calls an undefined function. + + If `--lint' has been specified (*note Command Line Options: +Options.), `gawk' will report about calls to undefined functions. + + Some `awk' implementations generate a run-time error if you use the +`next' statement (*note The `next' Statement: Next Statement.) inside +a user-defined function. `gawk' does not have this problem. + + +File: gawk.info, Node: Return Statement, Prev: Function Caveats, Up: User-defined + +The `return' Statement +====================== + + The body of a user-defined function can contain a `return' statement. +This statement returns control to the rest of the `awk' program. It +can also be used to return a value for use in the rest of the `awk' +program. It looks like this: + + return [EXPRESSION] + + The EXPRESSION part is optional. If it is omitted, then the returned +value is undefined and, therefore, unpredictable. + + A `return' statement with no value expression is assumed at the end +of every function definition. So if control reaches the end of the +function body, then the function returns an unpredictable value. `awk' +will _not_ warn you if you use the return value of such a function. + + Sometimes, you want to write a function for what it does, not for +what it returns. Such a function corresponds to a `void' function in C +or to a `procedure' in Pascal. Thus, it may be appropriate to not +return any value; you should simply bear in mind that if you use the +return value of such a function, you do so at your own risk. + + Here is an example of a user-defined function that returns a value +for the largest number among the elements of an array: + + function maxelt(vec, i, ret) + { + for (i in vec) { + if (ret == "" || vec[i] > ret) + ret = vec[i] + } + return ret + } + +You call `maxelt' with one argument, which is an array name. The local +variables `i' and `ret' are not intended to be arguments; while there +is nothing to stop you from passing two or three arguments to `maxelt', +the results would be strange. The extra space before `i' in the +function parameter list indicates that `i' and `ret' are not supposed +to be arguments. This is a convention that you should follow when you +define functions. + + Here is a program that uses our `maxelt' function. It loads an +array, calls `maxelt', and then reports the maximum number in that +array: + + awk ' + function maxelt(vec, i, ret) + { + for (i in vec) { + if (ret == "" || vec[i] > ret) + ret = vec[i] + } + return ret + } + + # Load all fields of each record into nums. + { + for(i = 1; i <= NF; i++) + nums[NR, i] = $i + } + + END { + print maxelt(nums) + }' + + Given the following input: + + 1 5 23 8 16 + 44 3 5 2 8 26 + 256 291 1396 2962 100 + -6 467 998 1101 + 99385 11 0 225 + +our program tells us (predictably) that `99385' is the largest number +in our array. + + +File: gawk.info, Node: Invoking Gawk, Next: Library Functions, Prev: User-defined, Up: Top + +Running `awk' +************* + + There are two ways to run `awk': with an explicit program, or with +one or more program files. Here are templates for both of them; items +enclosed in `[...]' in these templates are optional. + + Besides traditional one-letter POSIX-style options, `gawk' also +supports GNU long options. + + awk [OPTIONS] -f progfile [`--'] FILE ... + awk [OPTIONS] [`--'] 'PROGRAM' FILE ... + + It is possible to invoke `awk' with an empty program: + + $ awk '' datafile1 datafile2 + +Doing so makes little sense though; `awk' will simply exit silently +when given an empty program (d.c.). If `--lint' has been specified on +the command line, `gawk' will issue a warning that the program is empty. + +* Menu: + +* Options:: Command line options and their meanings. +* Other Arguments:: Input file names and variable assignments. +* AWKPATH Variable:: Searching directories for `awk' programs. +* Obsolete:: Obsolete Options and/or features. +* Undocumented:: Undocumented Options and Features. +* Known Bugs:: Known Bugs in `gawk'. + + +File: gawk.info, Node: Options, Next: Other Arguments, Prev: Invoking Gawk, Up: Invoking Gawk + +Command Line Options +==================== + + Options begin with a dash, and consist of a single character. GNU +style long options consist of two dashes and a keyword. The keyword +can be abbreviated, as long the abbreviation allows the option to be +uniquely identified. If the option takes an argument, then the keyword +is either immediately followed by an equals sign (`=') and the +argument's value, or the keyword and the argument's value are separated +by whitespace. For brevity, the discussion below only refers to the +traditional short options; however the long and short options are +interchangeable in all contexts. + + Each long option for `gawk' has a corresponding POSIX-style option. +The options and their meanings are as follows: + +`-F FS' +`--field-separator FS' + Sets the `FS' variable to FS (*note Specifying How Fields are + Separated: Field Separators.). + +`-f SOURCE-FILE' +`--file SOURCE-FILE' + Indicates that the `awk' program is to be found in SOURCE-FILE + instead of in the first non-option argument. + +`-v VAR=VAL' +`--assign VAR=VAL' + Sets the variable VAR to the value VAL *before* execution of the + program begins. Such variable values are available inside the + `BEGIN' rule (*note Other Command Line Arguments: Other + Arguments.). + + The `-v' option can only set one variable, but you can use it more + than once, setting another variable each time, like this: `awk + -v foo=1 -v bar=2 ...'. + +`-mf NNN' +`-mr NNN' + Set various memory limits to the value NNN. The `f' flag sets the + maximum number of fields, and the `r' flag sets the maximum record + size. These two flags and the `-m' option are from the Bell Labs + research version of Unix `awk'. They are provided for + compatibility, but otherwise ignored by `gawk', since `gawk' has + no predefined limits. + +`-W GAWK-OPT' + Following the POSIX standard, options that are implementation + specific are supplied as arguments to the `-W' option. These + options also have corresponding GNU style long options. See below. + +`--' + Signals the end of the command line options. The following + arguments are not treated as options even if they begin with `-'. + This interpretation of `--' follows the POSIX argument parsing + conventions. + + This is useful if you have file names that start with `-', or in + shell scripts, if you have file names that will be specified by + the user which could start with `-'. + + The following `gawk'-specific options are available: + +`-W traditional' +`-W compat' +`--traditional' +`--compat' + Specifies "compatibility mode", in which the GNU extensions to the + `awk' language are disabled, so that `gawk' behaves just like the + Bell Labs research version of Unix `awk'. `--traditional' is the + preferred form of this option. *Note Extensions in `gawk' Not in + POSIX `awk': POSIX/GNU, which summarizes the extensions. Also see + *Note Downward Compatibility and Debugging: Compatibility Mode. + +`-W copyleft' +`-W copyright' +`--copyleft' +`--copyright' + Print the short version of the General Public License, and then + exit. This option may disappear in a future version of `gawk'. + +`-W help' +`-W usage' +`--help' +`--usage' + Print a "usage" message summarizing the short and long style + options that `gawk' accepts, and then exit. + +`-W lint' +`--lint' + Warn about constructs that are dubious or non-portable to other + `awk' implementations. Some warnings are issued when `gawk' first + reads your program. Others are issued at run-time, as your + program executes. + +`-W lint-old' +`--lint-old' + Warn about constructs that are not available in the original + Version 7 Unix version of `awk' (*note Major Changes between V7 + and SVR3.1: V7/SVR3.1.). + +`-W posix' +`--posix' + Operate in strict POSIX mode. This disables all `gawk' extensions + (just like `--traditional'), and adds the following additional + restrictions: + + * `\x' escape sequences are not recognized (*note Escape + Sequences::). + + * Newlines do not act as whitespace to separate fields when + `FS' is equal to a single space. + + * The synonym `func' for the keyword `function' is not + recognized (*note Function Definition Syntax: Definition + Syntax.). + + * The operators `**' and `**=' cannot be used in place of `^' + and `^=' (*note Arithmetic Operators: Arithmetic Ops., and + also *note Assignment Expressions: Assignment Ops.). + + * Specifying `-Ft' on the command line does not set the value + of `FS' to be a single tab character (*note Specifying How + Fields are Separated: Field Separators.). + + * The `fflush' built-in function is not supported (*note + Built-in Functions for Input/Output: I/O Functions.). + + If you supply both `--traditional' and `--posix' on the command + line, `--posix' will take precedence. `gawk' will also issue a + warning if both options are supplied. + +`-W re-interval' +`--re-interval' + Allow interval expressions (*note Regular Expression Operators: + Regexp Operators.), in regexps. Because interval expressions were + traditionally not available in `awk', `gawk' does not provide them + by default. This prevents old `awk' programs from breaking. + +`-W source PROGRAM-TEXT' +`--source PROGRAM-TEXT' + Program source code is taken from the PROGRAM-TEXT. This option + allows you to mix source code in files with source code that you + enter on the command line. This is particularly useful when you + have library functions that you wish to use from your command line + programs (*note The `AWKPATH' Environment Variable: AWKPATH + Variable.). + +`-W version' +`--version' + Prints version information for this particular copy of `gawk'. + This allows you to determine if your copy of `gawk' is up to date + with respect to whatever the Free Software Foundation is currently + distributing. It is also useful for bug reports (*note Reporting + Problems and Bugs: Bugs.). + + Any other options are flagged as invalid with a warning message, but +are otherwise ignored. + + In compatibility mode, as a special case, if the value of FS supplied +to the `-F' option is `t', then `FS' is set to the tab character +(`"\t"'). This is only true for `--traditional', and not for `--posix' +(*note Specifying How Fields are Separated: Field Separators.). + + The `-f' option may be used more than once on the command line. If +it is, `awk' reads its program source from all of the named files, as +if they had been concatenated together into one big file. This is +useful for creating libraries of `awk' functions. Useful functions can +be written once, and then retrieved from a standard place, instead of +having to be included into each individual program. + + You can type in a program at the terminal and still use library +functions, by specifying `-f /dev/tty'. `awk' will read a file from +the terminal to use as part of the `awk' program. After typing your +program, type `Control-d' (the end-of-file character) to terminate it. +(You may also use `-f -' to read program source from the standard +input, but then you will not be able to also use the standard input as a +source of data.) + + Because it is clumsy using the standard `awk' mechanisms to mix +source file and command line `awk' programs, `gawk' provides the +`--source' option. This does not require you to pre-empt the standard +input for your source code, and allows you to easily mix command line +and library source code (*note The `AWKPATH' Environment Variable: +AWKPATH Variable.). + + If no `-f' or `--source' option is specified, then `gawk' will use +the first non-option command line argument as the text of the program +source code. + + If the environment variable `POSIXLY_CORRECT' exists, then `gawk' +will behave in strict POSIX mode, exactly as if you had supplied the +`--posix' command line option. Many GNU programs look for this +environment variable to turn on strict POSIX mode. If you supply +`--lint' on the command line, and `gawk' turns on POSIX mode because of +`POSIXLY_CORRECT', then it will print a warning message indicating that +POSIX mode is in effect. + + You would typically set this variable in your shell's startup file. +For a Bourne compatible shell (such as Bash), you would add these lines +to the `.profile' file in your home directory. + + POSIXLY_CORRECT=true + export POSIXLY_CORRECT + + For a `csh' compatible shell,(1) you would add this line to the +`.login' file in your home directory. + + setenv POSIXLY_CORRECT true + + ---------- Footnotes ---------- + + (1) Not recommended. + + +File: gawk.info, Node: Other Arguments, Next: AWKPATH Variable, Prev: Options, Up: Invoking Gawk + +Other Command Line Arguments +============================ + + Any additional arguments on the command line are normally treated as +input files to be processed in the order specified. However, an +argument that has the form `VAR=VALUE', assigns the value VALUE to the +variable VAR--it does not specify a file at all. + + All these arguments are made available to your `awk' program in the +`ARGV' array (*note Built-in Variables::). Command line options and +the program text (if present) are omitted from `ARGV'. All other +arguments, including variable assignments, are included. As each +element of `ARGV' is processed, `gawk' sets the variable `ARGIND' to +the index in `ARGV' of the current element. + + The distinction between file name arguments and variable-assignment +arguments is made when `awk' is about to open the next input file. At +that point in execution, it checks the "file name" to see whether it is +really a variable assignment; if so, `awk' sets the variable instead of +reading a file. + + Therefore, the variables actually receive the given values after all +previously specified files have been read. In particular, the values of +variables assigned in this fashion are _not_ available inside a `BEGIN' +rule (*note The `BEGIN' and `END' Special Patterns: BEGIN/END.), since +such rules are run before `awk' begins scanning the argument list. + + The variable values given on the command line are processed for +escape sequences (d.c.) (*note Escape Sequences::). + + In some earlier implementations of `awk', when a variable assignment +occurred before any file names, the assignment would happen _before_ +the `BEGIN' rule was executed. `awk''s behavior was thus inconsistent; +some command line assignments were available inside the `BEGIN' rule, +while others were not. However, some applications came to depend upon +this "feature." When `awk' was changed to be more consistent, the `-v' +option was added to accommodate applications that depended upon the old +behavior. + + The variable assignment feature is most useful for assigning to +variables such as `RS', `OFS', and `ORS', which control input and +output formats, before scanning the data files. It is also useful for +controlling state if multiple passes are needed over a data file. For +example: + + awk 'pass == 1 { PASS 1 STUFF } + pass == 2 { PASS 2 STUFF }' pass=1 mydata pass=2 mydata + + Given the variable assignment feature, the `-F' option for setting +the value of `FS' is not strictly necessary. It remains for historical +compatibility. + + +File: gawk.info, Node: AWKPATH Variable, Next: Obsolete, Prev: Other Arguments, Up: Invoking Gawk + +The `AWKPATH' Environment Variable +================================== + + The previous section described how `awk' program files can be named +on the command line with the `-f' option. In most `awk' +implementations, you must supply a precise path name for each program +file, unless the file is in the current directory. + + But in `gawk', if the file name supplied to the `-f' option does not +contain a `/', then `gawk' searches a list of directories (called the +"search path"), one by one, looking for a file with the specified name. + + The search path is a string consisting of directory names separated +by colons. `gawk' gets its search path from the `AWKPATH' environment +variable. If that variable does not exist, `gawk' uses a default path, +which is `.:/usr/local/share/awk'.(1) (Programs written for use by +system administrators should use an `AWKPATH' variable that does not +include the current directory, `.'.) + + The search path feature is particularly useful for building up +libraries of useful `awk' functions. The library files can be placed +in a standard directory that is in the default path, and then specified +on the command line with a short file name. Otherwise, the full file +name would have to be typed for each file. + + By using both the `--source' and `-f' options, your command line +`awk' programs can use facilities in `awk' library files. *Note A +Library of `awk' Functions: Library Functions. + + Path searching is not done if `gawk' is in compatibility mode. This +is true for both `--traditional' and `--posix'. *Note Command Line +Options: Options. + + *Note:* if you want files in the current directory to be found, you +must include the current directory in the path, either by including `.' +explicitly in the path, or by writing a null entry in the path. (A +null entry is indicated by starting or ending the path with a colon, or +by placing two colons next to each other (`::').) If the current +directory is not included in the path, then files cannot be found in +the current directory. This path search mechanism is identical to the +shell's. + + Starting with version 3.0, if `AWKPATH' is not defined in the +environment, `gawk' will place its default search path into +`ENVIRON["AWKPATH"]'. This makes it easy to determine the actual search +path `gawk' will use. + + ---------- Footnotes ---------- + + (1) Your version of `gawk' may use a directory that is different +than `/usr/local/share/awk'; it will depend upon how `gawk' was built +and installed. The actual directory will be the value of `$(datadir)' +generated when `gawk' was configured. You probably don't need to worry +about this though. + + +File: gawk.info, Node: Obsolete, Next: Undocumented, Prev: AWKPATH Variable, Up: Invoking Gawk + +Obsolete Options and/or Features +================================ + + This section describes features and/or command line options from +previous releases of `gawk' that are either not available in the +current version, or that are still supported but deprecated (meaning +that they will _not_ be in the next release). + + For version 3.0.1 of `gawk', there are no command line options or +other deprecated features from the previous version of `gawk'. This +node is thus essentially a place holder, in case some option becomes +obsolete in a future version of `gawk'. + + +File: gawk.info, Node: Undocumented, Next: Known Bugs, Prev: Obsolete, Up: Invoking Gawk + +Undocumented Options and Features +================================= + + This section intentionally left blank. + + +File: gawk.info, Node: Known Bugs, Prev: Undocumented, Up: Invoking Gawk + +Known Bugs in `gawk' +==================== + + * The `-F' option for changing the value of `FS' (*note Command Line + Options: Options.) is not necessary given the command line + variable assignment feature; it remains only for backwards + compatibility. + + * If your system actually has support for `/dev/fd' and the + associated `/dev/stdin', `/dev/stdout', and `/dev/stderr' files, + you may get different output from `gawk' than you would get on a + system without those files. When `gawk' interprets these files + internally, it synchronizes output to the standard output with + output to `/dev/stdout', while on a system with those files, the + output is actually to different open files (*note Special File + Names in `gawk': Special Files.). + + * Syntactically invalid single character programs tend to overflow + the parse stack, generating a rather unhelpful message. Such + programs are surprisingly difficult to diagnose in the completely + general case, and the effort to do so really is not worth it. + + +File: gawk.info, Node: Library Functions, Next: Sample Programs, Prev: Invoking Gawk, Up: Top + +A Library of `awk' Functions +**************************** + + This chapter presents a library of useful `awk' functions. The +sample programs presented later (*note Practical `awk' Programs: Sample +Programs.) use these functions. The functions are presented here in a +progression from simple to complex. + + *Note Extracting Programs from Texinfo Source Files: Extract Program, +presents a program that you can use to extract the source code for +these example library functions and programs from the Texinfo source +for this Info file. (This has already been done as part of the `gawk' +distribution.) + + If you have written one or more useful, general purpose `awk' +functions, and would like to contribute them for a subsequent edition +of this Info file, please contact the author. *Note Reporting Problems +and Bugs: Bugs, for information on doing this. Don't just send code, +as you will be required to either place your code in the public domain, +publish it under the GPL (*note GNU GENERAL PUBLIC LICENSE: Copying.), +or assign the copyright in it to the Free Software Foundation. + +* Menu: + +* Portability Notes:: What to do if you don't have `gawk'. +* Nextfile Function:: Two implementations of a `nextfile' + function. +* Assert Function:: A function for assertions in `awk' + programs. +* Round Function:: A function for rounding if `sprintf' does + not do it correctly. +* Ordinal Functions:: Functions for using characters as numbers and + vice versa. +* Join Function:: A function to join an array into a string. +* Mktime Function:: A function to turn a date into a timestamp. +* Gettimeofday Function:: A function to get formatted times. +* Filetrans Function:: A function for handling data file transitions. +* Getopt Function:: A function for processing command line + arguments. +* Passwd Functions:: Functions for getting user information. +* Group Functions:: Functions for getting group information. +* Library Names:: How to best name private global variables in + library functions. + + +File: gawk.info, Node: Portability Notes, Next: Nextfile Function, Prev: Library Functions, Up: Library Functions + +Simulating `gawk'-specific Features +=================================== + + The programs in this chapter and in *Note Practical `awk' Programs: +Sample Programs, freely use features that are specific to `gawk'. This +section briefly discusses how you can rewrite these programs for +different implementations of `awk'. + + Diagnostic error messages are sent to `/dev/stderr'. Use `| "cat +1>&2"' instead of `> "/dev/stderr"', if your system does not have a +`/dev/stderr', or if you cannot use `gawk'. + + A number of programs use `nextfile' (*note The `nextfile' Statement: +Nextfile Statement.), to skip any remaining input in the input file. +*Note Implementing `nextfile' as a Function: Nextfile Function, shows +you how to write a function that will do the same thing. + + Finally, some of the programs choose to ignore upper-case and +lower-case distinctions in their input. They do this by assigning one +to `IGNORECASE'. You can achieve the same effect by adding the +following rule to the beginning of the program: + + # ignore case + { $0 = tolower($0) } + +Also, verify that all regexp and string constants used in comparisons +only use lower-case letters. + + +File: gawk.info, Node: Nextfile Function, Next: Assert Function, Prev: Portability Notes, Up: Library Functions + +Implementing `nextfile' as a Function +===================================== + + The `nextfile' statement presented in *Note The `nextfile' +Statement: Nextfile Statement, is a `gawk'-specific extension. It is +not available in other implementations of `awk'. This section shows +two versions of a `nextfile' function that you can use to simulate +`gawk''s `nextfile' statement if you cannot use `gawk'. + + Here is a first attempt at writing a `nextfile' function. + + # nextfile --- skip remaining records in current file + + # this should be read in before the "main" awk program + + function nextfile() { _abandon_ = FILENAME; next } + + _abandon_ == FILENAME { next } + + This file should be included before the main program, because it +supplies a rule that must be executed first. This rule compares the +current data file's name (which is always in the `FILENAME' variable) +to a private variable named `_abandon_'. If the file name matches, +then the action part of the rule executes a `next' statement, to go on +to the next record. (The use of `_' in the variable name is a +convention. It is discussed more fully in *Note Naming Library +Function Global Variables: Library Names.) + + The use of the `next' statement effectively creates a loop that reads +all the records from the current data file. Eventually, the end of the +file is reached, and a new data file is opened, changing the value of +`FILENAME'. Once this happens, the comparison of `_abandon_' to +`FILENAME' fails, and execution continues with the first rule of the +"real" program. + + The `nextfile' function itself simply sets the value of `_abandon_' +and then executes a `next' statement to start the loop going.(1) + + This initial version has a subtle problem. What happens if the same +data file is listed _twice_ on the command line, one right after the +other, or even with just a variable assignment between the two +occurrences of the file name? + + In such a case, this code will skip right through the file, a second +time, even though it should stop when it gets to the end of the first +occurrence. Here is a second version of `nextfile' that remedies this +problem. + + # nextfile --- skip remaining records in current file + # correctly handle successive occurrences of the same file + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May, 1993 + + # this should be read in before the "main" awk program + + function nextfile() { _abandon_ = FILENAME; next } + + _abandon_ == FILENAME { + if (FNR == 1) + _abandon_ = "" + else + next + } + + The `nextfile' function has not changed. It sets `_abandon_' equal +to the current file name and then executes a `next' satement. The +`next' statement reads the next record and increments `FNR', so `FNR' +is guaranteed to have a value of at least two. However, if `nextfile' +is called for the last record in the file, then `awk' will close the +current data file and move on to the next one. Upon doing so, +`FILENAME' will be set to the name of the new file, and `FNR' will be +reset to one. If this next file is the same as the previous one, +`_abandon_' will still be equal to `FILENAME'. However, `FNR' will be +equal to one, telling us that this is a new occurrence of the file, and +not the one we were reading when the `nextfile' function was executed. +In that case, `_abandon_' is reset to the empty string, so that further +executions of this rule will fail (until the next time that `nextfile' +is called). + + If `FNR' is not one, then we are still in the original data file, +and the program executes a `next' statement to skip through it. + + An important question to ask at this point is: "Given that the +functionality of `nextfile' can be provided with a library file, why is +it built into `gawk'?" This is an important question. Adding features +for little reason leads to larger, slower programs that are harder to +maintain. + + The answer is that building `nextfile' into `gawk' provides +significant gains in efficiency. If the `nextfile' function is executed +at the beginning of a large data file, `awk' still has to scan the +entire file, splitting it up into records, just to skip over it. The +built-in `nextfile' can simply close the file immediately and proceed +to the next one, saving a lot of time. This is particularly important +in `awk', since `awk' programs are generally I/O bound (i.e. they +spend most of their time doing input and output, instead of performing +computations). + + ---------- Footnotes ---------- + + (1) Some implementations of `awk' do not allow you to execute `next' +from within a function body. Some other work-around will be necessary +if you use such a version. + + +File: gawk.info, Node: Assert Function, Next: Round Function, Prev: Nextfile Function, Up: Library Functions + +Assertions +========== + + When writing large programs, it is often useful to be able to know +that a condition or set of conditions is true. Before proceeding with a +particular computation, you make a statement about what you believe to +be the case. Such a statement is known as an "assertion." The C +language provides an `<assert.h>' header file and corresponding +`assert' macro that the programmer can use to make assertions. If an +assertion fails, the `assert' macro arranges to print a diagnostic +message describing the condition that should have been true but was +not, and then it kills the program. In C, using `assert' looks this: + + #include <assert.h> + + int myfunc(int a, double b) + { + assert(a <= 5 && b >= 17); + ... + } + + If the assertion failed, the program would print a message similar to +this: + + prog.c:5: assertion failed: a <= 5 && b >= 17 + + The ANSI C language makes it possible to turn the condition into a +string for use in printing the diagnostic message. This is not +possible in `awk', so this `assert' function also requires a string +version of the condition that is being tested. + + # assert --- assert that a condition is true. Otherwise exit. + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May, 1993 + + function assert(condition, string) + { + if (! condition) { + printf("%s:%d: assertion failed: %s\n", + FILENAME, FNR, string) > "/dev/stderr" + _assert_exit = 1 + exit 1 + } + } + + END { + if (_assert_exit) + exit 1 + } + + The `assert' function tests the `condition' parameter. If it is +false, it prints a message to standard error, using the `string' +parameter to describe the failed condition. It then sets the variable +`_assert_exit' to one, and executes the `exit' statement. The `exit' +statement jumps to the `END' rule. If the `END' rules finds +`_assert_exit' to be true, then it exits immediately. + + The purpose of the `END' rule with its test is to keep any other +`END' rules from running. When an assertion fails, the program should +exit immediately. If no assertions fail, then `_assert_exit' will +still be false when the `END' rule is run normally, and the rest of the +program's `END' rules will execute. For all of this to work correctly, +`assert.awk' must be the first source file read by `awk'. + + You would use this function in your programs this way: + + function myfunc(a, b) + { + assert(a <= 5 && b >= 17, "a <= 5 && b >= 17") + ... + } + +If the assertion failed, you would see a message like this: + + mydata:1357: assertion failed: a <= 5 && b >= 17 + + There is a problem with this version of `assert', that it may not be +possible to work around. An `END' rule is automatically added to the +program calling `assert'. Normally, if a program consists of just a +`BEGIN' rule, the input files and/or standard input are not read. +However, now that the program has an `END' rule, `awk' will attempt to +read the input data files, or standard input (*note Startup and Cleanup +Actions: Using BEGIN/END.), most likely causing the program to hang, +waiting for input. + + +File: gawk.info, Node: Round Function, Next: Ordinal Functions, Prev: Assert Function, Up: Library Functions + +Rounding Numbers +================ + + The way `printf' and `sprintf' (*note Using `printf' Statements for +Fancier Printing: Printf.) do rounding will often depend upon the +system's C `sprintf' subroutine. On many machines, `sprintf' rounding +is "unbiased," which means it doesn't always round a trailing `.5' up, +contrary to naive expectations. In unbiased rounding, `.5' rounds to +even, rather than always up, so 1.5 rounds to 2 but 4.5 rounds to 4. +The result is that if you are using a format that does rounding (e.g., +`"%.0f"') you should check what your system does. The following +function does traditional rounding; it might be useful if your awk's +`printf' does unbiased rounding. + + # round --- do normal rounding + # + # Arnold Robbins, arnold@gnu.ai.mit.edu, August, 1996 + # Public Domain + + function round(x, ival, aval, fraction) + { + ival = int(x) # integer part, int() truncates + + # see if fractional part + if (ival == x) # no fraction + return x + + if (x < 0) { + aval = -x # absolute value + ival = int(aval) + fraction = aval - ival + if (fraction >= .5) + return int(x) - 1 # -2.5 --> -3 + else + return int(x) # -2.3 --> -2 + } else { + fraction = x - ival + if (fraction >= .5) + return ival + 1 + else + return ival + } + } + + # test harness + { print $0, round($0) } + + +File: gawk.info, Node: Ordinal Functions, Next: Join Function, Prev: Round Function, Up: Library Functions + +Translating Between Characters and Numbers +========================================== + + One commercial implementation of `awk' supplies a built-in function, +`ord', which takes a character and returns the numeric value for that +character in the machine's character set. If the string passed to +`ord' has more than one character, only the first one is used. + + The inverse of this function is `chr' (from the function of the same +name in Pascal), which takes a number and returns the corresponding +character. + + Both functions can be written very nicely in `awk'; there is no real +reason to build them into the `awk' interpreter. + + # ord.awk --- do ord and chr + # + # Global identifiers: + # _ord_: numerical values indexed by characters + # _ord_init: function to initialize _ord_ + # + # Arnold Robbins + # arnold@gnu.ai.mit.edu + # Public Domain + # 16 January, 1992 + # 20 July, 1992, revised + + BEGIN { _ord_init() } + + function _ord_init( low, high, i, t) + { + low = sprintf("%c", 7) # BEL is ascii 7 + if (low == "\a") { # regular ascii + low = 0 + high = 127 + } else if (sprintf("%c", 128 + 7) == "\a") { + # ascii, mark parity + low = 128 + high = 255 + } else { # ebcdic(!) + low = 0 + high = 255 + } + + for (i = low; i <= high; i++) { + t = sprintf("%c", i) + _ord_[t] = i + } + } + + Some explanation of the numbers used by `chr' is worthwhile. The +most prominent character set in use today is ASCII. Although an +eight-bit byte can hold 256 distinct values (from zero to 255), ASCII +only defines characters that use the values from zero to 127.(1) At +least one computer manufacturer that we know of uses ASCII, but with +mark parity, meaning that the leftmost bit in the byte is always one. +What this means is that on those systems, characters have numeric +values from 128 to 255. Finally, large mainframe systems use the +EBCDIC character set, which uses all 256 values. While there are other +character sets in use on some older systems, they are not really worth +worrying about. + + function ord(str, c) + { + # only first character is of interest + c = substr(str, 1, 1) + return _ord_[c] + } + + function chr(c) + { + # force c to be numeric by adding 0 + return sprintf("%c", c + 0) + } + + #### test code #### + # BEGIN \ + # { + # for (;;) { + # printf("enter a character: ") + # if (getline var <= 0) + # break + # printf("ord(%s) = %d\n", var, ord(var)) + # } + # } + + An obvious improvement to these functions would be to move the code +for the `_ord_init' function into the body of the `BEGIN' rule. It was +written this way initially for ease of development. + + There is a "test program" in a `BEGIN' rule, for testing the +function. It is commented out for production use. + + ---------- Footnotes ---------- + + (1) ASCII has been extended in many countries to use the values from +128 to 255 for country-specific characters. If your system uses these +extensions, you can simplify `_ord_init' to simply loop from zero to +255. + + +File: gawk.info, Node: Join Function, Next: Mktime Function, Prev: Ordinal Functions, Up: Library Functions + +Merging an Array Into a String +============================== + + When doing string processing, it is often useful to be able to join +all the strings in an array into one long string. The following +function, `join', accomplishes this task. It is used later in several +of the application programs (*note Practical `awk' Programs: Sample +Programs.). + + Good function design is important; this function needs to be +general, but it should also have a reasonable default behavior. It is +called with an array and the beginning and ending indices of the +elements in the array to be merged. This assumes that the array +indices are numeric--a reasonable assumption since the array was likely +created with `split' (*note Built-in Functions for String Manipulation: +String Functions.). + + # join.awk --- join an array into a string + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + function join(array, start, end, sep, result, i) + { + if (sep == "") + sep = " " + else if (sep == SUBSEP) # magic value + sep = "" + result = array[start] + for (i = start + 1; i <= end; i++) + result = result sep array[i] + return result + } + + An optional additional argument is the separator to use when joining +the strings back together. If the caller supplies a non-empty value, +`join' uses it. If it is not supplied, it will have a null value. In +this case, `join' uses a single blank as a default separator for the +strings. If the value is equal to `SUBSEP', then `join' joins the +strings with no separator between them. `SUBSEP' serves as a "magic" +value to indicate that there should be no separation between the +component strings. + + It would be nice if `awk' had an assignment operator for +concatenation. The lack of an explicit operator for concatenation +makes string operations more difficult than they really need to be. + + +File: gawk.info, Node: Mktime Function, Next: Gettimeofday Function, Prev: Join Function, Up: Library Functions + +Turning Dates Into Timestamps +============================= + + The `systime' function built in to `gawk' returns the current time +of day as a timestamp in "seconds since the Epoch." This timestamp can +be converted into a printable date of almost infinitely variable format +using the built-in `strftime' function. (For more information on +`systime' and `strftime', *note Functions for Dealing with Time Stamps: +Time Functions..) + + An interesting but difficult problem is to convert a readable +representation of a date back into a timestamp. The ANSI C library +provides a `mktime' function that does the basic job, converting a +canonical representation of a date into a timestamp. + + It would appear at first glance that `gawk' would have to supply a +`mktime' built-in function that was simply a "hook" to the C language +version. In fact though, `mktime' can be implemented entirely in `awk'. + + Here is a version of `mktime' for `awk'. It takes a simple +representation of the date and time, and converts it into a timestamp. + + The code is presented here intermixed with explanatory prose. In +*Note Extracting Programs from Texinfo Source Files: Extract Program, +you will see how the Texinfo source file for this Info file can be +processed to extract the code into a single source file. + + The program begins with a descriptive comment and a `BEGIN' rule +that initializes a table `_tm_months'. This table is a two-dimensional +array that has the lengths of the months. The first index is zero for +regular years, and one for leap years. The values are the same for all +the months in both kinds of years, except for February; thus the use of +multiple assignment. + + # mktime.awk --- convert a canonical date representation + # into a timestamp + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + BEGIN \ + { + # Initialize table of month lengths + _tm_months[0,1] = _tm_months[1,1] = 31 + _tm_months[0,2] = 28; _tm_months[1,2] = 29 + _tm_months[0,3] = _tm_months[1,3] = 31 + _tm_months[0,4] = _tm_months[1,4] = 30 + _tm_months[0,5] = _tm_months[1,5] = 31 + _tm_months[0,6] = _tm_months[1,6] = 30 + _tm_months[0,7] = _tm_months[1,7] = 31 + _tm_months[0,8] = _tm_months[1,8] = 31 + _tm_months[0,9] = _tm_months[1,9] = 30 + _tm_months[0,10] = _tm_months[1,10] = 31 + _tm_months[0,11] = _tm_months[1,11] = 30 + _tm_months[0,12] = _tm_months[1,12] = 31 + } + + The benefit of merging multiple `BEGIN' rules (*note The `BEGIN' and +`END' Special Patterns: BEGIN/END.) is particularly clear when writing +library files. Functions in library files can cleanly initialize their +own private data and also provide clean-up actions in private `END' +rules. + + The next function is a simple one that computes whether a given year +is or is not a leap year. If a year is evenly divisible by four, but +not evenly divisible by 100, or if it is evenly divisible by 400, then +it is a leap year. Thus, 1904 was a leap year, 1900 was not, but 2000 +will be. + + # decide if a year is a leap year + function _tm_isleap(year, ret) + { + ret = (year % 4 == 0 && year % 100 != 0) || + (year % 400 == 0) + + return ret + } + + This function is only used a few times in this file, and its +computation could have been written "in-line" (at the point where it's +used). Making it a separate function made the original development +easier, and also avoids the possibility of typing errors when +duplicating the code in multiple places. + + The next function is more interesting. It does most of the work of +generating a timestamp, which is converting a date and time into some +number of seconds since the Epoch. The caller passes an array (rather +imaginatively named `a') containing six values: the year including +century, the month as a number between one and 12, the day of the +month, the hour as a number between zero and 23, the minute in the +hour, and the seconds within the minute. + + The function uses several local variables to precompute the number of +seconds in an hour, seconds in a day, and seconds in a year. Often, +similar C code simply writes out the expression in-line, expecting the +compiler to do "constant folding". E.g., most C compilers would turn +`60 * 60' into `3600' at compile time, instead of recomputing it every +time at run time. Precomputing these values makes the function more +efficient. + + # convert a date into seconds + function _tm_addup(a, total, yearsecs, daysecs, + hoursecs, i, j) + { + hoursecs = 60 * 60 + daysecs = 24 * hoursecs + yearsecs = 365 * daysecs + + total = (a[1] - 1970) * yearsecs + + # extra day for leap years + for (i = 1970; i < a[1]; i++) + if (_tm_isleap(i)) + total += daysecs + + j = _tm_isleap(a[1]) + for (i = 1; i < a[2]; i++) + total += _tm_months[j, i] * daysecs + + total += (a[3] - 1) * daysecs + total += a[4] * hoursecs + total += a[5] * 60 + total += a[6] + + return total + } + + The function starts with a first approximation of all the seconds +between Midnight, January 1, 1970,(1) and the beginning of the current +year. It then goes through all those years, and for every leap year, +adds an additional day's worth of seconds. + + The variable `j' holds either one or zero, if the current year is or +is not a leap year. For every month in the current year prior to the +current month, it adds the number of seconds in the month, using the +appropriate entry in the `_tm_months' array. + + Finally, it adds in the seconds for the number of days prior to the +current day, and the number of hours, minutes, and seconds in the +current day. + + The result is a count of seconds since January 1, 1970. This value +is not yet what is needed though. The reason why is described shortly. + + The main `mktime' function takes a single character string argument. +This string is a representation of a date and time in a "canonical" +(fixed) form. This string should be `"YEAR MONTH DAY HOUR MINUTE +SECOND"'. + + # mktime --- convert a date into seconds, + # compensate for time zone + + function mktime(str, res1, res2, a, b, i, j, t, diff) + { + i = split(str, a, " ") # don't rely on FS + + if (i != 6) + return -1 + + # force numeric + for (j in a) + a[j] += 0 + + # validate + if (a[1] < 1970 || + a[2] < 1 || a[2] > 12 || + a[3] < 1 || a[3] > 31 || + a[4] < 0 || a[4] > 23 || + a[5] < 0 || a[5] > 59 || + a[6] < 0 || a[6] > 60 ) + return -1 + + res1 = _tm_addup(a) + t = strftime("%Y %m %d %H %M %S", res1) + + if (_tm_debug) + printf("(%s) -> (%s)\n", str, t) > "/dev/stderr" + + split(t, b, " ") + res2 = _tm_addup(b) + + diff = res1 - res2 + + if (_tm_debug) + printf("diff = %d seconds\n", diff) > "/dev/stderr" + + res1 += diff + + return res1 + } + + The function first splits the string into an array, using spaces and +tabs as separators. If there are not six elements in the array, it +returns an error, signaled as the value -1. Next, it forces each +element of the array to be numeric, by adding zero to it. The +following `if' statement then makes sure that each element is within an +allowable range. (This checking could be extended further, e.g., to +make sure that the day of the month is within the correct range for the +particular month supplied.) All of this is essentially preliminary +set-up and error checking. + + Recall that `_tm_addup' generated a value in seconds since Midnight, +January 1, 1970. This value is not directly usable as the result we +want, _since the calculation does not account for the local timezone_. +In other words, the value represents the count in seconds since the +Epoch, but only for UTC (Universal Coordinated Time). If the local +timezone is east or west of UTC, then some number of hours should be +either added to, or subtracted from the resulting timestamp. + + For example, 6:23 p.m. in Atlanta, Georgia (USA), is normally five +hours west of (behind) UTC. It is only four hours behind UTC if +daylight savings time is in effect. If you are calling `mktime' in +Atlanta, with the argument `"1993 5 23 18 23 12"', the result from +`_tm_addup' will be for 6:23 p.m. UTC, which is only 2:23 p.m. in +Atlanta. It is necessary to add another four hours worth of seconds to +the result. + + How can `mktime' determine how far away it is from UTC? This is +surprisingly easy. The returned timestamp represents the time passed to +`mktime' _as UTC_. This timestamp can be fed back to `strftime', which +will format it as a _local_ time; i.e. as if it already had the UTC +difference added in to it. This is done by giving +`"%Y %m %d %H %M %S"' to `strftime' as the format argument. It returns +the computed timestamp in the original string format. The result +represents a time that accounts for the UTC difference. When the new +time is converted back to a timestamp, the difference between the two +timestamps is the difference (in seconds) between the local timezone +and UTC. This difference is then added back to the original result. +An example demonstrating this is presented below. + + Finally, there is a "main" program for testing the function. + + BEGIN { + if (_tm_test) { + printf "Enter date as yyyy mm dd hh mm ss: " + getline _tm_test_date + + t = mktime(_tm_test_date) + r = strftime("%Y %m %d %H %M %S", t) + printf "Got back (%s)\n", r + } + } + + The entire program uses two variables that can be set on the command +line to control debugging output and to enable the test in the final +`BEGIN' rule. Here is the result of a test run. (Note that debugging +output is to standard error, and test output is to standard output.) + + $ gawk -f mktime.awk -v _tm_test=1 -v _tm_debug=1 + -| Enter date as yyyy mm dd hh mm ss: 1993 5 23 15 35 10 + error--> (1993 5 23 15 35 10) -> (1993 05 23 11 35 10) + error--> diff = 14400 seconds + -| Got back (1993 05 23 15 35 10) + + The time entered was 3:35 p.m. (15:35 on a 24-hour clock), on May +23, 1993. The first line of debugging output shows the resulting time +as UTC--four hours ahead of the local time zone. The second line shows +that the difference is 14400 seconds, which is four hours. (The +difference is only four hours, since daylight savings time is in effect +during May.) The final line of test output shows that the timezone +compensation algorithm works; the returned time is the same as the +entered time. + + This program does not solve the general problem of turning an +arbitrary date representation into a timestamp. That problem is very +involved. However, the `mktime' function provides a foundation upon +which to build. Other software can convert month names into numeric +months, and AM/PM times into 24-hour clocks, to generate the +"canonical" format that `mktime' requires. + + ---------- Footnotes ---------- + + (1) This is the Epoch on POSIX systems. It may be different on +other systems. + + +File: gawk.info, Node: Gettimeofday Function, Next: Filetrans Function, Prev: Mktime Function, Up: Library Functions + +Managing the Time of Day +======================== + + The `systime' and `strftime' functions described in *Note Functions +for Dealing with Time Stamps: Time Functions, provide the minimum +functionality necessary for dealing with the time of day in human +readable form. While `strftime' is extensive, the control formats are +not necessarily easy to remember or intuitively obvious when reading a +program. + + The following function, `gettimeofday', populates a user-supplied +array with pre-formatted time information. It returns a string with +the current time formatted in the same way as the `date' utility. + + # gettimeofday --- get the time of day in a usable format + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain, May 1993 + # + # Returns a string in the format of output of date(1) + # Populates the array argument time with individual values: + # time["second"] -- seconds (0 - 59) + # time["minute"] -- minutes (0 - 59) + # time["hour"] -- hours (0 - 23) + # time["althour"] -- hours (0 - 12) + # time["monthday"] -- day of month (1 - 31) + # time["month"] -- month of year (1 - 12) + # time["monthname"] -- name of the month + # time["shortmonth"] -- short name of the month + # time["year"] -- year within century (0 - 99) + # time["fullyear"] -- year with century (19xx or 20xx) + # time["weekday"] -- day of week (Sunday = 0) + # time["altweekday"] -- day of week (Monday = 0) + # time["weeknum"] -- week number, Sunday first day + # time["altweeknum"] -- week number, Monday first day + # time["dayname"] -- name of weekday + # time["shortdayname"] -- short name of weekday + # time["yearday"] -- day of year (0 - 365) + # time["timezone"] -- abbreviation of timezone name + # time["ampm"] -- AM or PM designation + + function gettimeofday(time, ret, now, i) + { + # get time once, avoids unnecessary system calls + now = systime() + + # return date(1)-style output + ret = strftime("%a %b %d %H:%M:%S %Z %Y", now) + + # clear out target array + for (i in time) + delete time[i] + + # fill in values, force numeric values to be + # numeric by adding 0 + time["second"] = strftime("%S", now) + 0 + time["minute"] = strftime("%M", now) + 0 + time["hour"] = strftime("%H", now) + 0 + time["althour"] = strftime("%I", now) + 0 + time["monthday"] = strftime("%d", now) + 0 + time["month"] = strftime("%m", now) + 0 + time["monthname"] = strftime("%B", now) + time["shortmonth"] = strftime("%b", now) + time["year"] = strftime("%y", now) + 0 + time["fullyear"] = strftime("%Y", now) + 0 + time["weekday"] = strftime("%w", now) + 0 + time["altweekday"] = strftime("%u", now) + 0 + time["dayname"] = strftime("%A", now) + time["shortdayname"] = strftime("%a", now) + time["yearday"] = strftime("%j", now) + 0 + time["timezone"] = strftime("%Z", now) + time["ampm"] = strftime("%p", now) + time["weeknum"] = strftime("%U", now) + 0 + time["altweeknum"] = strftime("%W", now) + 0 + + return ret + } + + The string indices are easier to use and read than the various +formats required by `strftime'. The `alarm' program presented in *Note +An Alarm Clock Program: Alarm Program, uses this function. + + The `gettimeofday' function is presented above as it was written. A +more general design for this function would have allowed the user to +supply an optional timestamp value that would have been used instead of +the current time. + + +File: gawk.info, Node: Filetrans Function, Next: Getopt Function, Prev: Gettimeofday Function, Up: Library Functions + +Noting Data File Boundaries +=========================== + + The `BEGIN' and `END' rules are each executed exactly once, at the +beginning and end respectively of your `awk' program (*note The `BEGIN' +and `END' Special Patterns: BEGIN/END.). We (the `gawk' authors) once +had a user who mistakenly thought that the `BEGIN' rule was executed at +the beginning of each data file and the `END' rule was executed at the +end of each data file. When informed that this was not the case, the +user requested that we add new special patterns to `gawk', named +`BEGIN_FILE' and `END_FILE', that would have the desired behavior. He +even supplied us the code to do so. + + However, after a little thought, I came up with the following +library program. It arranges to call two user-supplied functions, +`beginfile' and `endfile', at the beginning and end of each data file. +Besides solving the problem in only nine(!) lines of code, it does so +_portably_; this will work with any implementation of `awk'. + + # transfile.awk + # + # Give the user a hook for filename transitions + # + # The user must supply functions beginfile() and endfile() + # that each take the name of the file being started or + # finished, respectively. + # + # Arnold Robbins, arnold@gnu.ai.mit.edu, January 1992 + # Public Domain + + FILENAME != _oldfilename \ + { + if (_oldfilename != "") + endfile(_oldfilename) + _oldfilename = FILENAME + beginfile(FILENAME) + } + + END { endfile(FILENAME) } + + This file must be loaded before the user's "main" program, so that +the rule it supplies will be executed first. + + This rule relies on `awk''s `FILENAME' variable that automatically +changes for each new data file. The current file name is saved in a +private variable, `_oldfilename'. If `FILENAME' does not equal +`_oldfilename', then a new data file is being processed, and it is +necessary to call `endfile' for the old file. Since `endfile' should +only be called if a file has been processed, the program first checks +to make sure that `_oldfilename' is not the null string. The program +then assigns the current file name to `_oldfilename', and calls +`beginfile' for the file. Since, like all `awk' variables, +`_oldfilename' will be initialized to the null string, this rule +executes correctly even for the first data file. + + The program also supplies an `END' rule, to do the final processing +for the last file. Since this `END' rule comes before any `END' rules +supplied in the "main" program, `endfile' will be called first. Once +again the value of multiple `BEGIN' and `END' rules should be clear. + + This version has same problem as the first version of `nextfile' +(*note Implementing `nextfile' as a Function: Nextfile Function.). If +the same data file occurs twice in a row on command line, then +`endfile' and `beginfile' will not be executed at the end of the first +pass and at the beginning of the second pass. This version solves the +problem. + + # ftrans.awk --- handle data file transitions + # + # user supplies beginfile() and endfile() functions + # + # Arnold Robbins, arnold@gnu.ai.mit.edu. November 1992 + # Public Domain + + FNR == 1 { + if (_filename_ != "") + endfile(_filename_) + _filename_ = FILENAME + beginfile(FILENAME) + } + + END { endfile(_filename_) } + + In *Note Counting Things: Wc Program, you will see how this library +function can be used, and how it simplifies writing the main program. + + +File: gawk.info, Node: Getopt Function, Next: Passwd Functions, Prev: Filetrans Function, Up: Library Functions + +Processing Command Line Options +=============================== + + Most utilities on POSIX compatible systems take options or +"switches" on the command line that can be used to change the way a +program behaves. `awk' is an example of such a program (*note Command +Line Options: Options.). Often, options take "arguments", data that +the program needs to correctly obey the command line option. For +example, `awk''s `-F' option requires a string to use as the field +separator. The first occurrence on the command line of either `--' or a +string that does not begin with `-' ends the options. + + Most Unix systems provide a C function named `getopt' for processing +command line arguments. The programmer provides a string describing +the one letter options. If an option requires an argument, it is +followed in the string with a colon. `getopt' is also passed the count +and values of the command line arguments, and is called in a loop. +`getopt' processes the command line arguments for option letters. Each +time around the loop, it returns a single character representing the +next option letter that it found, or `?' if it found an invalid option. +When it returns -1, there are no options left on the command line. + + When using `getopt', options that do not take arguments can be +grouped together. Furthermore, options that take arguments require +that the argument be present. The argument can immediately follow the +option letter, or it can be a separate command line argument. + + Given a hypothetical program that takes three command line options, +`-a', `-b', and `-c', and `-b' requires an argument, all of the +following are valid ways of invoking the program: + + prog -a -b foo -c data1 data2 data3 + prog -ac -bfoo -- data1 data2 data3 + prog -acbfoo data1 data2 data3 + + Notice that when the argument is grouped with its option, the rest of +the command line argument is considered to be the option's argument. +In the above example, `-acbfoo' indicates that all of the `-a', `-b', +and `-c' options were supplied, and that `foo' is the argument to the +`-b' option. + + `getopt' provides four external variables that the programmer can +use. + +`optind' + The index in the argument value array (`argv') where the first + non-option command line argument can be found. + +`optarg' + The string value of the argument to an option. + +`opterr' + Usually `getopt' prints an error message when it finds an invalid + option. Setting `opterr' to zero disables this feature. (An + application might wish to print its own error message.) + +`optopt' + The letter representing the command line option. While not + usually documented, most versions supply this variable. + + The following C fragment shows how `getopt' might process command +line arguments for `awk'. + + int + main(int argc, char *argv[]) + { + ... + /* print our own message */ + opterr = 0; + while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) { + switch (c) { + case 'f': /* file */ + ... + break; + case 'F': /* field separator */ + ... + break; + case 'v': /* variable assignment */ + ... + break; + case 'W': /* extension */ + ... + break; + case '?': + default: + usage(); + break; + } + } + ... + } + + As a side point, `gawk' actually uses the GNU `getopt_long' function +to process both normal and GNU-style long options (*note Command Line +Options: Options.). + + The abstraction provided by `getopt' is very useful, and would be +quite handy in `awk' programs as well. Here is an `awk' version of +`getopt'. This function highlights one of the greatest weaknesses in +`awk', which is that it is very poor at manipulating single characters. +Repeated calls to `substr' are necessary for accessing individual +characters (*note Built-in Functions for String Manipulation: String +Functions.). + + The discussion walks through the code a bit at a time. + + # getopt --- do C library getopt(3) function in awk + # + # arnold@gnu.ai.mit.edu + # Public domain + # + # Initial version: March, 1991 + # Revised: May, 1993 + + # External variables: + # Optind -- index of ARGV for first non-option argument + # Optarg -- string value of argument to current option + # Opterr -- if non-zero, print our own diagnostic + # Optopt -- current option letter + + # Returns + # -1 at end of options + # ? for unrecognized option + # <c> a character representing the current option + + # Private Data + # _opti index in multi-flag option, e.g., -abc + + The function starts out with some documentation: who wrote the code, +and when it was revised, followed by a list of the global variables it +uses, what the return values are and what they mean, and any global +variables that are "private" to this library function. Such +documentation is essential for any program, and particularly for +library functions. + + function getopt(argc, argv, options, optl, thisopt, i) + { + optl = length(options) + if (optl == 0) # no options given + return -1 + + if (argv[Optind] == "--") { # all done + Optind++ + _opti = 0 + return -1 + } else if (argv[Optind] !~ /^-[^: \t\n\f\r\v\b]/) { + _opti = 0 + return -1 + } + + The function first checks that it was indeed called with a string of +options (the `options' parameter). If `options' has a zero length, +`getopt' immediately returns -1. + + The next thing to check for is the end of the options. A `--' ends +the command line options, as does any command line argument that does +not begin with a `-'. `Optind' is used to step through the array of +command line arguments; it retains its value across calls to `getopt', +since it is a global variable. + + The regexp used, `/^-[^: \t\n\f\r\v\b]/', is perhaps a bit of +overkill; it checks for a `-' followed by anything that is not +whitespace and not a colon. If the current command line argument does +not match this pattern, it is not an option, and it ends option +processing. + + if (_opti == 0) + _opti = 2 + thisopt = substr(argv[Optind], _opti, 1) + Optopt = thisopt + i = index(options, thisopt) + if (i == 0) { + if (Opterr) + printf("%c -- invalid option\n", + thisopt) > "/dev/stderr" + if (_opti >= length(argv[Optind])) { + Optind++ + _opti = 0 + } else + _opti++ + return "?" + } + + The `_opti' variable tracks the position in the current command line +argument (`argv[Optind]'). In the case that multiple options were +grouped together with one `-' (e.g., `-abx'), it is necessary to return +them to the user one at a time. + + If `_opti' is equal to zero, it is set to two, the index in the +string of the next character to look at (we skip the `-', which is at +position one). The variable `thisopt' holds the character, obtained +with `substr'. It is saved in `Optopt' for the main program to use. + + If `thisopt' is not in the `options' string, then it is an invalid +option. If `Opterr' is non-zero, `getopt' prints an error message on +the standard error that is similar to the message from the C version of +`getopt'. + + Since the option is invalid, it is necessary to skip it and move on +to the next option character. If `_opti' is greater than or equal to +the length of the current command line argument, then it is necessary +to move on to the next one, so `Optind' is incremented and `_opti' is +reset to zero. Otherwise, `Optind' is left alone and `_opti' is merely +incremented. + + In any case, since the option was invalid, `getopt' returns `?'. +The main program can examine `Optopt' if it needs to know what the +invalid option letter actually was. + + if (substr(options, i + 1, 1) == ":") { + # get option argument + if (length(substr(argv[Optind], _opti + 1)) > 0) + Optarg = substr(argv[Optind], _opti + 1) + else + Optarg = argv[++Optind] + _opti = 0 + } else + Optarg = "" + + If the option requires an argument, the option letter is followed by +a colon in the `options' string. If there are remaining characters in +the current command line argument (`argv[Optind]'), then the rest of +that string is assigned to `Optarg'. Otherwise, the next command line +argument is used (`-xFOO' vs. `-x FOO'). In either case, `_opti' is +reset to zero, since there are no more characters left to examine in +the current command line argument. + + if (_opti == 0 || _opti >= length(argv[Optind])) { + Optind++ + _opti = 0 + } else + _opti++ + return thisopt + } + + Finally, if `_opti' is either zero or greater than the length of the +current command line argument, it means this element in `argv' is +through being processed, so `Optind' is incremented to point to the +next element in `argv'. If neither condition is true, then only +`_opti' is incremented, so that the next option letter can be processed +on the next call to `getopt'. + + BEGIN { + Opterr = 1 # default is to diagnose + Optind = 1 # skip ARGV[0] + + # test program + if (_getopt_test) { + while ((_go_c = getopt(ARGC, ARGV, "ab:cd")) != -1) + printf("c = <%c>, optarg = <%s>\n", + _go_c, Optarg) + printf("non-option arguments:\n") + for (; Optind < ARGC; Optind++) + printf("\tARGV[%d] = <%s>\n", + Optind, ARGV[Optind]) + } + } + + The `BEGIN' rule initializes both `Opterr' and `Optind' to one. +`Opterr' is set to one, since the default behavior is for `getopt' to +print a diagnostic message upon seeing an invalid option. `Optind' is +set to one, since there's no reason to look at the program name, which +is in `ARGV[0]'. + + The rest of the `BEGIN' rule is a simple test program. Here is the +result of two sample runs of the test program. + + $ awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x + -| c = <a>, optarg = <> + -| c = <c>, optarg = <> + -| c = <b>, optarg = <ARG> + -| non-option arguments: + -| ARGV[3] = <bax> + -| ARGV[4] = <-x> + + $ awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc + -| c = <a>, optarg = <> + error--> x -- invalid option + -| c = <?>, optarg = <> + -| non-option arguments: + -| ARGV[4] = <xyz> + -| ARGV[5] = <abc> + + The first `--' terminates the arguments to `awk', so that it does +not try to interpret the `-a' etc. as its own options. + + Several of the sample programs presented in *Note Practical `awk' +Programs: Sample Programs, use `getopt' to process their arguments. + + +File: gawk.info, Node: Passwd Functions, Next: Group Functions, Prev: Getopt Function, Up: Library Functions + +Reading the User Database +========================= + + The `/dev/user' special file (*note Special File Names in `gawk': +Special Files.) provides access to the current user's real and +effective user and group id numbers, and if available, the user's +supplementary group set. However, since these are numbers, they do not +provide very useful information to the average user. There needs to be +some way to find the user information associated with the user and +group numbers. This section presents a suite of functions for +retrieving information from the user database. *Note Reading the Group +Database: Group Functions, for a similar suite that retrieves +information from the group database. + + The POSIX standard does not define the file where user information is +kept. Instead, it provides the `<pwd.h>' header file and several C +language subroutines for obtaining user information. The primary +function is `getpwent', for "get password entry." The "password" comes +from the original user database file, `/etc/passwd', which kept user +information, along with the encrypted passwords (hence the name). + + While an `awk' program could simply read `/etc/passwd' directly (the +format is well known), because of the way password files are handled on +networked systems, this file may not contain complete information about +the system's set of users. + + To be sure of being able to produce a readable, complete version of +the user database, it is necessary to write a small C program that +calls `getpwent'. `getpwent' is defined to return a pointer to a +`struct passwd'. Each time it is called, it returns the next entry in +the database. When there are no more entries, it returns `NULL', the +null pointer. When this happens, the C program should call `endpwent' +to close the database. Here is `pwcat', a C program that "cats" the +password database. + + /* + * pwcat.c + * + * Generate a printable version of the password database + * + * Arnold Robbins + * arnold@gnu.ai.mit.edu + * May 1993 + * Public Domain + */ + + #include <stdio.h> + #include <pwd.h> + + int + main(argc, argv) + int argc; + char **argv; + { + struct passwd *p; + + while ((p = getpwent()) != NULL) + printf("%s:%s:%d:%d:%s:%s:%s\n", + p->pw_name, p->pw_passwd, p->pw_uid, + p->pw_gid, p->pw_gecos, p->pw_dir, p->pw_shell); + + endpwent(); + exit(0); + } + + If you don't understand C, don't worry about it. The output from +`pwcat' is the user database, in the traditional `/etc/passwd' format +of colon-separated fields. The fields are: + +Login name + The user's login name. + +Encrypted password + The user's encrypted password. This may not be available on some + systems. + +User-ID + The user's numeric user-id number. + +Group-ID + The user's numeric group-id number. + +Full name + The user's full name, and perhaps other information associated + with the user. + +Home directory + The user's login, or "home" directory (familiar to shell + programmers as `$HOME'). + +Login shell + The program that will be run when the user logs in. This is + usually a shell, such as Bash (the Gnu Bourne-Again shell). + + Here are a few lines representative of `pwcat''s output. + + $ pwcat + -| root:3Ov02d5VaUPB6:0:1:Operator:/:/bin/sh + -| nobody:*:65534:65534::/: + -| daemon:*:1:1::/: + -| sys:*:2:2::/:/bin/csh + -| bin:*:3:3::/bin: + -| arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh + -| miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh + -| andy:abcca2:113:10:Andy Jacobs:/home/andy:/bin/sh + ... + + With that introduction, here is a group of functions for getting user +information. There are several functions here, corresponding to the C +functions of the same name. + + # passwd.awk --- access password file information + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + BEGIN { + # tailor this to suit your system + _pw_awklib = "/usr/local/libexec/awk/" + } + + function _pw_init( oldfs, oldrs, olddol0, pwcat) + { + if (_pw_inited) + return + oldfs = FS + oldrs = RS + olddol0 = $0 + FS = ":" + RS = "\n" + pwcat = _pw_awklib "pwcat" + while ((pwcat | getline) > 0) { + _pw_byname[$1] = $0 + _pw_byuid[$3] = $0 + _pw_bycount[++_pw_total] = $0 + } + close(pwcat) + _pw_count = 0 + _pw_inited = 1 + FS = oldfs + RS = oldrs + $0 = olddol0 + } + + The `BEGIN' rule sets a private variable to the directory where +`pwcat' is stored. Since it is used to help out an `awk' library +routine, we have chosen to put it in `/usr/local/libexec/awk'. You +might want it to be in a different directory on your system. + + The function `_pw_init' keeps three copies of the user information +in three associative arrays. The arrays are indexed by user name +(`_pw_byname'), by user-id number (`_pw_byuid'), and by order of +occurrence (`_pw_bycount'). + + The variable `_pw_inited' is used for efficiency; `_pw_init' only +needs to be called once. + + Since this function uses `getline' to read information from `pwcat', +it first saves the values of `FS', `RS', and `$0'. Doing so is +necessary, since these functions could be called from anywhere within a +user's program, and the user may have his or her own values for `FS' +and `RS'. + + The main part of the function uses a loop to read database lines, +split the line into fields, and then store the line into each array as +necessary. When the loop is done, `_pw_init' cleans up by closing the +pipeline, setting `_pw_inited' to one, and restoring `FS', `RS', and +`$0'. The use of `_pw_count' will be explained below. + + function getpwnam(name) + { + _pw_init() + if (name in _pw_byname) + return _pw_byname[name] + return "" + } + + The `getpwnam' function takes a user name as a string argument. If +that user is in the database, it returns the appropriate line. +Otherwise it returns the null string. + + function getpwuid(uid) + { + _pw_init() + if (uid in _pw_byuid) + return _pw_byuid[uid] + return "" + } + + Similarly, the `getpwuid' function takes a user-id number argument. +If that user number is in the database, it returns the appropriate +line. Otherwise it returns the null string. + + function getpwent() + { + _pw_init() + if (_pw_count < _pw_total) + return _pw_bycount[++_pw_count] + return "" + } + + The `getpwent' function simply steps through the database, one entry +at a time. It uses `_pw_count' to track its current position in the +`_pw_bycount' array. + + function endpwent() + { + _pw_count = 0 + } + + The `endpwent' function resets `_pw_count' to zero, so that +subsequent calls to `getpwent' will start over again. + + A conscious design decision in this suite is that each subroutine +calls `_pw_init' to initialize the database arrays. The overhead of +running a separate process to generate the user database, and the I/O +to scan it, will only be incurred if the user's main program actually +calls one of these functions. If this library file is loaded along +with a user's program, but none of the routines are ever called, then +there is no extra run-time overhead. (The alternative would be to move +the body of `_pw_init' into a `BEGIN' rule, which would always run +`pwcat'. This simplifies the code but runs an extra process that may +never be needed.) + + In turn, calling `_pw_init' is not too expensive, since the +`_pw_inited' variable keeps the program from reading the data more than +once. If you are worried about squeezing every last cycle out of your +`awk' program, the check of `_pw_inited' could be moved out of +`_pw_init' and duplicated in all the other functions. In practice, +this is not necessary, since most `awk' programs are I/O bound, and it +would clutter up the code. + + The `id' program in *Note Printing Out User Information: Id Program, +uses these functions. + + +File: gawk.info, Node: Group Functions, Next: Library Names, Prev: Passwd Functions, Up: Library Functions + +Reading the Group Database +========================== + + Much of the discussion presented in *Note Reading the User Database: +Passwd Functions, applies to the group database as well. Although +there has traditionally been a well known file, `/etc/group', in a well +known format, the POSIX standard only provides a set of C library +routines (`<grp.h>' and `getgrent') for accessing the information. +Even though this file may exist, it likely does not have complete +information. Therefore, as with the user database, it is necessary to +have a small C program that generates the group database as its output. + + Here is `grcat', a C program that "cats" the group database. + + /* + * grcat.c + * + * Generate a printable version of the group database + * + * Arnold Robbins, arnold@gnu.ai.mit.edu + * May 1993 + * Public Domain + */ + + #include <stdio.h> + #include <grp.h> + + int + main(argc, argv) + int argc; + char **argv; + { + struct group *g; + int i; + + while ((g = getgrent()) != NULL) { + printf("%s:%s:%d:", g->gr_name, g->gr_passwd, + g->gr_gid); + for (i = 0; g->gr_mem[i] != NULL; i++) { + printf("%s", g->gr_mem[i]); + if (g->gr_mem[i+1] != NULL) + putchar(','); + } + putchar('\n'); + } + endgrent(); + exit(0); + } + + Each line in the group database represent one group. The fields are +separated with colons, and represent the following information. + +Group Name + The name of the group. + +Group Password + The encrypted group password. In practice, this field is never + used. It is usually empty, or set to `*'. + +Group ID Number + The numeric group-id number. This number should be unique within + the file. + +Group Member List + A comma-separated list of user names. These users are members of + the group. Most Unix systems allow users to be members of several + groups simultaneously. If your system does, then reading + `/dev/user' will return those group-id numbers in `$5' through + `$NF'. (Note that `/dev/user' is a `gawk' extension; *note + Special File Names in `gawk': Special Files..) + + Here is what running `grcat' might produce: + + $ grcat + -| wheel:*:0:arnold + -| nogroup:*:65534: + -| daemon:*:1: + -| kmem:*:2: + -| staff:*:10:arnold,miriam,andy + -| other:*:20: + ... + + Here are the functions for obtaining information from the group +database. There are several, modeled after the C library functions of +the same names. + + # group.awk --- functions for dealing with the group file + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + BEGIN \ + { + # Change to suit your system + _gr_awklib = "/usr/local/libexec/awk/" + } + + function _gr_init( oldfs, oldrs, olddol0, grcat, n, a, i) + { + if (_gr_inited) + return + + oldfs = FS + oldrs = RS + olddol0 = $0 + FS = ":" + RS = "\n" + + grcat = _gr_awklib "grcat" + while ((grcat | getline) > 0) { + if ($1 in _gr_byname) + _gr_byname[$1] = _gr_byname[$1] "," $4 + else + _gr_byname[$1] = $0 + if ($3 in _gr_bygid) + _gr_bygid[$3] = _gr_bygid[$3] "," $4 + else + _gr_bygid[$3] = $0 + + n = split($4, a, "[ \t]*,[ \t]*") + for (i = 1; i <= n; i++) + if (a[i] in _gr_groupsbyuser) + _gr_groupsbyuser[a[i]] = \ + _gr_groupsbyuser[a[i]] " " $1 + else + _gr_groupsbyuser[a[i]] = $1 + + _gr_bycount[++_gr_count] = $0 + } + close(grcat) + _gr_count = 0 + _gr_inited++ + FS = oldfs + RS = oldrs + $0 = olddol0 + } + + The `BEGIN' rule sets a private variable to the directory where +`grcat' is stored. Since it is used to help out an `awk' library +routine, we have chosen to put it in `/usr/local/libexec/awk'. You +might want it to be in a different directory on your system. + + These routines follow the same general outline as the user database +routines (*note Reading the User Database: Passwd Functions.). The +`_gr_inited' variable is used to ensure that the database is scanned no +more than once. The `_gr_init' function first saves `FS', `RS', and +`$0', and then sets `FS' and `RS' to the correct values for scanning +the group information. + + The group information is stored is several associative arrays. The +arrays are indexed by group name (`_gr_byname'), by group-id number +(`_gr_bygid'), and by position in the database (`_gr_bycount'). There +is an additional array indexed by user name (`_gr_groupsbyuser'), that +is a space separated list of groups that each user belongs to. + + Unlike the user database, it is possible to have multiple records in +the database for the same group. This is common when a group has a +large number of members. Such a pair of entries might look like: + + tvpeople:*:101:johny,jay,arsenio + tvpeople:*:101:david,conan,tom,joan + + For this reason, `_gr_init' looks to see if a group name or group-id +number has already been seen. If it has, then the user names are +simply concatenated onto the previous list of users. (There is +actually a subtle problem with the code presented above. Suppose that +the first time there were no names. This code adds the names with a +leading comma. It also doesn't check that there is a `$4'.) + + Finally, `_gr_init' closes the pipeline to `grcat', restores `FS', +`RS', and `$0', initializes `_gr_count' to zero (it is used later), and +makes `_gr_inited' non-zero. + + function getgrnam(group) + { + _gr_init() + if (group in _gr_byname) + return _gr_byname[group] + return "" + } + + The `getgrnam' function takes a group name as its argument, and if +that group exists, it is returned. Otherwise, `getgrnam' returns the +null string. + + function getgrgid(gid) + { + _gr_init() + if (gid in _gr_bygid) + return _gr_bygid[gid] + return "" + } + + The `getgrgid' function is similar, it takes a numeric group-id, and +looks up the information associated with that group-id. + + function getgruser(user) + { + _gr_init() + if (user in _gr_groupsbyuser) + return _gr_groupsbyuser[user] + return "" + } + + The `getgruser' function does not have a C counterpart. It takes a +user name, and returns the list of groups that have the user as a +member. + + function getgrent() + { + _gr_init() + if (++gr_count in _gr_bycount) + return _gr_bycount[_gr_count] + return "" + } + + The `getgrent' function steps through the database one entry at a +time. It uses `_gr_count' to track its position in the list. + + function endgrent() + { + _gr_count = 0 + } + + `endgrent' resets `_gr_count' to zero so that `getgrent' can start +over again. + + As with the user database routines, each function calls `_gr_init' to +initialize the arrays. Doing so only incurs the extra overhead of +running `grcat' if these functions are used (as opposed to moving the +body of `_gr_init' into a `BEGIN' rule). + + Most of the work is in scanning the database and building the various +associative arrays. The functions that the user calls are themselves +very simple, relying on `awk''s associative arrays to do work. + + The `id' program in *Note Printing Out User Information: Id Program, +uses these functions. + + +File: gawk.info, Node: Library Names, Prev: Group Functions, Up: Library Functions + +Naming Library Function Global Variables +======================================== + + Due to the way the `awk' language evolved, variables are either +"global" (usable by the entire program), or "local" (usable just by a +specific function). There is no intermediate state analogous to +`static' variables in C. + + Library functions often need to have global variables that they can +use to preserve state information between calls to the function. For +example, `getopt''s variable `_opti' (*note Processing Command Line +Options: Getopt Function.), and the `_tm_months' array used by `mktime' +(*note Turning Dates Into Timestamps: Mktime Function.). Such +variables are called "private", since the only functions that need to +use them are the ones in the library. + + When writing a library function, you should try to choose names for +your private variables so that they will not conflict with any +variables used by either another library function or a user's main +program. For example, a name like `i' or `j' is not a good choice, +since user programs often use variable names like these for their own +purposes. + + The example programs shown in this chapter all start the names of +their private variables with an underscore (`_'). Users generally +don't use leading underscores in their variable names, so this +convention immediately decreases the chances that the variable name +will be accidentally shared with the user's program. + + In addition, several of the library functions use a prefix that helps +indicate what function or set of functions uses the variables. For +example, `_tm_months' in `mktime' (*note Turning Dates Into Timestamps: +Mktime Function.), and `_pw_byname' in the user data base routines +(*note Reading the User Database: Passwd Functions.). This convention +is recommended, since it even further decreases the chance of +inadvertent conflict among variable names. Note that this convention +can be used equally well both for variable names and for private +function names too. + + While I could have re-written all the library routines to use this +convention, I did not do so, in order to show how my own `awk' +programming style has evolved, and to provide some basis for this +discussion. + + As a final note on variable naming, if a function makes global +variables available for use by a main program, it is a good convention +to start that variable's name with a capital letter. For example, +`getopt''s `Opterr' and `Optind' variables (*note Processing Command +Line Options: Getopt Function.). The leading capital letter indicates +that it is global, while the fact that the variable name is not all +capital letters indicates that the variable is not one of `awk''s +built-in variables, like `FS'. + + It is also important that _all_ variables in library functions that +do not need to save state are in fact declared local. If this is not +done, the variable could accidentally be used in the user's program, +leading to bugs that are very difficult to track down. + + function lib_func(x, y, l1, l2) + { + ... + USE VARIABLE some_var # some_var could be local + ... # but is not by oversight + } + + A different convention, common in the Tcl community, is to use a +single associative array to hold the values needed by the library +function(s), or "package." This significantly decreases the number of +actual global names in use. For example, the functions described in +*Note Reading the User Database: Passwd Functions, might have used +`PW_data["inited"]', `PW_data["total"]', `PW_data["count"]' and +`PW_data["awklib"]', instead of `_pw_inited', `_pw_awklib', `_pw_total', +and `_pw_count'. + + The conventions presented in this section are exactly that, +conventions. You are not required to write your programs this way, we +merely recommend that you do so. + + +File: gawk.info, Node: Sample Programs, Next: Language History, Prev: Library Functions, Up: Top + +Practical `awk' Programs +************************ + + This chapter presents a potpourri of `awk' programs for your reading +enjoyment. + + Many of these programs use the library functions presented in *Note +A Library of `awk' Functions: Library Functions. + +* Menu: + +* Clones:: Clones of common utilities. +* Miscellaneous Programs:: Some interesting `awk' programs. + + +File: gawk.info, Node: Clones, Next: Miscellaneous Programs, Prev: Sample Programs, Up: Sample Programs + +Re-inventing Wheels for Fun and Profit +====================================== + + This section presents a number of POSIX utilities that are +implemented in `awk'. Re-inventing these programs in `awk' is often +enjoyable, since the algorithms can be very clearly expressed, and +usually the code is very concise and simple. This is true because +`awk' does so much for you. + + It should be noted that these programs are not necessarily intended +to replace the installed versions on your system. Instead, their +purpose is to illustrate `awk' language programming for "real world" +tasks. + + The programs are presented in alphabetical order. + +* Menu: + +* Cut Program:: The `cut' utility. +* Egrep Program:: The `egrep' utility. +* Id Program:: The `id' utility. +* Split Program:: The `split' utility. +* Tee Program:: The `tee' utility. +* Uniq Program:: The `uniq' utility. +* Wc Program:: The `wc' utility. + + +File: gawk.info, Node: Cut Program, Next: Egrep Program, Prev: Clones, Up: Clones + +Cutting Out Fields and Columns +------------------------------ + + The `cut' utility selects, or "cuts," either characters or fields +from its standard input and sends them to its standard output. `cut' +can cut out either a list of characters, or a list of fields. By +default, fields are separated by tabs, but you may supply a command +line option to change the field "delimiter", i.e. the field separator +character. `cut''s definition of fields is less general than `awk''s. + + A common use of `cut' might be to pull out just the login name of +logged-on users from the output of `who'. For example, the following +pipeline generates a sorted, unique list of the logged on users: + + who | cut -c1-8 | sort | uniq + + The options for `cut' are: + +`-c LIST' + Use LIST as the list of characters to cut out. Items within the + list may be separated by commas, and ranges of characters can be + separated with dashes. The list `1-8,15,22-35' specifies + characters one through eight, 15, and 22 through 35. + +`-f LIST' + Use LIST as the list of fields to cut out. + +`-d DELIM' + Use DELIM as the field separator character instead of the tab + character. + +`-s' + Suppress printing of lines that do not contain the field delimiter. + + The `awk' implementation of `cut' uses the `getopt' library function +(*note Processing Command Line Options: Getopt Function.), and the +`join' library function (*note Merging an Array Into a String: Join +Function.). + + The program begins with a comment describing the options and a +`usage' function which prints out a usage message and exits. `usage' +is called if invalid arguments are supplied. + + # cut.awk --- implement cut in awk + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + # Options: + # -f list Cut fields + # -d c Field delimiter character + # -c list Cut characters + # + # -s Suppress lines without the delimiter character + + function usage( e1, e2) + { + e1 = "usage: cut [-f list] [-d c] [-s] [files...]" + e2 = "usage: cut [-c list] [files...]" + print e1 > "/dev/stderr" + print e2 > "/dev/stderr" + exit 1 + } + +The variables `e1' and `e2' are used so that the function fits nicely +on the screen. + + Next comes a `BEGIN' rule that parses the command line options. It +sets `FS' to a single tab character, since that is `cut''s default +field separator. The output field separator is also set to be the same +as the input field separator. Then `getopt' is used to step through +the command line options. One or the other of the variables +`by_fields' or `by_chars' is set to true, to indicate that processing +should be done by fields or by characters respectively. When cutting +by characters, the output field separator is set to the null string. + + BEGIN \ + { + FS = "\t" # default + OFS = FS + while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) { + if (c == "f") { + by_fields = 1 + fieldlist = Optarg + } else if (c == "c") { + by_chars = 1 + fieldlist = Optarg + OFS = "" + } else if (c == "d") { + if (length(Optarg) > 1) { + printf("Using first character of %s" \ + " for delimiter\n", Optarg) > "/dev/stderr" + Optarg = substr(Optarg, 1, 1) + } + FS = Optarg + OFS = FS + if (FS == " ") # defeat awk semantics + FS = "[ ]" + } else if (c == "s") + suppress++ + else + usage() + } + + for (i = 1; i < Optind; i++) + ARGV[i] = "" + + Special care is taken when the field delimiter is a space. Using +`" "' (a single space) for the value of `FS' is incorrect--`awk' would +separate fields with runs of spaces, tabs and/or newlines, and we want +them to be separated with individual spaces. Also, note that after +`getopt' is through, we have to clear out all the elements of `ARGV' +from one to `Optind', so that `awk' will not try to process the command +line options as file names. + + After dealing with the command line options, the program verifies +that the options make sense. Only one or the other of `-c' and `-f' +should be used, and both require a field list. Then either +`set_fieldlist' or `set_charlist' is called to pull apart the list of +fields or characters. + + if (by_fields && by_chars) + usage() + + if (by_fields == 0 && by_chars == 0) + by_fields = 1 # default + + if (fieldlist == "") { + print "cut: needs list for -c or -f" > "/dev/stderr" + exit 1 + } + + if (by_fields) + set_fieldlist() + else + set_charlist() + } + + Here is `set_fieldlist'. It first splits the field list apart at +the commas, into an array. Then, for each element of the array, it +looks to see if it is actually a range, and if so splits it apart. The +range is verified to make sure the first number is smaller than the +second. Each number in the list is added to the `flist' array, which +simply lists the fields that will be printed. Normal field splitting +is used. The program lets `awk' handle the job of doing the field +splitting. + + function set_fieldlist( n, m, i, j, k, f, g) + { + n = split(fieldlist, f, ",") + j = 1 # index in flist + for (i = 1; i <= n; i++) { + if (index(f[i], "-") != 0) { # a range + m = split(f[i], g, "-") + if (m != 2 || g[1] >= g[2]) { + printf("bad field list: %s\n", + f[i]) > "/dev/stderr" + exit 1 + } + for (k = g[1]; k <= g[2]; k++) + flist[j++] = k + } else + flist[j++] = f[i] + } + nfields = j - 1 + } + + The `set_charlist' function is more complicated than `set_fieldlist'. +The idea here is to use `gawk''s `FIELDWIDTHS' variable (*note Reading +Fixed-width Data: Constant Size.), which describes constant width +input. When using a character list, that is exactly what we have. + + Setting up `FIELDWIDTHS' is more complicated than simply listing the +fields that need to be printed. We have to keep track of the fields to +be printed, and also the intervening characters that have to be skipped. +For example, suppose you wanted characters one through eight, 15, and +22 through 35. You would use `-c 1-8,15,22-35'. The necessary value +for `FIELDWIDTHS' would be `"8 6 1 6 14"'. This gives us five fields, +and what should be printed are `$1', `$3', and `$5'. The intermediate +fields are "filler," stuff in between the desired data. + + `flist' lists the fields to be printed, and `t' tracks the complete +field list, including filler fields. + + function set_charlist( field, i, j, f, g, t, + filler, last, len) + { + field = 1 # count total fields + n = split(fieldlist, f, ",") + j = 1 # index in flist + for (i = 1; i <= n; i++) { + if (index(f[i], "-") != 0) { # range + m = split(f[i], g, "-") + if (m != 2 || g[1] >= g[2]) { + printf("bad character list: %s\n", + f[i]) > "/dev/stderr" + exit 1 + } + len = g[2] - g[1] + 1 + if (g[1] > 1) # compute length of filler + filler = g[1] - last - 1 + else + filler = 0 + if (filler) + t[field++] = filler + t[field++] = len # length of field + last = g[2] + flist[j++] = field - 1 + } else { + if (f[i] > 1) + filler = f[i] - last - 1 + else + filler = 0 + if (filler) + t[field++] = filler + t[field++] = 1 + last = f[i] + flist[j++] = field - 1 + } + } + FIELDWIDTHS = join(t, 1, field - 1) + nfields = j - 1 + } + + Here is the rule that actually processes the data. If the `-s' +option was given, then `suppress' will be true. The first `if' +statement makes sure that the input record does have the field +separator. If `cut' is processing fields, `suppress' is true, and the +field separator character is not in the record, then the record is +skipped. + + If the record is valid, then at this point, `gawk' has split the data +into fields, either using the character in `FS' or using fixed-length +fields and `FIELDWIDTHS'. The loop goes through the list of fields +that should be printed. If the corresponding field has data in it, it +is printed. If the next field also has data, then the separator +character is written out in between the fields. + + { + if (by_fields && suppress && $0 !~ FS) + next + + for (i = 1; i <= nfields; i++) { + if ($flist[i] != "") { + printf "%s", $flist[i] + if (i < nfields && $flist[i+1] != "") + printf "%s", OFS + } + } + print "" + } + + This version of `cut' relies on `gawk''s `FIELDWIDTHS' variable to +do the character-based cutting. While it would be possible in other +`awk' implementations to use `substr' (*note Built-in Functions for +String Manipulation: String Functions.), it would also be extremely +painful to do so. The `FIELDWIDTHS' variable supplies an elegant +solution to the problem of picking the input line apart by characters. + + +File: gawk.info, Node: Egrep Program, Next: Id Program, Prev: Cut Program, Up: Clones + +Searching for Regular Expressions in Files +------------------------------------------ + + The `egrep' utility searches files for patterns. It uses regular +expressions that are almost identical to those available in `awk' +(*note Regular Expression Constants: Regexp Constants.). It is used +this way: + + egrep [ OPTIONS ] 'PATTERN' FILES ... + + The PATTERN is a regexp. In typical usage, the regexp is quoted to +prevent the shell from expanding any of the special characters as file +name wildcards. Normally, `egrep' prints the lines that matched. If +multiple file names are provided on the command line, each output line +is preceded by the name of the file and a colon. + + The options are: + +`-c' + Print out a count of the lines that matched the pattern, instead + of the lines themselves. + +`-s' + Be silent. No output is produced, and the exit value indicates + whether or not the pattern was matched. + +`-v' + Invert the sense of the test. `egrep' prints the lines that do + _not_ match the pattern, and exits successfully if the pattern was + not matched. + +`-i' + Ignore case distinctions in both the pattern and the input data. + +`-l' + Only print the names of the files that matched, not the lines that + matched. + +`-e PATTERN' + Use PATTERN as the regexp to match. The purpose of the `-e' + option is to allow patterns that start with a `-'. + + This version uses the `getopt' library function (*note Processing +Command Line Options: Getopt Function.), and the file transition +library program (*note Noting Data File Boundaries: Filetrans +Function.). + + The program begins with a descriptive comment, and then a `BEGIN' +rule that processes the command line arguments with `getopt'. The `-i' +(ignore case) option is particularly easy with `gawk'; we just use the +`IGNORECASE' built in variable (*note Built-in Variables::). + + # egrep.awk --- simulate egrep in awk + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + # Options: + # -c count of lines + # -s silent - use exit value + # -v invert test, success if no match + # -i ignore case + # -l print filenames only + # -e argument is pattern + + BEGIN { + while ((c = getopt(ARGC, ARGV, "ce:svil")) != -1) { + if (c == "c") + count_only++ + else if (c == "s") + no_print++ + else if (c == "v") + invert++ + else if (c == "i") + IGNORECASE = 1 + else if (c == "l") + filenames_only++ + else if (c == "e") + pattern = Optarg + else + usage() + } + + Next comes the code that handles the `egrep' specific behavior. If no +pattern was supplied with `-e', the first non-option on the command +line is used. The `awk' command line arguments up to `ARGV[Optind]' +are cleared, so that `awk' won't try to process them as files. If no +files were specified, the standard input is used, and if multiple files +were specified, we make sure to note this so that the file names can +precede the matched lines in the output. + + The last two lines are commented out, since they are not needed in +`gawk'. They should be uncommented if you have to use another version +of `awk'. + + if (pattern == "") + pattern = ARGV[Optind++] + + for (i = 1; i < Optind; i++) + ARGV[i] = "" + if (Optind >= ARGC) { + ARGV[1] = "-" + ARGC = 2 + } else if (ARGC - Optind > 1) + do_filenames++ + + # if (IGNORECASE) + # pattern = tolower(pattern) + } + + The next set of lines should be uncommented if you are not using +`gawk'. This rule translates all the characters in the input line into +lower-case if the `-i' option was specified. The rule is commented out +since it is not necessary with `gawk'. + + #{ + # if (IGNORECASE) + # $0 = tolower($0) + #} + + The `beginfile' function is called by the rule in `ftrans.awk' when +each new file is processed. In this case, it is very simple; all it +does is initialize a variable `fcount' to zero. `fcount' tracks how +many lines in the current file matched the pattern. + + function beginfile(junk) + { + fcount = 0 + } + + The `endfile' function is called after each file has been processed. +It is used only when the user wants a count of the number of lines that +matched. `no_print' will be true only if the exit status is desired. +`count_only' will be true if line counts are desired. `egrep' will +therefore only print line counts if printing and counting are enabled. +The output format must be adjusted depending upon the number of files +to be processed. Finally, `fcount' is added to `total', so that we +know how many lines altogether matched the pattern. + + function endfile(file) + { + if (! no_print && count_only) + if (do_filenames) + print file ":" fcount + else + print fcount + + total += fcount + } + + This rule does most of the work of matching lines. The variable +`matches' will be true if the line matched the pattern. If the user +wants lines that did not match, the sense of the `matches' is inverted +using the `!' operator. `fcount' is incremented with the value of +`matches', which will be either one or zero, depending upon a +successful or unsuccessful match. If the line did not match, the +`next' statement just moves on to the next record. + + There are several optimizations for performance in the following few +lines of code. If the user only wants exit status (`no_print' is true), +and we don't have to count lines, then it is enough to know that one +line in this file matched, and we can skip on to the next file with +`nextfile'. Along similar lines, if we are only printing file names, +and we don't need to count lines, we can print the file name, and then +skip to the next file with `nextfile'. + + Finally, each line is printed, with a leading filename and colon if +necessary. + + { + matches = ($0 ~ pattern) + if (invert) + matches = ! matches + + fcount += matches # 1 or 0 + + if (! matches) + next + + if (no_print && ! count_only) + nextfile + + if (filenames_only && ! count_only) { + print FILENAME + nextfile + } + + if (do_filenames && ! count_only) + print FILENAME ":" $0 + else if (! count_only) + print + } + + The `END' rule takes care of producing the correct exit status. If +there were no matches, the exit status is one, otherwise it is zero. + + END \ + { + if (total == 0) + exit 1 + exit 0 + } + + The `usage' function prints a usage message in case of invalid +options and then exits. + + function usage( e) + { + e = "Usage: egrep [-csvil] [-e pat] [files ...]" + print e > "/dev/stderr" + exit 1 + } + + The variable `e' is used so that the function fits nicely on the +printed page. + + Just a note on programming style. You may have noticed that the `END' +rule uses backslash continuation, with the open brace on a line by +itself. This is so that it more closely resembles the way functions +are written. Many of the examples use this style. You can decide for +yourself if you like writing your `BEGIN' and `END' rules this way, or +not. + + +File: gawk.info, Node: Id Program, Next: Split Program, Prev: Egrep Program, Up: Clones + +Printing Out User Information +----------------------------- + + The `id' utility lists a user's real and effective user-id numbers, +real and effective group-id numbers, and the user's group set, if any. +`id' will only print the effective user-id and group-id if they are +different from the real ones. If possible, `id' will also supply the +corresponding user and group names. The output might look like this: + + $ id + -| uid=2076(arnold) gid=10(staff) groups=10(staff),4(tty) + + This information is exactly what is provided by `gawk''s `/dev/user' +special file (*note Special File Names in `gawk': Special Files.). +However, the `id' utility provides a more palatable output than just a +string of numbers. + + Here is a simple version of `id' written in `awk'. It uses the user +database library functions (*note Reading the User Database: Passwd +Functions.), and the group database library functions (*note Reading +the Group Database: Group Functions.). + + The program is fairly straightforward. All the work is done in the +`BEGIN' rule. The user and group id numbers are obtained from +`/dev/user'. If there is no support for `/dev/user', the program gives +up. + + The code is repetitive. The entry in the user database for the real +user-id number is split into parts at the `:'. The name is the first +field. Similar code is used for the effective user-id number, and the +group numbers. + + # id.awk --- implement id in awk + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + # output is: + # uid=12(foo) euid=34(bar) gid=3(baz) \ + # egid=5(blat) groups=9(nine),2(two),1(one) + + BEGIN \ + { + if ((getline < "/dev/user") < 0) { + err = "id: no /dev/user support - cannot run" + print err > "/dev/stderr" + exit 1 + } + close("/dev/user") + + uid = $1 + euid = $2 + gid = $3 + egid = $4 + + printf("uid=%d", uid) + pw = getpwuid(uid) + if (pw != "") { + split(pw, a, ":") + printf("(%s)", a[1]) + } + + if (euid != uid) { + printf(" euid=%d", euid) + pw = getpwuid(euid) + if (pw != "") { + split(pw, a, ":") + printf("(%s)", a[1]) + } + } + + printf(" gid=%d", gid) + pw = getgrgid(gid) + if (pw != "") { + split(pw, a, ":") + printf("(%s)", a[1]) + } + + if (egid != gid) { + printf(" egid=%d", egid) + pw = getgrgid(egid) + if (pw != "") { + split(pw, a, ":") + printf("(%s)", a[1]) + } + } + + if (NF > 4) { + printf(" groups="); + for (i = 5; i <= NF; i++) { + printf("%d", $i) + pw = getgrgid($i) + if (pw != "") { + split(pw, a, ":") + printf("(%s)", a[1]) + } + if (i < NF) + printf(",") + } + } + print "" + } + + +File: gawk.info, Node: Split Program, Next: Tee Program, Prev: Id Program, Up: Clones + +Splitting a Large File Into Pieces +---------------------------------- + + The `split' program splits large text files into smaller pieces. By +default, the output files are named `xaa', `xab', and so on. Each file +has 1000 lines in it, with the likely exception of the last file. To +change the number of lines in each file, you supply a number on the +command line preceded with a minus, e.g., `-500' for files with 500 +lines in them instead of 1000. To change the name of the output files +to something like `myfileaa', `myfileab', and so on, you supply an +additional argument that specifies the filename. + + Here is a version of `split' in `awk'. It uses the `ord' and `chr' +functions presented in *Note Translating Between Characters and +Numbers: Ordinal Functions. + + The program first sets its defaults, and then tests to make sure +there are not too many arguments. It then looks at each argument in +turn. The first argument could be a minus followed by a number. If it +is, this happens to look like a negative number, so it is made +positive, and that is the count of lines. The data file name is +skipped over, and the final argument is used as the prefix for the +output file names. + + # split.awk --- do split in awk + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + # usage: split [-num] [file] [outname] + + BEGIN \ + { + outfile = "x" # default + count = 1000 + if (ARGC > 4) + usage() + + i = 1 + if (ARGV[i] ~ /^-[0-9]+$/) { + count = -ARGV[i] + ARGV[i] = "" + i++ + } + # test argv in case reading from stdin instead of file + if (i in ARGV) + i++ # skip data file name + if (i in ARGV) { + outfile = ARGV[i] + ARGV[i] = "" + } + + s1 = s2 = "a" + out = (outfile s1 s2) + } + + The next rule does most of the work. `tcount' (temporary count) +tracks how many lines have been printed to the output file so far. If +it is greater than `count', it is time to close the current file and +start a new one. `s1' and `s2' track the current suffixes for the file +name. If they are both `z', the file is just too big. Otherwise, `s1' +moves to the next letter in the alphabet and `s2' starts over again at +`a'. + + { + if (++tcount > count) { + close(out) + if (s2 == "z") { + if (s1 == "z") { + printf("split: %s is too large to split\n", \ + FILENAME) > "/dev/stderr" + exit 1 + } + s1 = chr(ord(s1) + 1) + s2 = "a" + } else + s2 = chr(ord(s2) + 1) + out = (outfile s1 s2) + tcount = 1 + } + print > out + } + + The `usage' function simply prints an error message and exits. + + function usage( e) + { + e = "usage: split [-num] [file] [outname]" + print e > "/dev/stderr" + exit 1 + } + +The variable `e' is used so that the function fits nicely on the screen. + + This program is a bit sloppy; it relies on `awk' to close the last +file for it automatically, instead of doing it in an `END' rule. + + +File: gawk.info, Node: Tee Program, Next: Uniq Program, Prev: Split Program, Up: Clones + +Duplicating Output Into Multiple Files +-------------------------------------- + + The `tee' program is known as a "pipe fitting." `tee' copies its +standard input to its standard output, and also duplicates it to the +files named on the command line. Its usage is: + + tee [-a] file ... + + The `-a' option tells `tee' to append to the named files, instead of +truncating them and starting over. + + The `BEGIN' rule first makes a copy of all the command line +arguments, into an array named `copy'. `ARGV[0]' is not copied, since +it is not needed. `tee' cannot use `ARGV' directly, since `awk' will +attempt to process each file named in `ARGV' as input data. + + If the first argument is `-a', then the flag variable `append' is +set to true, and both `ARGV[1]' and `copy[1]' are deleted. If `ARGC' is +less than two, then no file names were supplied, and `tee' prints a +usage message and exits. Finally, `awk' is forced to read the standard +input by setting `ARGV[1]' to `"-"', and `ARGC' to two. + + # tee.awk --- tee in awk + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + # Revised December 1995 + + BEGIN \ + { + for (i = 1; i < ARGC; i++) + copy[i] = ARGV[i] + + if (ARGV[1] == "-a") { + append = 1 + delete ARGV[1] + delete copy[1] + ARGC-- + } + if (ARGC < 2) { + print "usage: tee [-a] file ..." > "/dev/stderr" + exit 1 + } + ARGV[1] = "-" + ARGC = 2 + } + + The single rule does all the work. Since there is no pattern, it is +executed for each line of input. The body of the rule simply prints the +line into each file on the command line, and then to the standard +output. + + { + # moving the if outside the loop makes it run faster + if (append) + for (i in copy) + print >> copy[i] + else + for (i in copy) + print > copy[i] + print + } + + It would have been possible to code the loop this way: + + for (i in copy) + if (append) + print >> copy[i] + else + print > copy[i] + +This is more concise, but it is also less efficient. The `if' is +tested for each record and for each output file. By duplicating the +loop body, the `if' is only tested once for each input record. If +there are N input records and M input files, the first method only +executes N `if' statements, while the second would execute N`*'M `if' +statements. + + Finally, the `END' rule cleans up, by closing all the output files. + + END \ + { + for (i in copy) + close(copy[i]) + } + + +File: gawk.info, Node: Uniq Program, Next: Wc Program, Prev: Tee Program, Up: Clones + +Printing Non-duplicated Lines of Text +------------------------------------- + + The `uniq' utility reads sorted lines of data on its standard input, +and (by default) removes duplicate lines. In other words, only unique +lines are printed, hence the name. `uniq' has a number of options. The +usage is: + + uniq [-udc [-N]] [+N] [ INPUT FILE [ OUTPUT FILE ]] + + The option meanings are: + +`-d' + Only print repeated lines. + +`-u' + Only print non-repeated lines. + +`-c' + Count lines. This option overrides `-d' and `-u'. Both repeated + and non-repeated lines are counted. + +`-N' + Skip N fields before comparing lines. The definition of fields is + similar to `awk''s default: non-whitespace characters separated by + runs of spaces and/or tabs. + +`+N' + Skip N characters before comparing lines. Any fields specified + with `-N' are skipped first. + +`INPUT FILE' + Data is read from the input file named on the command line, + instead of from the standard input. + +`OUTPUT FILE' + The generated output is sent to the named output file, instead of + to the standard output. + + Normally `uniq' behaves as if both the `-d' and `-u' options had +been provided. + + Here is an `awk' implementation of `uniq'. It uses the `getopt' +library function (*note Processing Command Line Options: Getopt +Function.), and the `join' library function (*note Merging an Array +Into a String: Join Function.). + + The program begins with a `usage' function and then a brief outline +of the options and their meanings in a comment. + + The `BEGIN' rule deals with the command line arguments and options. +It uses a trick to get `getopt' to handle options of the form `-25', +treating such an option as the option letter `2' with an argument of +`5'. If indeed two or more digits were supplied (`Optarg' looks like a +number), `Optarg' is concatenated with the option digit, and then +result is added to zero to make it into a number. If there is only one +digit in the option, then `Optarg' is not needed, and `Optind' must be +decremented so that `getopt' will process it next time. This code is +admittedly a bit tricky. + + If no options were supplied, then the default is taken, to print both +repeated and non-repeated lines. The output file, if provided, is +assigned to `outputfile'. Earlier, `outputfile' was initialized to the +standard output, `/dev/stdout'. + + # uniq.awk --- do uniq in awk + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + function usage( e) + { + e = "Usage: uniq [-udc [-n]] [+n] [ in [ out ]]" + print e > "/dev/stderr" + exit 1 + } + + # -c count lines. overrides -d and -u + # -d only repeated lines + # -u only non-repeated lines + # -n skip n fields + # +n skip n characters, skip fields first + + BEGIN \ + { + count = 1 + outputfile = "/dev/stdout" + opts = "udc0:1:2:3:4:5:6:7:8:9:" + while ((c = getopt(ARGC, ARGV, opts)) != -1) { + if (c == "u") + non_repeated_only++ + else if (c == "d") + repeated_only++ + else if (c == "c") + do_count++ + else if (index("0123456789", c) != 0) { + # getopt requires args to options + # this messes us up for things like -5 + if (Optarg ~ /^[0-9]+$/) + fcount = (c Optarg) + 0 + else { + fcount = c + 0 + Optind-- + } + } else + usage() + } + + if (ARGV[Optind] ~ /^\+[0-9]+$/) { + charcount = substr(ARGV[Optind], 2) + 0 + Optind++ + } + + for (i = 1; i < Optind; i++) + ARGV[i] = "" + + if (repeated_only == 0 && non_repeated_only == 0) + repeated_only = non_repeated_only = 1 + + if (ARGC - Optind == 2) { + outputfile = ARGV[ARGC - 1] + ARGV[ARGC - 1] = "" + } + } + + The following function, `are_equal', compares the current line, +`$0', to the previous line, `last'. It handles skipping fields and +characters. + + If no field count and no character count were specified, `are_equal' +simply returns one or zero depending upon the result of a simple string +comparison of `last' and `$0'. Otherwise, things get more complicated. + + If fields have to be skipped, each line is broken into an array using +`split' (*note Built-in Functions for String Manipulation: String +Functions.), and then the desired fields are joined back into a line +using `join'. The joined lines are stored in `clast' and `cline'. If +no fields are skipped, `clast' and `cline' are set to `last' and `$0' +respectively. + + Finally, if characters are skipped, `substr' is used to strip off the +leading `charcount' characters in `clast' and `cline'. The two strings +are then compared, and `are_equal' returns the result. + + function are_equal( n, m, clast, cline, alast, aline) + { + if (fcount == 0 && charcount == 0) + return (last == $0) + + if (fcount > 0) { + n = split(last, alast) + m = split($0, aline) + clast = join(alast, fcount+1, n) + cline = join(aline, fcount+1, m) + } else { + clast = last + cline = $0 + } + if (charcount) { + clast = substr(clast, charcount + 1) + cline = substr(cline, charcount + 1) + } + + return (clast == cline) + } + + The following two rules are the body of the program. The first one +is executed only for the very first line of data. It sets `last' equal +to `$0', so that subsequent lines of text have something to be compared +to. + + The second rule does the work. The variable `equal' will be one or +zero depending upon the results of `are_equal''s comparison. If `uniq' +is counting repeated lines, then the `count' variable is incremented if +the lines are equal. Otherwise the line is printed and `count' is +reset, since the two lines are not equal. + + If `uniq' is not counting, `count' is incremented if the lines are +equal. Otherwise, if `uniq' is counting repeated lines, and more than +one line has been seen, or if `uniq' is counting non-repeated lines, +and only one line has been seen, then the line is printed, and `count' +is reset. + + Finally, similar logic is used in the `END' rule to print the final +line of input data. + + NR == 1 { + last = $0 + next + } + + { + equal = are_equal() + + if (do_count) { # overrides -d and -u + if (equal) + count++ + else { + printf("%4d %s\n", count, last) > outputfile + last = $0 + count = 1 # reset + } + next + } + + if (equal) + count++ + else { + if ((repeated_only && count > 1) || + (non_repeated_only && count == 1)) + print last > outputfile + last = $0 + count = 1 + } + } + + END { + if (do_count) + printf("%4d %s\n", count, last) > outputfile + else if ((repeated_only && count > 1) || + (non_repeated_only && count == 1)) + print last > outputfile + } + + +File: gawk.info, Node: Wc Program, Prev: Uniq Program, Up: Clones + +Counting Things +--------------- + + The `wc' (word count) utility counts lines, words, and characters in +one or more input files. Its usage is: + + wc [-lwc] [ FILES ... ] + + If no files are specified on the command line, `wc' reads its +standard input. If there are multiple files, it will also print total +counts for all the files. The options and their meanings are: + +`-l' + Only count lines. + +`-w' + Only count words. A "word" is a contiguous sequence of + non-whitespace characters, separated by spaces and/or tabs. + Happily, this is the normal way `awk' separates fields in its + input data. + +`-c' + Only count characters. + + Implementing `wc' in `awk' is particularly elegant, since `awk' does +a lot of the work for us; it splits lines into words (i.e. fields) and +counts them, it counts lines (i.e. records) for us, and it can easily +tell us how long a line is. + + This version uses the `getopt' library function (*note Processing +Command Line Options: Getopt Function.), and the file transition +functions (*note Noting Data File Boundaries: Filetrans Function.). + + This version has one major difference from traditional versions of +`wc'. Our version always prints the counts in the order lines, words, +and characters. Traditional versions note the order of the `-l', `-w', +and `-c' options on the command line, and print the counts in that +order. + + The `BEGIN' rule does the argument processing. The variable +`print_total' will be true if more than one file was named on the +command line. + + # wc.awk --- count lines, words, characters + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + # Options: + # -l only count lines + # -w only count words + # -c only count characters + # + # Default is to count lines, words, characters + + BEGIN { + # let getopt print a message about + # invalid options. we ignore them + while ((c = getopt(ARGC, ARGV, "lwc")) != -1) { + if (c == "l") + do_lines = 1 + else if (c == "w") + do_words = 1 + else if (c == "c") + do_chars = 1 + } + for (i = 1; i < Optind; i++) + ARGV[i] = "" + + # if no options, do all + if (! do_lines && ! do_words && ! do_chars) + do_lines = do_words = do_chars = 1 + + print_total = (ARGC - i > 2) + } + + The `beginfile' function is simple; it just resets the counts of +lines, words, and characters to zero, and saves the current file name in +`fname'. + + The `endfile' function adds the current file's numbers to the running +totals of lines, words, and characters. It then prints out those +numbers for the file that was just read. It relies on `beginfile' to +reset the numbers for the following data file. + + function beginfile(file) + { + chars = lines = words = 0 + fname = FILENAME + } + + function endfile(file) + { + tchars += chars + tlines += lines + twords += words + if (do_lines) + printf "\t%d", lines + if (do_words) + printf "\t%d", words + if (do_chars) + printf "\t%d", chars + printf "\t%s\n", fname + } + + There is one rule that is executed for each line. It adds the length +of the record to `chars'. It has to add one, since the newline +character separating records (the value of `RS') is not part of the +record itself. `lines' is incremented for each line read, and `words' +is incremented by the value of `NF', the number of "words" on this +line.(1) + + Finally, the `END' rule simply prints the totals for all the files. + + # do per line + { + chars += length($0) + 1 # get newline + lines++ + words += NF + } + + END { + if (print_total) { + if (do_lines) + printf "\t%d", tlines + if (do_words) + printf "\t%d", twords + if (do_chars) + printf "\t%d", tchars + print "\ttotal" + } + } + + ---------- Footnotes ---------- + + (1) Examine the code in *Note Noting Data File Boundaries: Filetrans +Function. Why must `wc' use a separate `lines' variable, instead of +using the value of `FNR' in `endfile'? + + +File: gawk.info, Node: Miscellaneous Programs, Prev: Clones, Up: Sample Programs + +A Grab Bag of `awk' Programs +============================ + + This section is a large "grab bag" of miscellaneous programs. We +hope you find them both interesting and enjoyable. + +* Menu: + +* Dupword Program:: Finding duplicated words in a document. +* Alarm Program:: An alarm clock. +* Translate Program:: A program similar to the `tr' utility. +* Labels Program:: Printing mailing labels. +* Word Sorting:: A program to produce a word usage count. +* History Sorting:: Eliminating duplicate entries from a history + file. +* Extract Program:: Pulling out programs from Texinfo source + files. +* Simple Sed:: A Simple Stream Editor. +* Igawk Program:: A wrapper for `awk' that includes files. + + +File: gawk.info, Node: Dupword Program, Next: Alarm Program, Prev: Miscellaneous Programs, Up: Miscellaneous Programs + +Finding Duplicated Words in a Document +-------------------------------------- + + A common error when writing large amounts of prose is to accidentally +duplicate words. Often you will see this in text as something like "the +the program does the following ...." When the text is on-line, often +the duplicated words occur at the end of one line and the beginning of +another, making them very difficult to spot. + + This program, `dupword.awk', scans through a file one line at a time, +and looks for adjacent occurrences of the same word. It also saves the +last word on a line (in the variable `prev') for comparison with the +first word on the next line. + + The first two statements make sure that the line is all lower-case, +so that, for example, "The" and "the" compare equal to each other. The +second statement removes all non-alphanumeric and non-whitespace +characters from the line, so that punctuation does not affect the +comparison either. This sometimes leads to reports of duplicated words +that really are different, but this is unusual. + + # dupword --- find duplicate words in text + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # December 1991 + + { + $0 = tolower($0) + gsub(/[^A-Za-z0-9 \t]/, ""); + if ($1 == prev) + printf("%s:%d: duplicate %s\n", + FILENAME, FNR, $1) + for (i = 2; i <= NF; i++) + if ($i == $(i-1)) + printf("%s:%d: duplicate %s\n", + FILENAME, FNR, $i) + prev = $NF + } + + +File: gawk.info, Node: Alarm Program, Next: Translate Program, Prev: Dupword Program, Up: Miscellaneous Programs + +An Alarm Clock Program +---------------------- + + The following program is a simple "alarm clock" program. You give +it a time of day, and an optional message. At the given time, it +prints the message on the standard output. In addition, you can give it +the number of times to repeat the message, and also a delay between +repetitions. + + This program uses the `gettimeofday' function from *Note Managing +the Time of Day: Gettimeofday Function. + + All the work is done in the `BEGIN' rule. The first part is argument +checking and setting of defaults; the delay, the count, and the message +to print. If the user supplied a message, but it does not contain the +ASCII BEL character (known as the "alert" character, `\a'), then it is +added to the message. (On many systems, printing the ASCII BEL +generates some sort of audible alert. Thus, when the alarm goes off, +the system calls attention to itself, in case the user is not looking +at their computer or terminal.) + + # alarm --- set an alarm + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + # usage: alarm time [ "message" [ count [ delay ] ] ] + + BEGIN \ + { + # Initial argument sanity checking + usage1 = "usage: alarm time ['message' [count [delay]]]" + usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1]) + + if (ARGC < 2) { + print usage > "/dev/stderr" + exit 1 + } else if (ARGC == 5) { + delay = ARGV[4] + 0 + count = ARGV[3] + 0 + message = ARGV[2] + } else if (ARGC == 4) { + count = ARGV[3] + 0 + message = ARGV[2] + } else if (ARGC == 3) { + message = ARGV[2] + } else if (ARGV[1] !~ /[0-9]?[0-9]:[0-9][0-9]/) { + print usage1 > "/dev/stderr" + print usage2 > "/dev/stderr" + exit 1 + } + + # set defaults for once we reach the desired time + if (delay == 0) + delay = 180 # 3 minutes + if (count == 0) + count = 5 + if (message == "") + message = sprintf("\aIt is now %s!\a", ARGV[1]) + else if (index(message, "\a") == 0) + message = "\a" message "\a" + + The next section of code turns the alarm time into hours and minutes, +and converts it if necessary to a 24-hour clock. Then it turns that +time into a count of the seconds since midnight. Next it turns the +current time into a count of seconds since midnight. The difference +between the two is how long to wait before setting off the alarm. + + # split up dest time + split(ARGV[1], atime, ":") + hour = atime[1] + 0 # force numeric + minute = atime[2] + 0 # force numeric + + # get current broken down time + gettimeofday(now) + + # if time given is 12-hour hours and it's after that + # hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m., + # then add 12 to real hour + if (hour < 12 && now["hour"] > hour) + hour += 12 + + # set target time in seconds since midnight + target = (hour * 60 * 60) + (minute * 60) + + # get current time in seconds since midnight + current = (now["hour"] * 60 * 60) + \ + (now["minute"] * 60) + now["second"] + + # how long to sleep for + naptime = target - current + if (naptime <= 0) { + print "time is in the past!" > "/dev/stderr" + exit 1 + } + + Finally, the program uses the `system' function (*note Built-in +Functions for Input/Output: I/O Functions.) to call the `sleep' +utility. The `sleep' utility simply pauses for the given number of +seconds. If the exit status is not zero, the program assumes that +`sleep' was interrupted, and exits. If `sleep' exited with an OK status +(zero), then the program prints the message in a loop, again using +`sleep' to delay for however many seconds are necessary. + + # zzzzzz..... go away if interrupted + if (system(sprintf("sleep %d", naptime)) != 0) + exit 1 + + # time to notify! + command = sprintf("sleep %d", delay) + for (i = 1; i <= count; i++) { + print message + # if sleep command interrupted, go away + if (system(command) != 0) + break + } + + exit 0 + } + + +File: gawk.info, Node: Translate Program, Next: Labels Program, Prev: Alarm Program, Up: Miscellaneous Programs + +Transliterating Characters +-------------------------- + + The system `tr' utility transliterates characters. For example, it +is often used to map upper-case letters into lower-case, for further +processing. + + GENERATE DATA | tr '[A-Z]' '[a-z]' | PROCESS DATA ... + + You give `tr' two lists of characters enclosed in square brackets. +Usually, the lists are quoted to keep the shell from attempting to do a +filename expansion.(1) When processing the input, the first character +in the first list is replaced with the first character in the second +list, the second character in the first list is replaced with the +second character in the second list, and so on. If there are more +characters in the "from" list than in the "to" list, the last character +of the "to" list is used for the remaining characters in the "from" +list. + + Some time ago, a user proposed to us that we add a transliteration +function to `gawk'. Being opposed to "creeping featurism," I wrote the +following program to prove that character transliteration could be done +with a user-level function. This program is not as complete as the +system `tr' utility, but it will do most of the job. + + The `translate' program demonstrates one of the few weaknesses of +standard `awk': dealing with individual characters is very painful, +requiring repeated use of the `substr', `index', and `gsub' built-in +functions (*note Built-in Functions for String Manipulation: String +Functions.).(2) + + There are two functions. The first, `stranslate', takes three +arguments. + +`from' + A list of characters to translate from. + +`to' + A list of characters to translate to. + +`target' + The string to do the translation on. + + Associative arrays make the translation part fairly easy. `t_ar' +holds the "to" characters, indexed by the "from" characters. Then a +simple loop goes through `from', one character at a time. For each +character in `from', if the character appears in `target', `gsub' is +used to change it to the corresponding `to' character. + + The `translate' function simply calls `stranslate' using `$0' as the +target. The main program sets two global variables, `FROM' and `TO', +from the command line, and then changes `ARGV' so that `awk' will read +from the standard input. + + Finally, the processing rule simply calls `translate' for each +record. + + # translate --- do tr like stuff + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # August 1989 + + # bugs: does not handle things like: tr A-Z a-z, it has + # to be spelled out. However, if `to' is shorter than `from', + # the last character in `to' is used for the rest of `from'. + + function stranslate(from, to, target, lf, lt, t_ar, i, c) + { + lf = length(from) + lt = length(to) + for (i = 1; i <= lt; i++) + t_ar[substr(from, i, 1)] = substr(to, i, 1) + if (lt < lf) + for (; i <= lf; i++) + t_ar[substr(from, i, 1)] = substr(to, lt, 1) + for (i = 1; i <= lf; i++) { + c = substr(from, i, 1) + if (index(target, c) > 0) + gsub(c, t_ar[c], target) + } + return target + } + + function translate(from, to) + { + return $0 = stranslate(from, to, $0) + } + + # main program + BEGIN { + if (ARGC < 3) { + print "usage: translate from to" > "/dev/stderr" + exit + } + FROM = ARGV[1] + TO = ARGV[2] + ARGC = 2 + ARGV[1] = "-" + } + + { + translate(FROM, TO) + print + } + + While it is possible to do character transliteration in a user-level +function, it is not necessarily efficient, and we started to consider +adding a built-in function. However, shortly after writing this +program, we learned that the System V Release 4 `awk' had added the +`toupper' and `tolower' functions. These functions handle the vast +majority of the cases where character transliteration is necessary, and +so we chose to simply add those functions to `gawk' as well, and then +leave well enough alone. + + An obvious improvement to this program would be to set up the `t_ar' +array only once, in a `BEGIN' rule. However, this assumes that the +"from" and "to" lists will never change throughout the lifetime of the +program. + + ---------- Footnotes ---------- + + (1) On older, non-POSIX systems, `tr' often does not require that +the lists be enclosed in square brackets and quoted. This is a feature. + + (2) This program was written before `gawk' acquired the ability to +split each character in a string into separate array elements. How +might this ability simplify the program? + + +File: gawk.info, Node: Labels Program, Next: Word Sorting, Prev: Translate Program, Up: Miscellaneous Programs + +Printing Mailing Labels +----------------------- + + Here is a "real world"(1) program. This script reads lists of names +and addresses, and generates mailing labels. Each page of labels has +20 labels on it, two across and ten down. The addresses are guaranteed +to be no more than five lines of data. Each address is separated from +the next by a blank line. + + The basic idea is to read 20 labels worth of data. Each line of +each label is stored in the `line' array. The single rule takes care +of filling the `line' array and printing the page when 20 labels have +been read. + + The `BEGIN' rule simply sets `RS' to the empty string, so that `awk' +will split records at blank lines (*note How Input is Split into +Records: Records.). It sets `MAXLINES' to 100, since `MAXLINE' is the +maximum number of lines on the page (20 * 5 = 100). + + Most of the work is done in the `printpage' function. The label +lines are stored sequentially in the `line' array. But they have to be +printed horizontally; `line[1]' next to `line[6]', `line[2]' next to +`line[7]', and so on. Two loops are used to accomplish this. The +outer loop, controlled by `i', steps through every 10 lines of data; +this is each row of labels. The inner loop, controlled by `j', goes +through the lines within the row. As `j' goes from zero to four, `i+j' +is the `j''th line in the row, and `i+j+5' is the entry next to it. +The output ends up looking something like this: + + line 1 line 6 + line 2 line 7 + line 3 line 8 + line 4 line 9 + line 5 line 10 + + As a final note, at lines 21 and 61, an extra blank line is printed, +to keep the output lined up on the labels. This is dependent on the +particular brand of labels in use when the program was written. You +will also note that there are two blank lines at the top and two blank +lines at the bottom. + + The `END' rule arranges to flush the final page of labels; there may +not have been an even multiple of 20 labels in the data. + + # labels.awk + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # June 1992 + + # Program to print labels. Each label is 5 lines of data + # that may have blank lines. The label sheets have 2 + # blank lines at the top and 2 at the bottom. + + BEGIN { RS = "" ; MAXLINES = 100 } + + function printpage( i, j) + { + if (Nlines <= 0) + return + + printf "\n\n" # header + + for (i = 1; i <= Nlines; i += 10) { + if (i == 21 || i == 61) + print "" + for (j = 0; j < 5; j++) { + if (i + j > MAXLINES) + break + printf " %-41s %s\n", line[i+j], line[i+j+5] + } + print "" + } + + printf "\n\n" # footer + + for (i in line) + line[i] = "" + } + + # main rule + { + if (Count >= 20) { + printpage() + Count = 0 + Nlines = 0 + } + n = split($0, a, "\n") + for (i = 1; i <= n; i++) + line[++Nlines] = a[i] + for (; i <= 5; i++) + line[++Nlines] = "" + Count++ + } + + END \ + { + printpage() + } + + ---------- Footnotes ---------- + + (1) "Real world" is defined as "a program actually used to get +something done." + + +File: gawk.info, Node: Word Sorting, Next: History Sorting, Prev: Labels Program, Up: Miscellaneous Programs + +Generating Word Usage Counts +---------------------------- + + The following `awk' program prints the number of occurrences of each +word in its input. It illustrates the associative nature of `awk' +arrays by using strings as subscripts. It also demonstrates the `for X +in ARRAY' construction. Finally, it shows how `awk' can be used in +conjunction with other utility programs to do a useful task of some +complexity with a minimum of effort. Some explanations follow the +program listing. + + awk ' + # Print list of word frequencies + { + for (i = 1; i <= NF; i++) + freq[$i]++ + } + + END { + for (word in freq) + printf "%s\t%d\n", word, freq[word] + }' + + The first thing to notice about this program is that it has two +rules. The first rule, because it has an empty pattern, is executed on +every line of the input. It uses `awk''s field-accessing mechanism +(*note Examining Fields: Fields.) to pick out the individual words from +the line, and the built-in variable `NF' (*note Built-in Variables::) +to know how many fields are available. + + For each input word, an element of the array `freq' is incremented to +reflect that the word has been seen an additional time. + + The second rule, because it has the pattern `END', is not executed +until the input has been exhausted. It prints out the contents of the +`freq' table that has been built up inside the first action. + + This program has several problems that would prevent it from being +useful by itself on real text files: + + * Words are detected using the `awk' convention that fields are + separated by whitespace and that other characters in the input + (except newlines) don't have any special meaning to `awk'. This + means that punctuation characters count as part of words. + + * The `awk' language considers upper- and lower-case characters to be + distinct. Therefore, `bartender' and `Bartender' are not treated + as the same word. This is undesirable since, in normal text, words + are capitalized if they begin sentences, and a frequency analyzer + should not be sensitive to capitalization. + + * The output does not come out in any useful order. You're more + likely to be interested in which words occur most frequently, or + having an alphabetized table of how frequently each word occurs. + + The way to solve these problems is to use some of the more advanced +features of the `awk' language. First, we use `tolower' to remove case +distinctions. Next, we use `gsub' to remove punctuation characters. +Finally, we use the system `sort' utility to process the output of the +`awk' script. Here is the new version of the program: + + # Print list of word frequencies + { + $0 = tolower($0) # remove case distinctions + gsub(/[^a-z0-9_ \t]/, "", $0) # remove punctuation + for (i = 1; i <= NF; i++) + freq[$i]++ + } + + END { + for (word in freq) + printf "%s\t%d\n", word, freq[word] + } + + Assuming we have saved this program in a file named `wordfreq.awk', +and that the data is in `file1', the following pipeline + + awk -f wordfreq.awk file1 | sort +1 -nr + +produces a table of the words appearing in `file1' in order of +decreasing frequency. + + The `awk' program suitably massages the data and produces a word +frequency table, which is not ordered. + + The `awk' script's output is then sorted by the `sort' utility and +printed on the terminal. The options given to `sort' in this example +specify to sort using the second field of each input line (skipping one +field), that the sort keys should be treated as numeric quantities +(otherwise `15' would come before `5'), and that the sorting should be +done in descending (reverse) order. + + We could have even done the `sort' from within the program, by +changing the `END' action to: + + END { + sort = "sort +1 -nr" + for (word in freq) + printf "%s\t%d\n", word, freq[word] | sort + close(sort) + } + + You would have to use this way of sorting on systems that do not +have true pipes. + + See the general operating system documentation for more information +on how to use the `sort' program. + + +File: gawk.info, Node: History Sorting, Next: Extract Program, Prev: Word Sorting, Up: Miscellaneous Programs + +Removing Duplicates from Unsorted Text +-------------------------------------- + + The `uniq' program (*note Printing Non-duplicated Lines of Text: +Uniq Program.), removes duplicate lines from _sorted_ data. + + Suppose, however, you need to remove duplicate lines from a data +file, but that you wish to preserve the order the lines are in? A good +example of this might be a shell history file. The history file keeps +a copy of all the commands you have entered, and it is not unusual to +repeat a command several times in a row. Occasionally you might wish +to compact the history by removing duplicate entries. Yet it is +desirable to maintain the order of the original commands. + + This simple program does the job. It uses two arrays. The `data' +array is indexed by the text of each line. For each line, `data[$0]' +is incremented. + + If a particular line has not been seen before, then `data[$0]' will +be zero. In that case, the text of the line is stored in +`lines[count]'. Each element of `lines' is a unique command, and the +indices of `lines' indicate the order in which those lines were +encountered. The `END' rule simply prints out the lines, in order. + + # histsort.awk --- compact a shell history file + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + # Thanks to Byron Rakitzis for the general idea + { + if (data[$0]++ == 0) + lines[++count] = $0 + } + + END { + for (i = 1; i <= count; i++) + print lines[i] + } + + This program also provides a foundation for generating other useful +information. For example, using the following `print' satement in the +`END' rule would indicate how often a particular command was used. + + print data[lines[i]], lines[i] + + This works because `data[$0]' was incremented each time a line was +seen. + + +File: gawk.info, Node: Extract Program, Next: Simple Sed, Prev: History Sorting, Up: Miscellaneous Programs + +Extracting Programs from Texinfo Source Files +--------------------------------------------- + + The nodes *Note A Library of `awk' Functions: Library Functions, and +*Note Practical `awk' Programs: Sample Programs, are the top level +nodes for a large number of `awk' programs. If you wish to experiment +with these programs, it is tedious to have to type them in by hand. +Here we present a program that can extract parts of a Texinfo input +file into separate files. + + This Info file is written in Texinfo, the GNU project's document +formatting language. A single Texinfo source file can be used to +produce both printed and on-line documentation. The Texinfo language +is described fully, starting with *Note Introduction: (texi)Top. + + For our purposes, it is enough to know three things about Texinfo +input files. + + * The "at" symbol, `@', is special in Texinfo, much like `\' in C or + `awk'. Literal `@' symbols are represented in Texinfo source + files as `@@'. + + * Comments start with either `@c' or `@comment'. The file + extraction program will work by using special comments that start + at the beginning of a line. + + * Example text that should not be split across a page boundary is + bracketed between lines containing `@group' and `@end group' + commands. + + The following program, `extract.awk', reads through a Texinfo source +file, and does two things, based on the special comments. Upon seeing +`@c system ...', it runs a command, by extracting the command text from +the control line and passing it on to the `system' function (*note +Built-in Functions for Input/Output: I/O Functions.). Upon seeing `@c +file FILENAME', each subsequent line is sent to the file FILENAME, +until `@c endfile' is encountered. The rules in `extract.awk' will +match either `@c' or `@comment' by letting the `omment' part be +optional. Lines containing `@group' and `@end group' are simply +removed. `extract.awk' uses the `join' library function (*note Merging +an Array Into a String: Join Function.). + + The example programs in the on-line Texinfo source for `The GNU Awk +User's Guide' (`gawk.texi') have all been bracketed inside `file', and +`endfile' lines. The `gawk' distribution uses a copy of `extract.awk' +to extract the sample programs and install many of them in a standard +directory, where `gawk' can find them. + + `extract.awk' begins by setting `IGNORECASE' to one, so that mixed +upper-case and lower-case letters in the directives won't matter. + + The first rule handles calling `system', checking that a command was +given (`NF' is at least three), and also checking that the command +exited with a zero exit status, signifying OK. + + # extract.awk --- extract files and run programs + # from texinfo files + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # May 1993 + + BEGIN { IGNORECASE = 1 } + + /^@c(omment)?[ \t]+system/ \ + { + if (NF < 3) { + e = (FILENAME ":" FNR) + e = (e ": badly formed `system' line") + print e > "/dev/stderr" + next + } + $1 = "" + $2 = "" + stat = system($0) + if (stat != 0) { + e = (FILENAME ":" FNR) + e = (e ": warning: system returned " stat) + print e > "/dev/stderr" + } + } + +The variable `e' is used so that the function fits nicely on the screen. + + The second rule handles moving data into files. It verifies that a +file name was given in the directive. If the file named is not the +current file, then the current file is closed. This means that an `@c +endfile' was not given for that file. (We should probably print a +diagnostic in this case, although at the moment we do not.) + + The `for' loop does the work. It reads lines using `getline' (*note +Explicit Input with `getline': Getline.). For an unexpected end of +file, it calls the `unexpected_eof' function. If the line is an +"endfile" line, then it breaks out of the loop. If the line is an +`@group' or `@end group' line, then it ignores it, and goes on to the +next line. + + Most of the work is in the following few lines. If the line has no +`@' symbols, it can be printed directly. Otherwise, each leading `@' +must be stripped off. + + To remove the `@' symbols, the line is split into separate elements +of the array `a', using the `split' function (*note Built-in Functions +for String Manipulation: String Functions.). Each element of `a' that +is empty indicates two successive `@' symbols in the original line. +For each two empty elements (`@@' in the original file), we have to add +back in a single `@' symbol. + + When the processing of the array is finished, `join' is called with +the value of `SUBSEP', to rejoin the pieces back into a single line. +That line is then printed to the output file. + + /^@c(omment)?[ \t]+file/ \ + { + if (NF != 3) { + e = (FILENAME ":" FNR ": badly formed `file' line") + print e > "/dev/stderr" + next + } + if ($3 != curfile) { + if (curfile != "") + close(curfile) + curfile = $3 + } + + for (;;) { + if ((getline line) <= 0) + unexpected_eof() + if (line ~ /^@c(omment)?[ \t]+endfile/) + break + else if (line ~ /^@(end[ \t]+)?group/) + continue + if (index(line, "@") == 0) { + print line > curfile + continue + } + n = split(line, a, "@") + # if a[1] == "", means leading @, + # don't add one back in. + for (i = 2; i <= n; i++) { + if (a[i] == "") { # was an @@ + a[i] = "@" + if (a[i+1] == "") + i++ + } + } + print join(a, 1, n, SUBSEP) > curfile + } + } + + An important thing to note is the use of the `>' redirection. +Output done with `>' only opens the file once; it stays open and +subsequent output is appended to the file (*note Redirecting Output of +`print' and `printf': Redirection.). This allows us to easily mix +program text and explanatory prose for the same sample source file (as +has been done here!) without any hassle. The file is only closed when +a new data file name is encountered, or at the end of the input file. + + Finally, the function `unexpected_eof' prints an appropriate error +message and then exits. + + The `END' rule handles the final cleanup, closing the open file. + + function unexpected_eof() + { + printf("%s:%d: unexpected EOF or error\n", \ + FILENAME, FNR) > "/dev/stderr" + exit 1 + } + + END { + if (curfile) + close(curfile) + } + + +File: gawk.info, Node: Simple Sed, Next: Igawk Program, Prev: Extract Program, Up: Miscellaneous Programs + +A Simple Stream Editor +---------------------- + + The `sed' utility is a "stream editor," a program that reads a +stream of data, makes changes to it, and passes the modified data on. +It is often used to make global changes to a large file, or to a stream +of data generated by a pipeline of commands. + + While `sed' is a complicated program in its own right, its most +common use is to perform global substitutions in the middle of a +pipeline: + + command1 < orig.data | sed 's/old/new/g' | command2 > result + + Here, the `s/old/new/g' tells `sed' to look for the regexp `old' on +each input line, and replace it with the text `new', globally (i.e. all +the occurrences on a line). This is similar to `awk''s `gsub' function +(*note Built-in Functions for String Manipulation: String Functions.). + + The following program, `awksed.awk', accepts at least two command +line arguments; the pattern to look for and the text to replace it +with. Any additional arguments are treated as data file names to +process. If none are provided, the standard input is used. + + # awksed.awk --- do s/foo/bar/g using just print + # Thanks to Michael Brennan for the idea + + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # August 1995 + + function usage() + { + print "usage: awksed pat repl [files...]" > "/dev/stderr" + exit 1 + } + + BEGIN { + # validate arguments + if (ARGC < 3) + usage() + + RS = ARGV[1] + ORS = ARGV[2] + + # don't use arguments as files + ARGV[1] = ARGV[2] = "" + } + + # look ma, no hands! + { + if (RT == "") + printf "%s", $0 + else + print + } + + The program relies on `gawk''s ability to have `RS' be a regexp and +on the setting of `RT' to the actual text that terminated the record +(*note How Input is Split into Records: Records.). + + The idea is to have `RS' be the pattern to look for. `gawk' will +automatically set `$0' to the text between matches of the pattern. +This is text that we wish to keep, unmodified. Then, by setting `ORS' +to the replacement text, a simple `print' statement will output the +text we wish to keep, followed by the replacement text. + + There is one wrinkle to this scheme, which is what to do if the last +record doesn't end with text that matches `RS'? Using a `print' +statement unconditionally prints the replacement text, which is not +correct. + + However, if the file did not end in text that matches `RS', `RT' +will be set to the null string. In this case, we can print `$0' using +`printf' (*note Using `printf' Statements for Fancier Printing: +Printf.). + + The `BEGIN' rule handles the setup, checking for the right number of +arguments, and calling `usage' if there is a problem. Then it sets `RS' +and `ORS' from the command line arguments, and sets `ARGV[1]' and +`ARGV[2]' to the null string, so that they will not be treated as file +names (*note Using `ARGC' and `ARGV': ARGC and ARGV.). + + The `usage' function prints an error message and exits. + + Finally, the single rule handles the printing scheme outlined above, +using `print' or `printf' as appropriate, depending upon the value of +`RT'. + + +File: gawk.info, Node: Igawk Program, Prev: Simple Sed, Up: Miscellaneous Programs + +An Easy Way to Use Library Functions +------------------------------------ + + Using library functions in `awk' can be very beneficial. It +encourages code re-use and the writing of general functions. Programs +are smaller, and therefore clearer. However, using library functions +is only easy when writing `awk' programs; it is painful when running +them, requiring multiple `-f' options. If `gawk' is unavailable, then +so too is the `AWKPATH' environment variable and the ability to put +`awk' functions into a library directory (*note Command Line Options: +Options.). + + It would be nice to be able to write programs like so: + + # library functions + @include getopt.awk + @include join.awk + ... + + # main program + BEGIN { + while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1) + ... + ... + } + + The following program, `igawk.sh', provides this service. It +simulates `gawk''s searching of the `AWKPATH' variable, and also allows +"nested" includes; i.e. a file that has been included with `@include' +can contain further `@include' statements. `igawk' will make an effort +to only include files once, so that nested includes don't accidentally +include a library function twice. + + `igawk' should behave externally just like `gawk'. This means it +should accept all of `gawk''s command line arguments, including the +ability to have multiple source files specified via `-f', and the +ability to mix command line and library source files. + + The program is written using the POSIX Shell (`sh') command language. +The way the program works is as follows: + + 1. Loop through the arguments, saving anything that doesn't represent + `awk' source code for later, when the expanded program is run. + + 2. For any arguments that do represent `awk' text, put the arguments + into a temporary file that will be expanded. There are two cases. + + a. Literal text, provided with `--source' or `--source='. This + text is just echoed directly. The `echo' program will + automatically supply a trailing newline. + + b. File names provided with `-f'. We use a neat trick, and echo + `@include FILENAME' into the temporary file. Since the file + inclusion program will work the way `gawk' does, this will + get the text of the file included into the program at the + correct point. + + 3. Run an `awk' program (naturally) over the temporary file to expand + `@include' statements. The expanded program is placed in a second + temporary file. + + 4. Run the expanded program with `gawk' and any other original + command line arguments that the user supplied (such as the data + file names). + + The initial part of the program turns on shell tracing if the first +argument was `debug'. Otherwise, a shell `trap' statement arranges to +clean up any temporary files on program exit or upon an interrupt. + + The next part loops through all the command line arguments. There +are several cases of interest. + +`--' + This ends the arguments to `igawk'. Anything else should be + passed on to the user's `awk' program without being evaluated. + +`-W' + This indicates that the next option is specific to `gawk'. To make + argument processing easier, the `-W' is appended to the front of + the remaining arguments and the loop continues. (This is an `sh' + programming trick. Don't worry about it if you are not familiar + with `sh'.) + +`-v' +`-F' + These are saved and passed on to `gawk'. + +`-f' +`--file' +`--file=' +`-Wfile=' + The file name is saved to the temporary file `/tmp/ig.s.$$' with an + `@include' statement. The `sed' utility is used to remove the + leading option part of the argument (e.g., `--file='). + +`--source' +`--source=' +`-Wsource=' + The source text is echoed into `/tmp/ig.s.$$'. + +`--version' +`--version' +`-Wversion' + `igawk' prints its version number, and runs `gawk --version' to + get the `gawk' version information, and then exits. + + If none of `-f', `--file', `-Wfile', `--source', or `-Wsource', were +supplied, then the first non-option argument should be the `awk' +program. If there are no command line arguments left, `igawk' prints +an error message and exits. Otherwise, the first argument is echoed +into `/tmp/ig.s.$$'. + + In any case, after the arguments have been processed, `/tmp/ig.s.$$' +contains the complete text of the original `awk' program. + + The `$$' in `sh' represents the current process ID number. It is +often used in shell programs to generate unique temporary file names. +This allows multiple users to run `igawk' without worrying that the +temporary file names will clash. + + Here's the program: + + #! /bin/sh + + # igawk --- like gawk but do @include processing + # Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain + # July 1993 + + if [ "$1" = debug ] + then + set -x + shift + else + # cleanup on exit, hangup, interrupt, quit, termination + trap 'rm -f /tmp/ig.[se].$$' 0 1 2 3 15 + fi + + while [ $# -ne 0 ] # loop over arguments + do + case $1 in + --) shift; break;; + + -W) shift + set -- -W"$@" + continue;; + + -[vF]) opts="$opts $1 '$2'" + shift;; + + -[vF]*) opts="$opts '$1'" ;; + + -f) echo @include "$2" >> /tmp/ig.s.$$ + shift;; + + -f*) f=`echo "$1" | sed 's/-f//'` + echo @include "$f" >> /tmp/ig.s.$$ ;; + + -?file=*) # -Wfile or --file + f=`echo "$1" | sed 's/-.file=//'` + echo @include "$f" >> /tmp/ig.s.$$ ;; + + -?file) # get arg, $2 + echo @include "$2" >> /tmp/ig.s.$$ + shift;; + + -?source=*) # -Wsource or --source + t=`echo "$1" | sed 's/-.source=//'` + echo "$t" >> /tmp/ig.s.$$ ;; + + -?source) # get arg, $2 + echo "$2" >> /tmp/ig.s.$$ + shift;; + + -?version) + echo igawk: version 1.0 1>&2 + gawk --version + exit 0 ;; + + -[W-]*) opts="$opts '$1'" ;; + + *) break;; + esac + shift + done + + if [ ! -s /tmp/ig.s.$$ ] + then + if [ -z "$1" ] + then + echo igawk: no program! 1>&2 + exit 1 + else + echo "$1" > /tmp/ig.s.$$ + shift + fi + fi + + # at this point, /tmp/ig.s.$$ has the program + + The `awk' program to process `@include' directives reads through the +program, one line at a time using `getline' (*note Explicit Input with +`getline': Getline.). The input file names and `@include' statements +are managed using a stack. As each `@include' is encountered, the +current file name is "pushed" onto the stack, and the file named in the +`@include' directive becomes the current file name. As each file is +finished, the stack is "popped," and the previous input file becomes +the current input file again. The process is started by making the +original file the first one on the stack. + + The `pathto' function does the work of finding the full path to a +file. It simulates `gawk''s behavior when searching the `AWKPATH' +environment variable (*note The `AWKPATH' Environment Variable: AWKPATH +Variable.). If a file name has a `/' in it, no path search is done. +Otherwise, the file name is concatenated with the name of each +directory in the path, and an attempt is made to open the generated file +name. The only way in `awk' to test if a file can be read is to go +ahead and try to read it with `getline'; that is what `pathto' does. +If the file can be read, it is closed, and the file name is returned. + + gawk -- ' + # process @include directives + + function pathto(file, i, t, junk) + { + if (index(file, "/") != 0) + return file + + for (i = 1; i <= ndirs; i++) { + t = (pathlist[i] "/" file) + if ((getline junk < t) > 0) { + # found it + close(t) + return t + } + } + return "" + } + + The main program is contained inside one `BEGIN' rule. The first +thing it does is set up the `pathlist' array that `pathto' uses. After +splitting the path on `:', null elements are replaced with `"."', which +represents the current directory. + + BEGIN { + path = ENVIRON["AWKPATH"] + ndirs = split(path, pathlist, ":") + for (i = 1; i <= ndirs; i++) { + if (pathlist[i] == "") + pathlist[i] = "." + } + + The stack is initialized with `ARGV[1]', which will be +`/tmp/ig.s.$$'. The main loop comes next. Input lines are read in +succession. Lines that do not start with `@include' are printed +verbatim. + + If the line does start with `@include', the file name is in `$2'. +`pathto' is called to generate the full path. If it could not, then we +print an error message and continue. + + The next thing to check is if the file has been included already. +The `processed' array is indexed by the full file name of each included +file, and it tracks this information for us. If the file has been +seen, a warning message is printed. Otherwise, the new file name is +pushed onto the stack and processing continues. + + Finally, when `getline' encounters the end of the input file, the +file is closed and the stack is popped. When `stackptr' is less than +zero, the program is done. + + stackptr = 0 + input[stackptr] = ARGV[1] # ARGV[1] is first file + + for (; stackptr >= 0; stackptr--) { + while ((getline < input[stackptr]) > 0) { + if (tolower($1) != "@include") { + print + continue + } + fpath = pathto($2) + if (fpath == "") { + printf("igawk:%s:%d: cannot find %s\n", \ + input[stackptr], FNR, $2) > "/dev/stderr" + continue + } + if (! (fpath in processed)) { + processed[fpath] = input[stackptr] + input[++stackptr] = fpath + } else + print $2, "included in", input[stackptr], \ + "already included in", \ + processed[fpath] > "/dev/stderr" + } + close(input[stackptr]) + } + }' /tmp/ig.s.$$ > /tmp/ig.e.$$ + + The last step is to call `gawk' with the expanded program and the +original options and command line arguments that the user supplied. +`gawk''s exit status is passed back on to `igawk''s calling program. + + eval gawk -f /tmp/ig.e.$$ $opts -- "$@" + + exit $? + + This version of `igawk' represents my third attempt at this program. +There are three key simplifications that made the program work better. + + 1. Using `@include' even for the files named with `-f' makes building + the initial collected `awk' program much simpler; all the + `@include' processing can be done once. + + 2. The `pathto' function doesn't try to save the line read with + `getline' when testing for the file's accessibility. Trying to + save this line for use with the main program complicates things + considerably. + + 3. Using a `getline' loop in the `BEGIN' rule does it all in one + place. It is not necessary to call out to a separate loop for + processing nested `@include' statements. + + Also, this program illustrates that it is often worthwhile to combine +`sh' and `awk' programming together. You can usually accomplish quite +a lot, without having to resort to low-level programming in C or C++, +and it is frequently easier to do certain kinds of string and argument +manipulation using the shell than it is in `awk'. + + Finally, `igawk' shows that it is not always necessary to add new +features to a program; they can often be layered on top. With `igawk', +there is no real reason to build `@include' processing into `gawk' +itself. + + As an additional example of this, consider the idea of having two +files in a directory in the search path. + +`default.awk' + This file would contain a set of default library functions, such + as `getopt' and `assert'. + +`site.awk' + This file would contain library functions that are specific to a + site or installation, i.e. locally developed functions. Having a + separate file allows `default.awk' to change with new `gawk' + releases, without requiring the system administrator to update it + each time by adding the local functions. + + One user suggested that `gawk' be modified to automatically read +these files upon startup. Instead, it would be very simple to modify +`igawk' to do this. Since `igawk' can process nested `@include' +directives, `default.awk' could simply contain `@include' statements +for the desired library functions. + + +File: gawk.info, Node: Language History, Next: Gawk Summary, Prev: Sample Programs, Up: Top + +The Evolution of the `awk' Language +*********************************** + + This Info file describes the GNU implementation of `awk', which +follows the POSIX specification. Many `awk' users are only familiar +with the original `awk' implementation in Version 7 Unix. (This +implementation was the basis for `awk' in Berkeley Unix, through +4.3-Reno. The 4.4 release of Berkeley Unix uses `gawk' 2.15.2 for its +version of `awk'.) This chapter briefly describes the evolution of the +`awk' language, with cross references to other parts of the Info file +where you can find more information. + +* Menu: + +* V7/SVR3.1:: The major changes between V7 and System V + Release 3.1. +* SVR4:: Minor changes between System V Releases 3.1 + and 4. +* POSIX:: New features from the POSIX standard. +* BTL:: New features from the Bell Laboratories + version of `awk'. +* POSIX/GNU:: The extensions in `gawk' not in POSIX + `awk'. + + +File: gawk.info, Node: V7/SVR3.1, Next: SVR4, Prev: Language History, Up: Language History + +Major Changes between V7 and SVR3.1 +=================================== + + The `awk' language evolved considerably between the release of +Version 7 Unix (1978) and the new version first made generally +available in System V Release 3.1 (1987). This section summarizes the +changes, with cross-references to further details. + + * The requirement for `;' to separate rules on a line (*note `awk' + Statements Versus Lines: Statements/Lines.). + + * User-defined functions, and the `return' statement (*note + User-defined Functions: User-defined.). + + * The `delete' statement (*note The `delete' Statement: Delete.). + + * The `do'-`while' statement (*note The `do'-`while' Statement: Do + Statement.). + + * The built-in functions `atan2', `cos', `sin', `rand' and `srand' + (*note Numeric Built-in Functions: Numeric Functions.). + + * The built-in functions `gsub', `sub', and `match' (*note Built-in + Functions for String Manipulation: String Functions.). + + * The built-in functions `close', and `system' (*note Built-in + Functions for Input/Output: I/O Functions.). + + * The `ARGC', `ARGV', `FNR', `RLENGTH', `RSTART', and `SUBSEP' + built-in variables (*note Built-in Variables::). + + * The conditional expression using the ternary operator `?:' (*note + Conditional Expressions: Conditional Exp.). + + * The exponentiation operator `^' (*note Arithmetic Operators: + Arithmetic Ops.) and its assignment operator form `^=' (*note + Assignment Expressions: Assignment Ops.). + + * C-compatible operator precedence, which breaks some old `awk' + programs (*note Operator Precedence (How Operators Nest): + Precedence.). + + * Regexps as the value of `FS' (*note Specifying How Fields are + Separated: Field Separators.), and as the third argument to the + `split' function (*note Built-in Functions for String + Manipulation: String Functions.). + + * Dynamic regexps as operands of the `~' and `!~' operators (*note + How to Use Regular Expressions: Regexp Usage.). + + * The escape sequences `\b', `\f', and `\r' (*note Escape + Sequences::). (Some vendors have updated their old versions of + `awk' to recognize `\r', `\b', and `\f', but this is not something + you can rely on.) + + * Redirection of input for the `getline' function (*note Explicit + Input with `getline': Getline.). + + * Multiple `BEGIN' and `END' rules (*note The `BEGIN' and `END' + Special Patterns: BEGIN/END.). + + * Multi-dimensional arrays (*note Multi-dimensional Arrays: + Multi-dimensional.). + + +File: gawk.info, Node: SVR4, Next: POSIX, Prev: V7/SVR3.1, Up: Language History + +Changes between SVR3.1 and SVR4 +=============================== + + The System V Release 4 version of Unix `awk' added these features +(some of which originated in `gawk'): + + * The `ENVIRON' variable (*note Built-in Variables::). + + * Multiple `-f' options on the command line (*note Command Line + Options: Options.). + + * The `-v' option for assigning variables before program execution + begins (*note Command Line Options: Options.). + + * The `--' option for terminating command line options. + + * The `\a', `\v', and `\x' escape sequences (*note Escape + Sequences::). + + * A defined return value for the `srand' built-in function (*note + Numeric Built-in Functions: Numeric Functions.). + + * The `toupper' and `tolower' built-in string functions for case + translation (*note Built-in Functions for String Manipulation: + String Functions.). + + * A cleaner specification for the `%c' format-control letter in the + `printf' function (*note Format-Control Letters: Control Letters.). + + * The ability to dynamically pass the field width and precision + (`"%*.*d"') in the argument list of the `printf' function (*note + Format-Control Letters: Control Letters.). + + * The use of regexp constants such as `/foo/' as expressions, where + they are equivalent to using the matching operator, as in `$0 ~ + /foo/' (*note Using Regular Expression Constants: Using Constant + Regexps.). + + +File: gawk.info, Node: POSIX, Next: BTL, Prev: SVR4, Up: Language History + +Changes between SVR4 and POSIX `awk' +==================================== + + The POSIX Command Language and Utilities standard for `awk' +introduced the following changes into the language: + + * The use of `-W' for implementation-specific options. + + * The use of `CONVFMT' for controlling the conversion of numbers to + strings (*note Conversion of Strings and Numbers: Conversion.). + + * The concept of a numeric string, and tighter comparison rules to go + with it (*note Variable Typing and Comparison Expressions: Typing + and Comparison.). + + * More complete documentation of many of the previously undocumented + features of the language. + + The following common extensions are not permitted by the POSIX +standard: + + * `\x' escape sequences are not recognized (*note Escape + Sequences::). + + * Newlines do not act as whitespace to separate fields when `FS' is + equal to a single space. + + * The synonym `func' for the keyword `function' is not recognized + (*note Function Definition Syntax: Definition Syntax.). + + * The operators `**' and `**=' cannot be used in place of `^' and + `^=' (*note Arithmetic Operators: Arithmetic Ops., and also *note + Assignment Expressions: Assignment Ops.). + + * Specifying `-Ft' on the command line does not set the value of + `FS' to be a single tab character (*note Specifying How Fields are + Separated: Field Separators.). + + * The `fflush' built-in function is not supported (*note Built-in + Functions for Input/Output: I/O Functions.). + + +File: gawk.info, Node: BTL, Next: POSIX/GNU, Prev: POSIX, Up: Language History + +Extensions in the Bell Laboratories `awk' +========================================= + + Brian Kernighan, one of the original designers of Unix `awk', has +made his version available via anonymous `ftp' (*note Other Freely +Available `awk' Implementations: Other Versions.). This section +describes extensions in his version of `awk' that are not in POSIX +`awk'. + + * The `-mf NNN' and `-mr NNN' command line options to set the + maximum number of fields, and the maximum record size, respectively + (*note Command Line Options: Options.). + + * The `fflush' built-in function for flushing buffered output (*note + Built-in Functions for Input/Output: I/O Functions.). + + + +File: gawk.info, Node: POSIX/GNU, Prev: BTL, Up: Language History + +Extensions in `gawk' Not in POSIX `awk' +======================================= + + The GNU implementation, `gawk', adds a number of features. This +sections lists them in the order they were added to `gawk'. They can +all be disabled with either the `--traditional' or `--posix' options +(*note Command Line Options: Options.). + + Version 2.10 of `gawk' introduced these features: + + * The `AWKPATH' environment variable for specifying a path search for + the `-f' command line option (*note Command Line Options: + Options.). + + * The `IGNORECASE' variable and its effects (*note Case-sensitivity + in Matching: Case-sensitivity.). + + * The `/dev/stdin', `/dev/stdout', `/dev/stderr', and `/dev/fd/N' + file name interpretation (*note Special File Names in `gawk': + Special Files.). + + Version 2.13 of `gawk' introduced these features: + + * The `FIELDWIDTHS' variable and its effects (*note Reading + Fixed-width Data: Constant Size.). + + * The `systime' and `strftime' built-in functions for obtaining and + printing time stamps (*note Functions for Dealing with Time + Stamps: Time Functions.). + + * The `-W lint' option to provide source code and run time error and + portability checking (*note Command Line Options: Options.). + + * The `-W compat' option to turn off these extensions (*note Command + Line Options: Options.). + + * The `-W posix' option for full POSIX compliance (*note Command + Line Options: Options.). + + Version 2.14 of `gawk' introduced these features: + + * The `next file' statement for skipping to the next data file + (*note The `nextfile' Statement: Nextfile Statement.). + + Version 2.15 of `gawk' introduced these features: + + * The `ARGIND' variable, that tracks the movement of `FILENAME' + through `ARGV' (*note Built-in Variables::). + + * The `ERRNO' variable, that contains the system error message when + `getline' returns -1, or when `close' fails (*note Built-in + Variables::). + + * The ability to use GNU-style long named options that start with + `--' (*note Command Line Options: Options.). + + * The `--source' option for mixing command line and library file + source code (*note Command Line Options: Options.). + + * The `/dev/pid', `/dev/ppid', `/dev/pgrpid', and `/dev/user' file + name interpretation (*note Special File Names in `gawk': Special + Files.). + + Version 3.0 of `gawk' introduced these features: + + * The `next file' statement became `nextfile' (*note The `nextfile' + Statement: Nextfile Statement.). + + * The `--lint-old' option to warn about constructs that are not + available in the original Version 7 Unix version of `awk' (*note + Major Changes between V7 and SVR3.1: V7/SVR3.1.). + + * The `--traditional' option was added as a better name for + `--compat' (*note Command Line Options: Options.). + + * The ability for `FS' to be a null string, and for the third + argument to `split' to be the null string (*note Making Each + Character a Separate Field: Single Character Fields.). + + * The ability for `RS' to be a regexp (*note How Input is Split into + Records: Records.). + + * The `RT' variable (*note How Input is Split into Records: + Records.). + + * The `gensub' function for more powerful text manipulation (*note + Built-in Functions for String Manipulation: String Functions.). + + * The `strftime' function acquired a default time format, allowing + it to be called with no arguments (*note Functions for Dealing + with Time Stamps: Time Functions.). + + * Full support for both POSIX and GNU regexps (*note Regular + Expressions: Regexp.). + + * The `--re-interval' option to provide interval expressions in + regexps (*note Regular Expression Operators: Regexp Operators.). + + * `IGNORECASE' changed, now applying to string comparison as well as + regexp operations (*note Case-sensitivity in Matching: + Case-sensitivity.). + + * The `-m' option and the `fflush' function from the Bell Labs + research version of `awk' (*note Command Line Options: Options.; + also *note Built-in Functions for Input/Output: I/O Functions.). + + * The use of GNU Autoconf to control the configuration process + (*note Compiling `gawk' for Unix: Quick Installation.). + + * Amiga support (*note Installing `gawk' on an Amiga: Amiga + Installation.). + + + +File: gawk.info, Node: Gawk Summary, Next: Installation, Prev: Language History, Up: Top + +`gawk' Summary +************** + + This appendix provides a brief summary of the `gawk' command line +and the `awk' language. It is designed to serve as "quick reference." +It is therefore terse, but complete. + +* Menu: + +* Command Line Summary:: Recapitulation of the command line. +* Language Summary:: A terse review of the language. +* Variables/Fields:: Variables, fields, and arrays. +* Rules Summary:: Patterns and Actions, and their component + parts. +* Actions Summary:: Quick overview of actions. +* Functions Summary:: Defining and calling functions. +* Historical Features:: Some undocumented but supported ``features''. + + +File: gawk.info, Node: Command Line Summary, Next: Language Summary, Prev: Gawk Summary, Up: Gawk Summary + +Command Line Options Summary +============================ + + The command line consists of options to `gawk' itself, the `awk' +program text (if not supplied via the `-f' option), and values to be +made available in the `ARGC' and `ARGV' predefined `awk' variables: + + gawk [POSIX OR GNU STYLE OPTIONS] -f SOURCE-FILE [`--'] FILE ... + gawk [POSIX OR GNU STYLE OPTIONS] [`--'] 'PROGRAM' FILE ... + + The options that `gawk' accepts are: + +`-F FS' +`--field-separator FS' + Use FS for the input field separator (the value of the `FS' + predefined variable). + +`-f PROGRAM-FILE' +`--file PROGRAM-FILE' + Read the `awk' program source from the file PROGRAM-FILE, instead + of from the first command line argument. + +`-mf NNN' +`-mr NNN' + The `f' flag sets the maximum number of fields, and the `r' flag + sets the maximum record size. These options are ignored by + `gawk', since `gawk' has no predefined limits; they are only for + compatibility with the Bell Labs research version of Unix `awk'. + +`-v VAR=VAL' +`--assign VAR=VAL' + Assign the variable VAR the value VAL before program execution + begins. + +`-W traditional' +`-W compat' +`--traditional' +`--compat' + Use compatibility mode, in which `gawk' extensions are turned off. + +`-W copyleft' +`-W copyright' +`--copyleft' +`--copyright' + Print the short version of the General Public License on the + standard output, and exit. This option may disappear in a future + version of `gawk'. + +`-W help' +`-W usage' +`--help' +`--usage' + Print a relatively short summary of the available options on the + standard output, and exit. + +`-W lint' +`--lint' + Give warnings about dubious or non-portable `awk' constructs. + +`-W lint-old' +`--lint-old' + Warn about constructs that are not available in the original + Version 7 Unix version of `awk'. + +`-W posix' +`--posix' + Use POSIX compatibility mode, in which `gawk' extensions are + turned off and additional restrictions apply. + +`-W re-interval' +`--re-interval' + Allow interval expressions (*note Regular Expression Operators: + Regexp Operators.), in regexps. + +`-W source=PROGRAM-TEXT' +`--source PROGRAM-TEXT' + Use PROGRAM-TEXT as `awk' program source code. This option allows + mixing command line source code with source code from files, and is + particularly useful for mixing command line programs with library + functions. + +`-W version' +`--version' + Print version information for this particular copy of `gawk' on + the error output. + +`--' + Signal the end of options. This is useful to allow further + arguments to the `awk' program itself to start with a `-'. This + is mainly for consistency with POSIX argument parsing conventions. + + Any other options are flagged as invalid, but are otherwise ignored. +*Note Command Line Options: Options, for more details. + + +File: gawk.info, Node: Language Summary, Next: Variables/Fields, Prev: Command Line Summary, Up: Gawk Summary + +Language Summary +================ + + An `awk' program consists of a sequence of zero or more +pattern-action statements and optional function definitions. One or +the other of the pattern and action may be omitted. + + PATTERN { ACTION STATEMENTS } + PATTERN + { ACTION STATEMENTS } + + function NAME(PARAMETER LIST) { ACTION STATEMENTS } + + `gawk' first reads the program source from the PROGRAM-FILE(s), if +specified, or from the first non-option argument on the command line. +The `-f' option may be used multiple times on the command line. `gawk' +reads the program text from all the PROGRAM-FILE files, effectively +concatenating them in the order they are specified. This is useful for +building libraries of `awk' functions, without having to include them +in each new `awk' program that uses them. To use a library function in +a file from a program typed in on the command line, specify `--source +'PROGRAM'', and type your program in between the single quotes. *Note +Command Line Options: Options. + + The environment variable `AWKPATH' specifies a search path to use +when finding source files named with the `-f' option. The default +path, which is `.:/usr/local/share/awk'(1) is used if `AWKPATH' is not +set. If a file name given to the `-f' option contains a `/' character, +no path search is performed. *Note The `AWKPATH' Environment Variable: +AWKPATH Variable. + + `gawk' compiles the program into an internal form, and then proceeds +to read each file named in the `ARGV' array. The initial values of +`ARGV' come from the command line arguments. If there are no files +named on the command line, `gawk' reads the standard input. + + If a "file" named on the command line has the form `VAR=VAL', it is +treated as a variable assignment: the variable VAR is assigned the +value VAL. If any of the files have a value that is the null string, +that element in the list is skipped. + + For each record in the input, `gawk' tests to see if it matches any +PATTERN in the `awk' program. For each pattern that the record +matches, the associated ACTION is executed. + + ---------- Footnotes ---------- + + (1) The path may use a directory other than `/usr/local/share/awk', +depending upon how `gawk' was built and installed. + + +File: gawk.info, Node: Variables/Fields, Next: Rules Summary, Prev: Language Summary, Up: Gawk Summary + +Variables and Fields +==================== + + `awk' variables are not declared; they come into existence when they +are first used. Their values are either floating-point numbers or +strings. `awk' also has one-dimensional arrays; multiple-dimensional +arrays may be simulated. There are several predefined variables that +`awk' sets as a program runs; these are summarized below. + +* Menu: + +* Fields Summary:: Input field splitting. +* Built-in Summary:: `awk''s built-in variables. +* Arrays Summary:: Using arrays. +* Data Type Summary:: Values in `awk' are numbers or strings. + + +File: gawk.info, Node: Fields Summary, Next: Built-in Summary, Prev: Variables/Fields, Up: Variables/Fields + +Fields +------ + + As each input line is read, `gawk' splits the line into FIELDS, +using the value of the `FS' variable as the field separator. If `FS' +is a single character, fields are separated by that character. +Otherwise, `FS' is expected to be a full regular expression. In the +special case that `FS' is a single space, fields are separated by runs +of spaces, tabs and/or newlines.(1) If `FS' is the null string (`""'), +then each individual character in the record becomes a separate field. +Note that the value of `IGNORECASE' (*note Case-sensitivity in +Matching: Case-sensitivity.) also affects how fields are split when +`FS' is a regular expression. + + Each field in the input line may be referenced by its position, `$1', +`$2', and so on. `$0' is the whole line. The value of a field may be +assigned to as well. Field numbers need not be constants: + + n = 5 + print $n + +prints the fifth field in the input line. The variable `NF' is set to +the total number of fields in the input line. + + References to non-existent fields (i.e. fields after `$NF') return +the null string. However, assigning to a non-existent field (e.g., +`$(NF+2) = 5') increases the value of `NF', creates any intervening +fields with the null string as their value, and causes the value of +`$0' to be recomputed, with the fields being separated by the value of +`OFS'. Decrementing `NF' causes the values of fields past the new +value to be lost, and the value of `$0' to be recomputed, with the +fields being separated by the value of `OFS'. *Note Reading Input +Files: Reading Files. + + ---------- Footnotes ---------- + + (1) In POSIX `awk', newline does not separate fields. + + +File: gawk.info, Node: Built-in Summary, Next: Arrays Summary, Prev: Fields Summary, Up: Variables/Fields + +Built-in Variables +------------------ + + `gawk''s built-in variables are: + +`ARGC' + The number of elements in `ARGV'. See below for what is actually + included in `ARGV'. + +`ARGIND' + The index in `ARGV' of the current file being processed. When + `gawk' is processing the input data files, it is always true that + `FILENAME == ARGV[ARGIND]'. + +`ARGV' + The array of command line arguments. The array is indexed from + zero to `ARGC' - 1. Dynamically changing `ARGC' and the contents + of `ARGV' can control the files used for data. A null-valued + element in `ARGV' is ignored. `ARGV' does not include the options + to `awk' or the text of the `awk' program itself. + +`CONVFMT' + The conversion format to use when converting numbers to strings. + +`FIELDWIDTHS' + A space separated list of numbers describing the fixed-width input + data. + +`ENVIRON' + An array of environment variable values. The array is indexed by + variable name, each element being the value of that variable. + Thus, the environment variable `HOME' is `ENVIRON["HOME"]'. One + possible value might be `/home/arnold'. + + Changing this array does not affect the environment seen by + programs which `gawk' spawns via redirection or the `system' + function. (This may change in a future version of `gawk'.) + + Some operating systems do not have environment variables. The + `ENVIRON' array is empty when running on these systems. + +`ERRNO' + The system error message when an error occurs using `getline' or + `close'. + +`FILENAME' + The name of the current input file. If no files are specified on + the command line, the value of `FILENAME' is the null string. + +`FNR' + The input record number in the current input file. + +`FS' + The input field separator, a space by default. + +`IGNORECASE' + The case-sensitivity flag for string comparisons and regular + expression operations. If `IGNORECASE' has a non-zero value, then + pattern matching in rules, record separating with `RS', field + splitting with `FS', regular expression matching with `~' and + `!~', and the `gensub', `gsub', `index', `match', `split' and + `sub' built-in functions all ignore case when doing regular + expression operations, and all string comparisons are done + ignoring case. + +`NF' + The number of fields in the current input record. + +`NR' + The total number of input records seen so far. + +`OFMT' + The output format for numbers for the `print' statement, `"%.6g"' + by default. + +`OFS' + The output field separator, a space by default. + +`ORS' + The output record separator, by default a newline. + +`RS' + The input record separator, by default a newline. If `RS' is set + to the null string, then records are separated by blank lines. + When `RS' is set to the null string, then the newline character + always acts as a field separator, in addition to whatever value + `FS' may have. If `RS' is set to a multi-character string, it + denotes a regexp; input text matching the regexp separates records. + +`RT' + The input text that matched the text denoted by `RS', the record + separator. + +`RSTART' + The index of the first character last matched by `match'; zero if + no match. + +`RLENGTH' + The length of the string last matched by `match'; -1 if no match. + +`SUBSEP' + The string used to separate multiple subscripts in array elements, + by default `"\034"'. + + *Note Built-in Variables::, for more information. + + +File: gawk.info, Node: Arrays Summary, Next: Data Type Summary, Prev: Built-in Summary, Up: Variables/Fields + +Arrays +------ + + Arrays are subscripted with an expression between square brackets +(`[' and `]'). Array subscripts are _always_ strings; numbers are +converted to strings as necessary, following the standard conversion +rules (*note Conversion of Strings and Numbers: Conversion.). + + If you use multiple expressions separated by commas inside the square +brackets, then the array subscript is a string consisting of the +concatenation of the individual subscript values, converted to strings, +separated by the subscript separator (the value of `SUBSEP'). + + The special operator `in' may be used in a conditional context to +see if an array has an index consisting of a particular value. + + if (val in array) + print array[val] + + If the array has multiple subscripts, use `(i, j, ...) in ARRAY' to +test for existence of an element. + + The `in' construct may also be used in a `for' loop to iterate over +all the elements of an array. *Note Scanning All Elements of an Array: +Scanning an Array. + + You can remove an element from an array using the `delete' statement. + + You can clear an entire array using `delete ARRAY'. + + *Note Arrays in `awk': Arrays. + + +File: gawk.info, Node: Data Type Summary, Prev: Arrays Summary, Up: Variables/Fields + +Data Types +---------- + + The value of an `awk' expression is always either a number or a +string. + + Some contexts (such as arithmetic operators) require numeric values. +They convert strings to numbers by interpreting the text of the string +as a number. If the string does not look like a number, it converts to +zero. + + Other contexts (such as concatenation) require string values. They +convert numbers to strings by effectively printing them with `sprintf'. +*Note Conversion of Strings and Numbers: Conversion, for the details. + + To force conversion of a string value to a number, simply add zero +to it. If the value you start with is already a number, this does not +change it. + + To force conversion of a numeric value to a string, concatenate it +with the null string. + + Comparisons are done numerically if both operands are numeric, or if +one is numeric and the other is a numeric string. Otherwise one or +both operands are converted to strings and a string comparison is +performed. Fields, `getline' input, `FILENAME', `ARGV' elements, +`ENVIRON' elements and the elements of an array created by `split' are +the only items that can be numeric strings. String constants, such as +`"3.1415927"' are not numeric strings, they are string constants. The +full rules for comparisons are described in *Note Variable Typing and +Comparison Expressions: Typing and Comparison. + + Uninitialized variables have the string value `""' (the null, or +empty, string). In contexts where a number is required, this is +equivalent to zero. + + *Note Variables::, for more information on variable naming and +initialization; *note Conversion of Strings and Numbers: Conversion., +for more information on how variable values are interpreted. + + +File: gawk.info, Node: Rules Summary, Next: Actions Summary, Prev: Variables/Fields, Up: Gawk Summary + +Patterns +======== + +* Menu: + +* Pattern Summary:: Quick overview of patterns. +* Regexp Summary:: Quick overview of regular expressions. + + An `awk' program is mostly composed of rules, each consisting of a +pattern followed by an action. The action is enclosed in `{' and `}'. +Either the pattern may be missing, or the action may be missing, but +not both. If the pattern is missing, the action is executed for every +input record. A missing action is equivalent to `{ print }', which +prints the entire line. + + Comments begin with the `#' character, and continue until the end of +the line. Blank lines may be used to separate statements. Statements +normally end with a newline; however, this is not the case for lines +ending in a `,', `{', `?', `:', `&&', or `||'. Lines ending in `do' or +`else' also have their statements automatically continued on the +following line. In other cases, a line can be continued by ending it +with a `\', in which case the newline is ignored. + + Multiple statements may be put on one line by separating each one +with a `;'. This applies to both the statements within the action part +of a rule (the usual case), and to the rule statements. + + *Note Comments in `awk' Programs: Comments, for information on +`awk''s commenting convention; *note `awk' Statements Versus Lines: +Statements/Lines., for a description of the line continuation mechanism +in `awk'. + + +File: gawk.info, Node: Pattern Summary, Next: Regexp Summary, Prev: Rules Summary, Up: Rules Summary + +Pattern Summary +--------------- + + `awk' patterns may be one of the following: + + /REGULAR EXPRESSION/ + RELATIONAL EXPRESSION + PATTERN && PATTERN + PATTERN || PATTERN + PATTERN ? PATTERN : PATTERN + (PATTERN) + ! PATTERN + PATTERN1, PATTERN2 + BEGIN + END + + `BEGIN' and `END' are two special kinds of patterns that are not +tested against the input. The action parts of all `BEGIN' rules are +concatenated as if all the statements had been written in a single +`BEGIN' rule. They are executed before any of the input is read. +Similarly, all the `END' rules are concatenated, and executed when all +the input is exhausted (or when an `exit' statement is executed). +`BEGIN' and `END' patterns cannot be combined with other patterns in +pattern expressions. `BEGIN' and `END' rules cannot have missing +action parts. + + For `/REGULAR-EXPRESSION/' patterns, the associated statement is +executed for each input record that matches the regular expression. +Regular expressions are summarized below. + + A RELATIONAL EXPRESSION may use any of the operators defined below in +the section on actions. These generally test whether certain fields +match certain regular expressions. + + The `&&', `||', and `!' operators are logical "and," logical "or," +and logical "not," respectively, as in C. They do short-circuit +evaluation, also as in C, and are used for combining more primitive +pattern expressions. As in most languages, parentheses may be used to +change the order of evaluation. + + The `?:' operator is like the same operator in C. If the first +pattern matches, then the second pattern is matched against the input +record; otherwise, the third is matched. Only one of the second and +third patterns is matched. + + The `PATTERN1, PATTERN2' form of a pattern is called a range +pattern. It matches all input lines starting with a line that matches +PATTERN1, and continuing until a line that matches PATTERN2, inclusive. +A range pattern cannot be used as an operand of any of the pattern +operators. + + *Note Pattern Elements: Pattern Overview. + + +File: gawk.info, Node: Regexp Summary, Prev: Pattern Summary, Up: Rules Summary + +Regular Expressions +------------------- + + Regular expressions are based on POSIX EREs (extended regular +expressions). The escape sequences allowed in string constants are +also valid in regular expressions (*note Escape Sequences::). Regexps +are composed of characters as follows: + +`C' + matches the character C (assuming C is none of the characters + listed below). + +`\C' + matches the literal character C. + +`.' + matches any character, _including_ newline. In strict POSIX mode, + `.' does not match the NUL character, which is a character with + all bits equal to zero. + +`^' + matches the beginning of a string. + +`$' + matches the end of a string. + +`[ABC...]' + matches any of the characters ABC... (character list). + +`[[:CLASS:]]' + matches any character in the character class CLASS. Allowable + classes are `alnum', `alpha', `blank', `cntrl', `digit', `graph', + `lower', `print', `punct', `space', `upper', and `xdigit'. + +`[[.SYMBOL.]]' + matches the multi-character collating symbol SYMBOL. `gawk' does + not currently support collating symbols. + +`[[=CLASSNAME=]]' + matches any of the equivalent characters in the current locale + named by the equivalence class CLASSNAME. `gawk' does not + currently support equivalence classes. + +`[^ABC...]' + matches any character except ABC... (negated character list). + +`R1|R2' + matches either R1 or R2 (alternation). + +`R1R2' + matches R1, and then R2 (concatenation). + +`R+' + matches one or more R's. + +`R*' + matches zero or more R's. + +`R?' + matches zero or one R's. + +`(R)' + matches R (grouping). + +`R{N}' +`R{N,}' +`R{N,M}' + matches at least N, N to any number, or N to M occurrences of R + (interval expressions). + +`\y' + matches the empty string at either the beginning or the end of a + word. + +`\B' + matches the empty string within a word. + +`\<' + matches the empty string at the beginning of a word. + +`\>' + matches the empty string at the end of a word. + +`\w' + matches any word-constituent character (alphanumeric characters and + the underscore). + +`\W' + matches any character that is not word-constituent. + +`\`' + matches the empty string at the beginning of a buffer (same as a + string in `gawk'). + +`\'' + matches the empty string at the end of a buffer. + + The various command line options control how `gawk' interprets +characters in regexps. + +No options + In the default case, `gawk' provide all the facilities of POSIX + regexps and the GNU regexp operators described above. However, + interval expressions are not supported. + +`--posix' + Only POSIX regexps are supported, the GNU operators are not special + (e.g., `\w' matches a literal `w'). Interval expressions are + allowed. + +`--traditional' + Traditional Unix `awk' regexps are matched. The GNU operators are + not special, interval expressions are not available, and neither + are the POSIX character classes (`[[:alnum:]]' and so on). + Characters described by octal and hexadecimal escape sequences are + treated literally, even if they represent regexp metacharacters. + +`--re-interval' + Allow interval expressions in regexps, even if `--traditional' has + been provided. + + *Note Regular Expressions: Regexp. + + +File: gawk.info, Node: Actions Summary, Next: Functions Summary, Prev: Rules Summary, Up: Gawk Summary + +Actions +======= + + Action statements are enclosed in braces, `{' and `}'. A missing +action statement is equivalent to `{ print }'. + + Action statements consist of the usual assignment, conditional, and +looping statements found in most languages. The operators, control +statements, and Input/Output statements available are similar to those +in C. + + Comments begin with the `#' character, and continue until the end of +the line. Blank lines may be used to separate statements. Statements +normally end with a newline; however, this is not the case for lines +ending in a `,', `{', `?', `:', `&&', or `||'. Lines ending in `do' or +`else' also have their statements automatically continued on the +following line. In other cases, a line can be continued by ending it +with a `\', in which case the newline is ignored. + + Multiple statements may be put on one line by separating each one +with a `;'. This applies to both the statements within the action part +of a rule (the usual case), and to the rule statements. + + *Note Comments in `awk' Programs: Comments, for information on +`awk''s commenting convention; *note `awk' Statements Versus Lines: +Statements/Lines., for a description of the line continuation mechanism +in `awk'. + +* Menu: + +* Operator Summary:: `awk' operators. +* Control Flow Summary:: The control statements. +* I/O Summary:: The I/O statements. +* Printf Summary:: A summary of `printf'. +* Special File Summary:: Special file names interpreted internally. +* Built-in Functions Summary:: Built-in numeric and string functions. +* Time Functions Summary:: Built-in time functions. +* String Constants Summary:: Escape sequences in strings. + + +File: gawk.info, Node: Operator Summary, Next: Control Flow Summary, Prev: Actions Summary, Up: Actions Summary + +Operators +--------- + + The operators in `awk', in order of decreasing precedence, are: + +`(...)' + Grouping. + +`$' + Field reference. + +`++ --' + Increment and decrement, both prefix and postfix. + +`^' + Exponentiation (`**' may also be used, and `**=' for the assignment + operator, but they are not specified in the POSIX standard). + +`+ - !' + Unary plus, unary minus, and logical negation. + +`* / %' + Multiplication, division, and modulus. + +`+ -' + Addition and subtraction. + +`SPACE' + String concatenation. + +`< <= > >= != ==' + The usual relational operators. + +`~ !~' + Regular expression match, negated match. + +`in' + Array membership. + +`&&' + Logical "and". + +`||' + Logical "or". + +`?:' + A conditional expression. This has the form `EXPR1 ? EXPR2 : + EXPR3'. If EXPR1 is true, the value of the expression is EXPR2; + otherwise it is EXPR3. Only one of EXPR2 and EXPR3 is evaluated. + +`= += -= *= /= %= ^=' + Assignment. Both absolute assignment (`VAR=VALUE') and operator + assignment (the other forms) are supported. + + *Note Expressions::. + + +File: gawk.info, Node: Control Flow Summary, Next: I/O Summary, Prev: Operator Summary, Up: Actions Summary + +Control Statements +------------------ + + The control statements are as follows: + + if (CONDITION) STATEMENT [ else STATEMENT ] + while (CONDITION) STATEMENT + do STATEMENT while (CONDITION) + for (EXPR1; EXPR2; EXPR3) STATEMENT + for (VAR in ARRAY) STATEMENT + break + continue + delete ARRAY[INDEX] + delete ARRAY + exit [ EXPRESSION ] + { STATEMENTS } + + *Note Control Statements in Actions: Statements. + + +File: gawk.info, Node: I/O Summary, Next: Printf Summary, Prev: Control Flow Summary, Up: Actions Summary + +I/O Statements +-------------- + + The Input/Output statements are as follows: + +`getline' + Set `$0' from next input record; set `NF', `NR', `FNR'. *Note + Explicit Input with `getline': Getline. + +`getline <FILE' + Set `$0' from next record of FILE; set `NF'. + +`getline VAR' + Set VAR from next input record; set `NR', `FNR'. + +`getline VAR <FILE' + Set VAR from next record of FILE. + +`COMMAND | getline' + Run COMMAND, piping its output into `getline'; sets `$0', `NF', + `NR'. + +`COMMAND | getline `var'' + Run COMMAND, piping its output into `getline'; sets VAR. + +`next' + Stop processing the current input record. The next input record + is read and processing starts over with the first pattern in the + `awk' program. If the end of the input data is reached, the `END' + rule(s), if any, are executed. *Note The `next' Statement: Next + Statement. + +`nextfile' + Stop processing the current input file. The next input record + read comes from the next input file. `FILENAME' is updated, `FNR' + is set to one, `ARGIND' is incremented, and processing starts over + with the first pattern in the `awk' program. If the end of the + input data is reached, the `END' rule(s), if any, are executed. + Earlier versions of `gawk' used `next file'; this usage is still + supported, but is considered to be deprecated. *Note The + `nextfile' Statement: Nextfile Statement. + +`print' + Prints the current record. *Note Printing Output: Printing. + +`print EXPR-LIST' + Prints expressions. + +`print EXPR-LIST > FILE' + Prints expressions to FILE. If FILE does not exist, it is created. + If it does exist, its contents are deleted the first time the + `print' is executed. + +`print EXPR-LIST >> FILE' + Prints expressions to FILE. The previous contents of FILE are + retained, and the output of `print' is appended to the file. + +`print EXPR-LIST | COMMAND' + Prints expressions, sending the output down a pipe to COMMAND. + The pipeline to the command stays open until the `close' function + is called. + +`printf FMT, EXPR-LIST' + Format and print. + +`printf FMT, EXPR-LIST > file' + Format and print to FILE. If FILE does not exist, it is created. + If it does exist, its contents are deleted the first time the + `printf' is executed. + +`printf FMT, EXPR-LIST >> FILE' + Format and print to FILE. The previous contents of FILE are + retained, and the output of `printf' is appended to the file. + +`printf FMT, EXPR-LIST | COMMAND' + Format and print, sending the output down a pipe to COMMAND. The + pipeline to the command stays open until the `close' function is + called. + + `getline' returns zero on end of file, and -1 on an error. In the +event of an error, `getline' will set `ERRNO' to the value of a +system-dependent string that describes the error. + + +File: gawk.info, Node: Printf Summary, Next: Special File Summary, Prev: I/O Summary, Up: Actions Summary + +`printf' Summary +---------------- + + Conversion specification have the form +`%'[FLAG][WIDTH][`.'PREC]FORMAT. Items in brackets are optional. + + The `awk' `printf' statement and `sprintf' function accept the +following conversion specification formats: + +`%c' + An ASCII character. If the argument used for `%c' is numeric, it + is treated as a character and printed. Otherwise, the argument is + assumed to be a string, and the only first character of that + string is printed. + +`%d' +`%i' + A decimal number (the integer part). + +`%e' +`%E' + A floating point number of the form `[-]d.dddddde[+-]dd'. The + `%E' format uses `E' instead of `e'. + +`%f' + A floating point number of the form [`-']`ddd.dddddd'. + +`%g' +`%G' + Use either the `%e' or `%f' formats, whichever produces a shorter + string, with non-significant zeros suppressed. `%G' will use `%E' + instead of `%e'. + +`%o' + An unsigned octal number (again, an integer). + +`%s' + A character string. + +`%x' +`%X' + An unsigned hexadecimal number (an integer). The `%X' format uses + `A' through `F' instead of `a' through `f' for decimal 10 through + 15. + +`%%' + A single `%' character; no argument is converted. + + There are optional, additional parameters that may lie between the +`%' and the control letter: + +`-' + The expression should be left-justified within its field. + +`SPACE' + For numeric conversions, prefix positive values with a space, and + negative values with a minus sign. + +`+' + The plus sign, used before the width modifier (see below), says to + always supply a sign for numeric conversions, even if the data to + be formatted is positive. The `+' overrides the space modifier. + +`#' + Use an "alternate form" for certain control letters. For `o', + supply a leading zero. For `x', and `X', supply a leading `0x' or + `0X' for a non-zero result. For `e', `E', and `f', the result + will always contain a decimal point. For `g', and `G', trailing + zeros are not removed from the result. + +`0' + A leading `0' (zero) acts as a flag, that indicates output should + be padded with zeros instead of spaces. This applies even to + non-numeric output formats. This flag only has an effect when the + field width is wider than the value to be printed. + +`WIDTH' + The field should be padded to this width. The field is normally + padded with spaces. If the `0' flag has been used, it is padded + with zeros. + +`.PREC' + A number that specifies the precision to use when printing. For + the `e', `E', and `f' formats, this specifies the number of digits + you want printed to the right of the decimal point. For the `g', + and `G' formats, it specifies the maximum number of significant + digits. For the `d', `o', `i', `u', `x', and `X' formats, it + specifies the minimum number of digits to print. For the `s' + format, it specifies the maximum number of characters from the + string that should be printed. + + Either or both of the WIDTH and PREC values may be specified as `*'. +In that case, the particular value is taken from the argument list. + + *Note Using `printf' Statements for Fancier Printing: Printf. + + +File: gawk.info, Node: Special File Summary, Next: Built-in Functions Summary, Prev: Printf Summary, Up: Actions Summary + +Special File Names +------------------ + + When doing I/O redirection from either `print' or `printf' into a +file, or via `getline' from a file, `gawk' recognizes certain special +file names internally. These file names allow access to open file +descriptors inherited from `gawk''s parent process (usually the shell). +The file names are: + +`/dev/stdin' + The standard input. + +`/dev/stdout' + The standard output. + +`/dev/stderr' + The standard error output. + +`/dev/fd/N' + The file denoted by the open file descriptor N. + + In addition, reading the following files provides process related +information about the running `gawk' program. All returned records are +terminated with a newline. + +`/dev/pid' + Returns the process ID of the current process. + +`/dev/ppid' + Returns the parent process ID of the current process. + +`/dev/pgrpid' + Returns the process group ID of the current process. + +`/dev/user' + At least four space-separated fields, containing the return values + of the `getuid', `geteuid', `getgid', and `getegid' system calls. + If there are any additional fields, they are the group IDs + returned by `getgroups' system call. (Multiple groups may not be + supported on all systems.) + +These file names may also be used on the command line to name data +files. These file names are only recognized internally if you do not +actually have files with these names on your system. + + *Note Special File Names in `gawk': Special Files, for a longer +description that provides the motivation for this feature. + + +File: gawk.info, Node: Built-in Functions Summary, Next: Time Functions Summary, Prev: Special File Summary, Up: Actions Summary + +Built-in Functions +------------------ + + `awk' provides a number of built-in functions for performing numeric +operations, string related operations, and I/O related operations. + + The built-in arithmetic functions are: + +`atan2(Y, X)' + the arctangent of Y/X in radians. + +`cos(EXPR)' + the cosine of EXPR, which is in radians. + +`exp(EXPR)' + the exponential function (`e ^ EXPR'). + +`int(EXPR)' + truncates to integer. + +`log(EXPR)' + the natural logarithm of `expr'. + +`rand()' + a random number between zero and one. + +`sin(EXPR)' + the sine of EXPR, which is in radians. + +`sqrt(EXPR)' + the square root function. + +`srand([EXPR])' + use EXPR as a new seed for the random number generator. If no EXPR + is provided, the time of day is used. The return value is the + previous seed for the random number generator. + + `awk' has the following built-in string functions: + +`gensub(REGEX, SUBST, HOW [, TARGET])' + If HOW is a string beginning with `g' or `G', then replace each + match of REGEX in TARGET with SUBST. Otherwise, replace the + HOW'th occurrence. If TARGET is not supplied, use `$0'. The + return value is the changed string; the original TARGET is not + modified. Within SUBST, `\N', where N is a digit from one to nine, + can be used to indicate the text that matched the N'th + parenthesized subexpression. This function is `gawk'-specific. + +`gsub(REGEX, SUBST [, TARGET])' + for each substring matching the regular expression REGEX in the + string TARGET, substitute the string SUBST, and return the number + of substitutions. If TARGET is not supplied, use `$0'. + +`index(STR, SEARCH)' + returns the index of the string SEARCH in the string STR, or zero + if SEARCH is not present. + +`length([STR])' + returns the length of the string STR. The length of `$0' is + returned if no argument is supplied. + +`match(STR, REGEX)' + returns the position in STR where the regular expression REGEX + occurs, or zero if REGEX is not present, and sets the values of + `RSTART' and `RLENGTH'. + +`split(STR, ARR [, REGEX])' + splits the string STR into the array ARR on the regular expression + REGEX, and returns the number of elements. If REGEX is omitted, + `FS' is used instead. REGEX can be the null string, causing each + character to be placed into its own array element. The array ARR + is cleared first. + +`sprintf(FMT, EXPR-LIST)' + prints EXPR-LIST according to FMT, and returns the resulting + string. + +`sub(REGEX, SUBST [, TARGET])' + just like `gsub', but only the first matching substring is + replaced. + +`substr(STR, INDEX [, LEN])' + returns the LEN-character substring of STR starting at INDEX. If + LEN is omitted, the rest of STR is used. + +`tolower(STR)' + returns a copy of the string STR, with all the upper-case + characters in STR translated to their corresponding lower-case + counterparts. Non-alphabetic characters are left unchanged. + +`toupper(STR)' + returns a copy of the string STR, with all the lower-case + characters in STR translated to their corresponding upper-case + counterparts. Non-alphabetic characters are left unchanged. + + The I/O related functions are: + +`close(EXPR)' + Close the open file or pipe denoted by EXPR. + +`fflush([EXPR])' + Flush any buffered output for the output file or pipe denoted by + EXPR. If EXPR is omitted, standard output is flushed. If EXPR is + the null string (`""'), all output buffers are flushed. + +`system(CMD-LINE)' + Execute the command CMD-LINE, and return the exit status. If your + operating system does not support `system', calling it will + generate a fatal error. + + `system("")' can be used to force `awk' to flush any pending + output. This is more portable, but less obvious, than calling + `fflush'. + + +File: gawk.info, Node: Time Functions Summary, Next: String Constants Summary, Prev: Built-in Functions Summary, Up: Actions Summary + +Time Functions +-------------- + + The following two functions are available for getting the current +time of day, and for formatting time stamps. They are specific to +`gawk'. + +`systime()' + returns the current time of day as the number of seconds since a + particular epoch (Midnight, January 1, 1970 UTC, on POSIX systems). + +`strftime([FORMAT[, TIMESTAMP]])' + formats TIMESTAMP according to the specification in FORMAT. The + current time of day is used if no TIMESTAMP is supplied. A + default format equivalent to the output of the `date' utility is + used if no FORMAT is supplied. *Note Functions for Dealing with + Time Stamps: Time Functions, for the details on the conversion + specifiers that `strftime' accepts. + + +File: gawk.info, Node: String Constants Summary, Prev: Time Functions Summary, Up: Actions Summary + +String Constants +---------------- + + String constants in `awk' are sequences of characters enclosed in +double quotes (`"'). Within strings, certain "escape sequences" are +recognized, as in C. These are: + +`\\' + A literal backslash. + +`\a' + The "alert" character; usually the ASCII BEL character. + +`\b' + Backspace. + +`\f' + Formfeed. + +`\n' + Newline. + +`\r' + Carriage return. + +`\t' + Horizontal tab. + +`\v' + Vertical tab. + +`\xHEX DIGITS' + The character represented by the string of hexadecimal digits + following the `\x'. As in ANSI C, all following hexadecimal + digits are considered part of the escape sequence. E.g., `"\x1B"' + is a string containing the ASCII ESC (escape) character. (The `\x' + escape sequence is not in POSIX `awk'.) + +`\DDD' + The character represented by the one, two, or three digit sequence + of octal digits. Thus, `"\033"' is also a string containing the + ASCII ESC (escape) character. + +`\C' + The literal character C, if C is not one of the above. + + The escape sequences may also be used inside constant regular +expressions (e.g., the regexp `/[ \t\f\n\r\v]/' matches whitespace +characters). + + *Note Escape Sequences::. + + +File: gawk.info, Node: Functions Summary, Next: Historical Features, Prev: Actions Summary, Up: Gawk Summary + +User-defined Functions +====================== + + Functions in `awk' are defined as follows: + + function NAME(PARAMETER LIST) { STATEMENTS } + + Actual parameters supplied in the function call are used to +instantiate the formal parameters declared in the function. Arrays are +passed by reference, other variables are passed by value. + + If there are fewer arguments passed than there are names in +PARAMETER-LIST, the extra names are given the null string as their +value. Extra names have the effect of local variables. + + The open-parenthesis in a function call of a user-defined function +must immediately follow the function name, without any intervening +white space. This is to avoid a syntactic ambiguity with the +concatenation operator. + + The word `func' may be used in place of `function' (but not in POSIX +`awk'). + + Use the `return' statement to return a value from a function. + + *Note User-defined Functions: User-defined. + + +File: gawk.info, Node: Historical Features, Prev: Functions Summary, Up: Gawk Summary + +Historical Features +=================== + + There are two features of historical `awk' implementations that +`gawk' supports. + + First, it is possible to call the `length' built-in function not only +with no arguments, but even without parentheses! + + a = length + +is the same as either of + + a = length() + a = length($0) + +For example: + + $ echo abcdef | awk '{ print length }' + -| 6 + +This feature is marked as "deprecated" in the POSIX standard, and +`gawk' will issue a warning about its use if `--lint' is specified on +the command line. (The ability to use `length' this way was actually +an accident of the original Unix `awk' implementation. If any built-in +function used `$0' as its default argument, it was possible to call +that function without the parentheses. In particular, it was common +practice to use the `length' function in this fashion, and this usage +was documented in the `awk' manual page.) + + The other historical feature is the use of either the `break' +statement, or the `continue' statement outside the body of a `while', +`for', or `do' loop. Traditional `awk' implementations have treated +such usage as equivalent to the `next' statement. More recent versions +of Unix `awk' do not allow it. `gawk' supports this usage if +`--traditional' has been specified. + + *Note Command Line Options: Options, for more information about the +`--posix' and `--lint' options. + + +File: gawk.info, Node: Installation, Next: Notes, Prev: Gawk Summary, Up: Top + +Installing `gawk' +***************** + + This appendix provides instructions for installing `gawk' on the +various platforms that are supported by the developers. The primary +developers support Unix (and one day, GNU), while the other ports were +contributed. The file `ACKNOWLEDGMENT' in the `gawk' distribution +lists the electronic mail addresses of the people who did the +respective ports, and they are also provided in *Note Reporting +Problems and Bugs: Bugs. + +* Menu: + +* Gawk Distribution:: What is in the `gawk' distribution. +* Unix Installation:: Installing `gawk' under various versions + of Unix. +* VMS Installation:: Installing `gawk' on VMS. +* PC Installation:: Installing and Compiling `gawk' on MS-DOS + and OS/2 +* Atari Installation:: Installing `gawk' on the Atari ST. +* Amiga Installation:: Installing `gawk' on an Amiga. +* Bugs:: Reporting Problems and Bugs. +* Other Versions:: Other freely available `awk' + implementations. + + +File: gawk.info, Node: Gawk Distribution, Next: Unix Installation, Prev: Installation, Up: Installation + +The `gawk' Distribution +======================= + + This section first describes how to get the `gawk' distribution, how +to extract it, and then what is in the various files and subdirectories. + +* Menu: + +* Getting:: How to get the distribution. +* Extracting:: How to extract the distribution. +* Distribution contents:: What is in the distribution. + + +File: gawk.info, Node: Getting, Next: Extracting, Prev: Gawk Distribution, Up: Gawk Distribution + +Getting the `gawk' Distribution +------------------------------- + + There are three ways you can get GNU software. + + 1. You can copy it from someone else who already has it. + + 2. You can order `gawk' directly from the Free Software Foundation. + Software distributions are available for Unix, MS-DOS, and VMS, on + tape, CD-ROM, or floppies (MS-DOS only). The address is: + + Free Software Foundation + 59 Temple Place--Suite 330 + Boston, MA 02111-1307 USA + Phone: +1-617-542-5942 + Fax (including Japan): +1-617-542-2652 + E-mail: `gnu@prep.ai.mit.edu' + + Ordering from the FSF directly contributes to the support of the + foundation and to the production of more free software. + + 3. You can get `gawk' by using anonymous `ftp' to the Internet host + `ftp.gnu.ai.mit.edu', in the directory `/pub/gnu'. + + Here is a list of alternate `ftp' sites from which you can obtain + GNU software. When a site is listed as "SITE`:'DIRECTORY" the + DIRECTORY indicates the directory where GNU software is kept. You + should use a site that is geographically close to you. + + Asia: + + `cair-archive.kaist.ac.kr:/pub/gnu' + `ftp.cs.titech.ac.jp' + `ftp.nectec.or.th:/pub/mirrors/gnu' + `utsun.s.u-tokyo.ac.jp:/ftpsync/prep' + + Australia: + + `archie.au:/gnu' + (`archie.oz' or `archie.oz.au' for ACSnet) + + Africa: + + `ftp.sun.ac.za:/pub/gnu' + + Middle East: + + `ftp.technion.ac.il:/pub/unsupported/gnu' + + Europe: + + `archive.eu.net' + `ftp.denet.dk' + `ftp.eunet.ch' + `ftp.funet.fi:/pub/gnu' + `ftp.ieunet.ie:pub/gnu' + `ftp.informatik.rwth-aachen.de:/pub/gnu' + `ftp.informatik.tu-muenchen.de' + `ftp.luth.se:/pub/unix/gnu' + `ftp.mcc.ac.uk' + `ftp.stacken.kth.se' + `ftp.sunet.se:/pub/gnu' + `ftp.univ-lyon1.fr:pub/gnu' + `ftp.win.tue.nl:/pub/gnu' + `irisa.irisa.fr:/pub/gnu' + `isy.liu.se' + `nic.switch.ch:/mirror/gnu' + `src.doc.ic.ac.uk:/gnu' + `unix.hensa.ac.uk:/pub/uunet/systems/gnu' + + South America: + + `ftp.inf.utfsm.cl:/pub/gnu' + `ftp.unicamp.br:/pub/gnu' + + Western Canada: + + `ftp.cs.ubc.ca:/mirror2/gnu' + + USA: + + `col.hp.com:/mirrors/gnu' + `f.ms.uky.edu:/pub3/gnu' + `ftp.cc.gatech.edu:/pub/gnu' + `ftp.cs.columbia.edu:/archives/gnu/prep' + `ftp.digex.net:/pub/gnu' + `ftp.hawaii.edu:/mirrors/gnu' + `ftp.kpc.com:/pub/mirror/gnu' + + USA (continued): + `ftp.uu.net:/systems/gnu' + `gatekeeper.dec.com:/pub/GNU' + `jaguar.utah.edu:/gnustuff' + `labrea.stanford.edu' + `mrcnext.cso.uiuc.edu:/pub/gnu' + `vixen.cso.uiuc.edu:/gnu' + `wuarchive.wustl.edu:/systems/gnu' + + +File: gawk.info, Node: Extracting, Next: Distribution contents, Prev: Getting, Up: Gawk Distribution + +Extracting the Distribution +--------------------------- + + `gawk' is distributed as a `tar' file compressed with the GNU Zip +program, `gzip'. + + Once you have the distribution (for example, `gawk-3.0.1.tar.gz'), +first use `gzip' to expand the file, and then use `tar' to extract it. +You can use the following pipeline to produce the `gawk' distribution: + + # Under System V, add 'o' to the tar flags + gzip -d -c gawk-3.0.1.tar.gz | tar -xvpf - + +This will create a directory named `gawk-3.0.1' in the current +directory. + + The distribution file name is of the form `gawk-V.R.N.tar.gz'. The +V represents the major version of `gawk', the R represents the current +release of version V, and the N represents a "patch level", meaning +that minor bugs have been fixed in the release. The current patch +level is 0, but when retrieving distributions, you should get the +version with the highest version, release, and patch level. (Note that +release levels greater than or equal to 90 denote "beta," or +non-production software; you may not wish to retrieve such a version +unless you don't mind experimenting.) + + If you are not on a Unix system, you will need to make other +arrangements for getting and extracting the `gawk' distribution. You +should consult a local expert. + + +File: gawk.info, Node: Distribution contents, Prev: Extracting, Up: Gawk Distribution + +Contents of the `gawk' Distribution +----------------------------------- + + The `gawk' distribution has a number of C source files, +documentation files, subdirectories and files related to the +configuration process (*note Compiling and Installing `gawk' on Unix: +Unix Installation.), and several subdirectories related to different, +non-Unix, operating systems. + +various `.c', `.y', and `.h' files + These files are the actual `gawk' source code. + +`README' +`README_d/README.*' + Descriptive files: `README' for `gawk' under Unix, and the rest + for the various hardware and software combinations. + +`INSTALL' + A file providing an overview of the configuration and installation + process. + +`PORTS' + A list of systems to which `gawk' has been ported, and which have + successfully run the test suite. + +`ACKNOWLEDGMENT' + A list of the people who contributed major parts of the code or + documentation. + +`ChangeLog' + A detailed list of source code changes as bugs are fixed or + improvements made. + +`NEWS' + A list of changes to `gawk' since the last release or patch. + +`COPYING' + The GNU General Public License. + +`FUTURES' + A brief list of features and/or changes being contemplated for + future releases, with some indication of the time frame for the + feature, based on its difficulty. + +`LIMITATIONS' + A list of those factors that limit `gawk''s performance. Most of + these depend on the hardware or operating system software, and are + not limits in `gawk' itself. + +`POSIX.STD' + A description of one area where the POSIX standard for `awk' is + incorrect, and how `gawk' handles the problem. + +`PROBLEMS' + A file describing known problems with the current release. + +`doc/awkforai.txt' + A short article describing why `gawk' is a good language for AI + (Artificial Intelligence) programming. + +`doc/README.card' +`doc/ad.block' +`doc/awkcard.in' +`doc/cardfonts' +`doc/colors' +`doc/macros' +`doc/no.colors' +`doc/setter.outline' + The `troff' source for a five-color `awk' reference card. A + modern version of `troff', such as GNU Troff (`groff') is needed + to produce the color version. See the file `README.card' for + instructions if you have an older `troff'. + +`doc/gawk.1' + The `troff' source for a manual page describing `gawk'. This is + distributed for the convenience of Unix users. + +`doc/gawk.texi' + The Texinfo source file for this Info file. It should be + processed with TeX to produce a printed document, and with + `makeinfo' to produce an Info file. + +`doc/gawk.info' + The generated Info file for this Info file. + +`doc/igawk.1' + The `troff' source for a manual page describing the `igawk' + program presented in *Note An Easy Way to Use Library Functions: + Igawk Program. + +`doc/Makefile.in' + The input file used during the configuration process to generate + the actual `Makefile' for creating the documentation. + +`Makefile.in' +`acconfig.h' +`aclocal.m4' +`configh.in' +`configure.in' +`configure' +`custom.h' +`missing/*' + These files and subdirectory are used when configuring `gawk' for + various Unix systems. They are explained in detail in *Note + Compiling and Installing `gawk' on Unix: Unix Installation. + +`awklib/extract.awk' +`awklib/Makefile.in' + The `awklib' directory contains a copy of `extract.awk' (*note + Extracting Programs from Texinfo Source Files: Extract Program.), + which can be used to extract the sample programs from the Texinfo + source file for this Info file, and a `Makefile.in' file, which + `configure' uses to generate a `Makefile'. As part of the process + of building `gawk', the library functions from *Note A Library of + `awk' Functions: Library Functions, and the `igawk' program from + *Note An Easy Way to Use Library Functions: Igawk Program, are + extracted into ready to use files. They are installed as part of + the installation process. + +`amiga/*' + Files needed for building `gawk' on an Amiga. *Note Installing + `gawk' on an Amiga: Amiga Installation, for details. + +`atari/*' + Files needed for building `gawk' on an Atari ST. *Note Installing + `gawk' on the Atari ST: Atari Installation, for details. + +`pc/*' + Files needed for building `gawk' under MS-DOS and OS/2. *Note + MS-DOS and OS/2 Installation and Compilation: PC Installation, for + details. + +`vms/*' + Files needed for building `gawk' under VMS. *Note How to Compile + and Install `gawk' on VMS: VMS Installation, for details. + +`test/*' + A test suite for `gawk'. You can use `make check' from the top + level `gawk' directory to run your version of `gawk' against the + test suite. If `gawk' successfully passes `make check' then you + can be confident of a successful port. + + +File: gawk.info, Node: Unix Installation, Next: VMS Installation, Prev: Gawk Distribution, Up: Installation + +Compiling and Installing `gawk' on Unix +======================================= + + Usually, you can compile and install `gawk' by typing only two +commands. However, if you do use an unusual system, you may need to +configure `gawk' for your system yourself. + +* Menu: + +* Quick Installation:: Compiling `gawk' under Unix. +* Configuration Philosophy:: How it's all supposed to work. + + +File: gawk.info, Node: Quick Installation, Next: Configuration Philosophy, Prev: Unix Installation, Up: Unix Installation + +Compiling `gawk' for Unix +------------------------- + + After you have extracted the `gawk' distribution, `cd' to +`gawk-3.0.1'. Like most GNU software, `gawk' is configured +automatically for your Unix system by running the `configure' program. +This program is a Bourne shell script that was generated automatically +using GNU `autoconf'. (The `autoconf' software is described fully +starting with *Note Introduction: (autoconf)Top.) + + To configure `gawk', simply run `configure': + + sh ./configure + + This produces a `Makefile' and `config.h' tailored to your system. +The `config.h' file describes various facts about your system. You may +wish to edit the `Makefile' to change the `CFLAGS' variable, which +controls the command line options that are passed to the C compiler +(such as optimization levels, or compiling for debugging). + + Alternatively, you can add your own values for most `make' +variables, such as `CC' and `CFLAGS', on the command line when running +`configure': + + CC=cc CFLAGS=-g sh ./configure + +See the file `INSTALL' in the `gawk' distribution for all the details. + + After you have run `configure', and possibly edited the `Makefile', +type: + + make + +and shortly thereafter, you should have an executable version of `gawk'. +That's all there is to it! (If these steps do not work, please send in +a bug report; *note Reporting Problems and Bugs: Bugs..) + + +File: gawk.info, Node: Configuration Philosophy, Prev: Quick Installation, Up: Unix Installation + +The Configuration Process +------------------------- + + (This section is of interest only if you know something about using +the C language and the Unix operating system.) + + The source code for `gawk' generally attempts to adhere to formal +standards wherever possible. This means that `gawk' uses library +routines that are specified by the ANSI C standard and by the POSIX +operating system interface standard. When using an ANSI C compiler, +function prototypes are used to help improve the compile-time checking. + + Many Unix systems do not support all of either the ANSI or the POSIX +standards. The `missing' subdirectory in the `gawk' distribution +contains replacement versions of those subroutines that are most likely +to be missing. + + The `config.h' file that is created by the `configure' program +contains definitions that describe features of the particular operating +system where you are attempting to compile `gawk'. The three things +described by this file are what header files are available, so that +they can be correctly included, what (supposedly) standard functions +are actually available in your C libraries, and other miscellaneous +facts about your variant of Unix. For example, there may not be an +`st_blksize' element in the `stat' structure. In this case +`HAVE_ST_BLKSIZE' would be undefined. + + It is possible for your C compiler to lie to `configure'. It may do +so by not exiting with an error when a library function is not +available. To get around this, you can edit the file `custom.h'. Use +an `#ifdef' that is appropriate for your system, and either `#define' +any constants that `configure' should have defined but didn't, or +`#undef' any constants that `configure' defined and should not have. +`custom.h' is automatically included by `config.h'. + + It is also possible that the `configure' program generated by +`autoconf' will not work on your system in some other fashion. If you +do have a problem, the file `configure.in' is the input for `autoconf'. +You may be able to change this file, and generate a new version of +`configure' that will work on your system. *Note Reporting Problems +and Bugs: Bugs, for information on how to report problems in +configuring `gawk'. The same mechanism may be used to send in updates +to `configure.in' and/or `custom.h'. + + +File: gawk.info, Node: VMS Installation, Next: PC Installation, Prev: Unix Installation, Up: Installation + +How to Compile and Install `gawk' on VMS +======================================== + + This section describes how to compile and install `gawk' under VMS. + +* Menu: + +* VMS Compilation:: How to compile `gawk' under VMS. +* VMS Installation Details:: How to install `gawk' under VMS. +* VMS Running:: How to run `gawk' under VMS. +* VMS POSIX:: Alternate instructions for VMS POSIX. + + +File: gawk.info, Node: VMS Compilation, Next: VMS Installation Details, Prev: VMS Installation, Up: VMS Installation + +Compiling `gawk' on VMS +----------------------- + + To compile `gawk' under VMS, there is a `DCL' command procedure that +will issue all the necessary `CC' and `LINK' commands, and there is +also a `Makefile' for use with the `MMS' utility. From the source +directory, use either + + $ @[.VMS]VMSBUILD.COM + +or + + $ MMS/DESCRIPTION=[.VMS]DESCRIP.MMS GAWK + + Depending upon which C compiler you are using, follow one of the sets +of instructions in this table: + +VAX C V3.x + Use either `vmsbuild.com' or `descrip.mms' as is. These use + `CC/OPTIMIZE=NOLINE', which is essential for Version 3.0. + +VAX C V2.x + You must have Version 2.3 or 2.4; older ones won't work. Edit + either `vmsbuild.com' or `descrip.mms' according to the comments + in them. For `vmsbuild.com', this just entails removing two `!' + delimiters. Also edit `config.h' (which is a copy of file + `[.config]vms-conf.h') and comment out or delete the two lines + `#define __STDC__ 0' and `#define VAXC_BUILTINS' near the end. + +GNU C + Edit `vmsbuild.com' or `descrip.mms'; the changes are different + from those for VAX C V2.x, but equally straightforward. No + changes to `config.h' should be needed. + +DEC C + Edit `vmsbuild.com' or `descrip.mms' according to their comments. + No changes to `config.h' should be needed. + + `gawk' has been tested under VAX/VMS 5.5-1 using VAX C V3.2, GNU C +1.40 and 2.3. It should work without modifications for VMS V4.6 and up. + + +File: gawk.info, Node: VMS Installation Details, Next: VMS Running, Prev: VMS Compilation, Up: VMS Installation + +Installing `gawk' on VMS +------------------------ + + To install `gawk', all you need is a "foreign" command, which is a +`DCL' symbol whose value begins with a dollar sign. For example: + + $ GAWK :== $disk1:[gnubin]GAWK + +(Substitute the actual location of `gawk.exe' for `$disk1:[gnubin]'.) +The symbol should be placed in the `login.com' of any user who wishes +to run `gawk', so that it will be defined every time the user logs on. +Alternatively, the symbol may be placed in the system-wide +`sylogin.com' procedure, which will allow all users to run `gawk'. + + Optionally, the help entry can be loaded into a VMS help library: + + $ LIBRARY/HELP SYS$HELP:HELPLIB [.VMS]GAWK.HLP + +(You may want to substitute a site-specific help library rather than +the standard VMS library `HELPLIB'.) After loading the help text, + + $ HELP GAWK + +will provide information about both the `gawk' implementation and the +`awk' programming language. + + The logical name `AWK_LIBRARY' can designate a default location for +`awk' program files. For the `-f' option, if the specified filename +has no device or directory path information in it, `gawk' will look in +the current directory first, then in the directory specified by the +translation of `AWK_LIBRARY' if the file was not found. If after +searching in both directories, the file still is not found, then `gawk' +appends the suffix `.awk' to the filename and the file search will be +re-tried. If `AWK_LIBRARY' is not defined, that portion of the file +search will fail benignly. + + +File: gawk.info, Node: VMS Running, Next: VMS POSIX, Prev: VMS Installation Details, Up: VMS Installation + +Running `gawk' on VMS +--------------------- + + Command line parsing and quoting conventions are significantly +different on VMS, so examples in this Info file or from other sources +often need minor changes. They _are_ minor though, and all `awk' +programs should run correctly. + + Here are a couple of trivial tests: + + $ gawk -- "BEGIN {print ""Hello, World!""}" + $ gawk -"W" version + ! could also be -"W version" or "-W version" + +Note that upper-case and mixed-case text must be quoted. + + The VMS port of `gawk' includes a `DCL'-style interface in addition +to the original shell-style interface (see the help entry for details). +One side-effect of dual command line parsing is that if there is only a +single parameter (as in the quoted string program above), the command +becomes ambiguous. To work around this, the normally optional `--' +flag is required to force Unix style rather than `DCL' parsing. If any +other dash-type options (or multiple parameters such as data files to be +processed) are present, there is no ambiguity and `--' can be omitted. + + The default search path when looking for `awk' program files +specified by the `-f' option is `"SYS$DISK:[],AWK_LIBRARY:"'. The +logical name `AWKPATH' can be used to override this default. The format +of `AWKPATH' is a comma-separated list of directory specifications. +When defining it, the value should be quoted so that it retains a single +translation, and not a multi-translation `RMS' searchlist. + + +File: gawk.info, Node: VMS POSIX, Prev: VMS Running, Up: VMS Installation + +Building and Using `gawk' on VMS POSIX +-------------------------------------- + + Ignore the instructions above, although `vms/gawk.hlp' should still +be made available in a help library. The source tree should be unpacked +into a container file subsystem rather than into the ordinary VMS file +system. Make sure that the two scripts, `configure' and +`vms/posix-cc.sh', are executable; use `chmod +x' on them if necessary. +Then execute the following two commands: + + psx> CC=vms/posix-cc.sh configure + psx> make CC=c89 gawk + +The first command will construct files `config.h' and `Makefile' out of +templates, using a script to make the C compiler fit `configure''s +expectations. The second command will compile and link `gawk' using +the C compiler directly; ignore any warnings from `make' about being +unable to redefine `CC'. `configure' will take a very long time to +execute, but at least it provides incremental feedback as it runs. + + This has been tested with VAX/VMS V6.2, VMS POSIX V2.0, and DEC C +V5.2. + + Once built, `gawk' will work like any other shell utility. Unlike +the normal VMS port of `gawk', no special command line manipulation is +needed in the VMS POSIX environment. + + +File: gawk.info, Node: PC Installation, Next: Atari Installation, Prev: VMS Installation, Up: Installation + +MS-DOS and OS/2 Installation and Compilation +============================================ + + If you have received a binary distribution prepared by the DOS +maintainers, then `gawk' and the necessary support files will appear +under the `gnu' directory, with executables in `gnu/bin', libraries in +`gnu/lib/awk', and manual pages under `gnu/man'. This is designed for +easy installation to a `/gnu' directory on your drive, but the files +can be installed anywhere provided `AWKPATH' is set properly. +Regardless of the installation directory, the first line of `igawk.cmd' +and `igawk.bat' (in `gnu/bin') may need to be edited. + + The binary distribution will contain a separate file describing the +contents. In particular, it may include more than one version of the +`gawk' executable. OS/2 binary distributions may have a different +arrangement, but installation is similar. + + The OS/2 and MS-DOS versions of `gawk' search for program files as +described in *Note The `AWKPATH' Environment Variable: AWKPATH Variable. +However, semicolons (rather than colons) separate elements in the +`AWKPATH' variable. If `AWKPATH' is not set or is empty, then the +default search path is `".;c:/lib/awk;c:/gnu/lib/awk"'. + + An `sh'-like shell (as opposed to `command.com' under MS-DOS or +`cmd.exe' under OS/2) may be useful for `awk' programming. Ian +Stewartson has written an excellent shell for MS-DOS and OS/2, and a +`ksh' clone and GNU Bash are available for OS/2. The file +`README_d/README.pc' in the `gawk' distribution contains information on +these shells. Users of Stewartson's shell on DOS should examine its +documentation on handling of command-lines. In particular, the setting +for `gawk' in the shell configuration may need to be changed, and the +`ignoretype' option may also be of interest. + + `gawk' can be compiled for MS-DOS and OS/2 using the GNU development +tools from DJ Delorie (DJGPP, MS-DOS-only) or Eberhard Mattes (EMX, +MS-DOS and OS/2). Microsoft C can be used to build 16-bit versions for +MS-DOS and OS/2. The file `README_d/README.pc' in the `gawk' +distribution contains additional notes, and `pc/Makefile' contains +important notes on compilation options. + + To build `gawk', copy the files in the `pc' directory (_except_ for +`ChangeLog') to the directory with the rest of the `gawk' sources. The +`Makefile' contains a configuration section with comments, and may need +to be edited in order to work with your `make' utility. + + The `Makefile' contains a number of targets for building various +MS-DOS and OS/2 versions. A list of targets will be printed if the +`make' command is given without a target. As an example, to build `gawk' +using the DJGPP tools, enter `make djgpp'. + + Using `make' to run the standard tests and to install `gawk' +requires additional Unix-like tools, including `sh', `sed', and `cp'. +In order to run the tests, the `test/*.ok' files may need to be +converted so that they have the usual DOS-style end-of-line markers. +Most of the tests will work properly with Stewartson's shell along with +the companion utilities or appropriate GNU utilities. However, some +editing of `test/Makefile' is required. It is recommended that the file +`pc/Makefile.tst' be copied to `test/Makefile' as a replacement. +Details can be found in `README_d/README.pc'. + + +File: gawk.info, Node: Atari Installation, Next: Amiga Installation, Prev: PC Installation, Up: Installation + +Installing `gawk' on the Atari ST +================================= + + There are no substantial differences when installing `gawk' on +various Atari models. Compiled `gawk' executables do not require a +large amount of memory with most `awk' programs and should run on all +Motorola processor based models (called further ST, even if that is not +exactly right). + + In order to use `gawk', you need to have a shell, either text or +graphics, that does not map all the characters of a command line to +upper-case. Maintaining case distinction in option flags is very +important (*note Command Line Options: Options.). These days this is +the default, and it may only be a problem for some very old machines. +If your system does not preserve the case of option flags, you will +need to upgrade your tools. Support for I/O redirection is necessary +to make it easy to import `awk' programs from other environments. +Pipes are nice to have, but not vital. + +* Menu: + +* Atari Compiling:: Compiling `gawk' on Atari +* Atari Using:: Running `gawk' on Atari + + +File: gawk.info, Node: Atari Compiling, Next: Atari Using, Prev: Atari Installation, Up: Atari Installation + +Compiling `gawk' on the Atari ST +-------------------------------- + + A proper compilation of `gawk' sources when `sizeof(int)' differs +from `sizeof(void *)' requires an ANSI C compiler. An initial port was +done with `gcc'. You may actually prefer executables where `int's are +four bytes wide, but the other variant works as well. + + You may need quite a bit of memory when trying to recompile the +`gawk' sources, as some source files (`regex.c' in particular) are quite +big. If you run out of memory compiling such a file, try reducing the +optimization level for this particular file; this may help. + + With a reasonable shell (Bash will do), and in particular if you run +Linux, MiNT or a similar operating system, you have a pretty good +chance that the `configure' utility will succeed. Otherwise sample +versions of `config.h' and `Makefile.st' are given in the `atari' +subdirectory and can be edited and copied to the corresponding files in +the main source directory. Even if `configure' produced something, it +might be advisable to compare its results with the sample versions and +possibly make adjustments. + + Some `gawk' source code fragments depend on a preprocessor define +`atarist'. This basically assumes the TOS environment with `gcc'. +Modify these sections as appropriate if they are not right for your +environment. Also see the remarks about `AWKPATH' and `envsep' in +*Note Running `gawk' on the Atari ST: Atari Using. + + As shipped, the sample `config.h' claims that the `system' function +is missing from the libraries, which is not true, and an alternative +implementation of this function is provided in `atari/system.c'. +Depending upon your particular combination of shell and operating +system, you may wish to change the file to indicate that `system' is +available. + + +File: gawk.info, Node: Atari Using, Prev: Atari Compiling, Up: Atari Installation + +Running `gawk' on the Atari ST +------------------------------ + + An executable version of `gawk' should be placed, as usual, anywhere +in your `PATH' where your shell can find it. + + While executing, `gawk' creates a number of temporary files. When +using `gcc' libraries for TOS, `gawk' looks for either of the +environment variables `TEMP' or `TMPDIR', in that order. If either one +is found, its value is assumed to be a directory for temporary files. +This directory must exist, and if you can spare the memory, it is a +good idea to put it on a RAM drive. If neither `TEMP' nor `TMPDIR' are +found, then `gawk' uses the current directory for its temporary files. + + The ST version of `gawk' searches for its program files as described +in *Note The `AWKPATH' Environment Variable: AWKPATH Variable. The +default value for the `AWKPATH' variable is taken from `DEFPATH' +defined in `Makefile'. The sample `gcc'/TOS `Makefile' for the ST in +the distribution sets `DEFPATH' to `".,c:\lib\awk,c:\gnu\lib\awk"'. +The search path can be modified by explicitly setting `AWKPATH' to +whatever you wish. Note that colons cannot be used on the ST to +separate elements in the `AWKPATH' variable, since they have another, +reserved, meaning. Instead, you must use a comma to separate elements +in the path. When recompiling, the separating character can be +modified by initializing the `envsep' variable in `atari/gawkmisc.atr' +to another value. + + Although `awk' allows great flexibility in doing I/O redirections +from within a program, this facility should be used with care on the ST +running under TOS. In some circumstances the OS routines for file +handle pool processing lose track of certain events, causing the +computer to crash, and requiring a reboot. Often a warm reboot is +sufficient. Fortunately, this happens infrequently, and in rather +esoteric situations. In particular, avoid having one part of an `awk' +program using `print' statements explicitly redirected to +`"/dev/stdout"', while other `print' statements use the default +standard output, and a calling shell has redirected standard output to +a file. + + When `gawk' is compiled with the ST version of `gcc' and its usual +libraries, it will accept both `/' and `\' as path separators. While +this is convenient, it should be remembered that this removes one, +technically valid, character (`/') from your file names, and that it +may create problems for external programs, called via the `system' +function, which may not support this convention. Whenever it is +possible that a file created by `gawk' will be used by some other +program, use only backslashes. Also remember that in `awk', +backslashes in strings have to be doubled in order to get literal +backslashes (*note Escape Sequences::). + + +File: gawk.info, Node: Amiga Installation, Next: Bugs, Prev: Atari Installation, Up: Installation + +Installing `gawk' on an Amiga +============================= + + You can install `gawk' on an Amiga system using a Unix emulation +environment available via anonymous `ftp' from `wuarchive.wustl.edu' in +the directory `pub/aminet/dev/gcc'. This includes a shell based on +`pdksh'. The primary component of this environment is a Unix emulation +library, `ixemul.lib'. + + A more complete distribution for the Amiga is available on the +FreshFish CD-ROM from: + + CRONUS + 1840 E. Warner Road #105-265 + Tempe, AZ 85284 USA + US Toll Free: (800) 804-0833 + Phone: +1-602-491-0442 + FAX: +1-602-491-0048 + Email: `info@ninemoons.com' + WWW: `http://www.ninemoons.com' + Anonymous `ftp' site: `ftp.ninemoons.com' + + Once you have the distribution, you can configure `gawk' simply by +running `configure': + + configure -v m68k-cbm-amigados + + Then run `make', and you should be all set! (If these steps do not +work, please send in a bug report; *note Reporting Problems and Bugs: +Bugs..) + + +File: gawk.info, Node: Bugs, Next: Other Versions, Prev: Amiga Installation, Up: Installation + +Reporting Problems and Bugs +=========================== + + If you have problems with `gawk' or think that you have found a bug, +please report it to the developers; we cannot promise to do anything +but we might well want to fix it. + + Before reporting a bug, make sure you have actually found a real bug. +Carefully reread the documentation and see if it really says you can do +what you're trying to do. If it's not clear whether you should be able +to do something or not, report that too; it's a bug in the +documentation! + + Before reporting a bug or trying to fix it yourself, try to isolate +it to the smallest possible `awk' program and input data file that +reproduces the problem. Then send us the program and data file, some +idea of what kind of Unix system you're using, and the exact results +`gawk' gave you. Also say what you expected to occur; this will help +us decide whether the problem was really in the documentation. + + Once you have a precise problem, there are two e-mail addresses you +can send mail to. + +Internet: + `bug-gnu-utils@prep.ai.mit.edu' + +UUCP: + `uunet!prep.ai.mit.edu!bug-gnu-utils' + + Please include the version number of `gawk' you are using. You can +get this information with the command `gawk --version'. You should +send a carbon copy of your mail to Arnold Robbins, who can be reached +at `arnold@gnu.ai.mit.edu'. + + *Important!* Do _not_ try to report bugs in `gawk' by posting to the +Usenet/Internet newsgroup `comp.lang.awk'. While the `gawk' developers +do occasionally read this newsgroup, there is no guarantee that we will +see your posting. The steps described above are the official, +recognized ways for reporting bugs. + + Non-bug suggestions are always welcome as well. If you have +questions about things that are unclear in the documentation or are +just obscure features, ask Arnold Robbins; he will try to help you out, +although he may not have the time to fix the problem. You can send him +electronic mail at the Internet address above. + + If you find bugs in one of the non-Unix ports of `gawk', please send +an electronic mail message to the person who maintains that port. They +are listed below, and also in the `README' file in the `gawk' +distribution. Information in the `README' file should be considered +authoritative if it conflicts with this Info file. + + The people maintaining the non-Unix ports of `gawk' are: + +MS-DOS + Scott Deifik, `scottd@amgen.com', and Darrel Hankerson, + `hankedr@mail.auburn.edu'. + +OS/2 + Kai Uwe Rommel, `rommel@ars.de'. + +VMS + Pat Rankin, `rankin@eql.caltech.edu'. + +Atari ST + Michal Jaegermann, `michal@gortel.phys.ualberta.ca'. + +Amiga + Fred Fish, `fnf@ninemoons.com'. + + If your bug is also reproducible under Unix, please send copies of +your report to the general GNU bug list, as well as to Arnold Robbins, +at the addresses listed above. + + +File: gawk.info, Node: Other Versions, Prev: Bugs, Up: Installation + +Other Freely Available `awk' Implementations +============================================ + + It's kind of fun to put comments like this in your awk code. + `// Do C++ comments work? answer: yes! of course' + Michael Brennan + + There are two other freely available `awk' implementations. This +section briefly describes where to get them. + +Unix `awk' + Brian Kernighan has been able to make his implementation of `awk' + freely available. You can get it via anonymous `ftp' to the host + `netlib.att.com'. Change directory to `/netlib/research'. Use + "binary" or "image" mode, and retrieve `awk.bundle.Z'. + + This is a shell archive that has been compressed with the + `compress' utility. It can be uncompressed with either + `uncompress' or the GNU `gunzip' utility. + + This version requires an ANSI C compiler; GCC (the GNU C compiler) + works quite nicely. + +`mawk' + Michael Brennan has written an independent implementation of `awk', + called `mawk'. It is available under the GPL (*note GNU GENERAL + PUBLIC LICENSE: Copying.), just as `gawk' is. + + You can get it via anonymous `ftp' to the host `ftp.whidbey.net'. + Change directory to `/pub/brennan'. Use "binary" or "image" mode, + and retrieve `mawk1.3.3.tar.gz' (or the latest version that is + there). + + `gunzip' may be used to decompress this file. Installation is + similar to `gawk''s (*note Compiling and Installing `gawk' on + Unix: Unix Installation.). + + +File: gawk.info, Node: Notes, Next: Glossary, Prev: Installation, Up: Top + +Implementation Notes +******************** + + This appendix contains information mainly of interest to +implementors and maintainers of `gawk'. Everything in it applies +specifically to `gawk', and not to other implementations. + +* Menu: + +* Compatibility Mode:: How to disable certain `gawk' extensions. +* Additions:: Making Additions To `gawk'. +* Future Extensions:: New features that may be implemented one day. +* Improvements:: Suggestions for improvements by volunteers. + + +File: gawk.info, Node: Compatibility Mode, Next: Additions, Prev: Notes, Up: Notes + +Downward Compatibility and Debugging +==================================== + + *Note Extensions in `gawk' Not in POSIX `awk': POSIX/GNU, for a +summary of the GNU extensions to the `awk' language and program. All +of these features can be turned off by invoking `gawk' with the +`--traditional' option, or with the `--posix' option. + + If `gawk' is compiled for debugging with `-DDEBUG', then there is +one more option available on the command line: + +`-W parsedebug' +`--parsedebug' + Print out the parse stack information as the program is being + parsed. + + This option is intended only for serious `gawk' developers, and not +for the casual user. It probably has not even been compiled into your +version of `gawk', since it slows down execution. + + +File: gawk.info, Node: Additions, Next: Future Extensions, Prev: Compatibility Mode, Up: Notes + +Making Additions to `gawk' +========================== + + If you should find that you wish to enhance `gawk' in a significant +fashion, you are perfectly free to do so. That is the point of having +free software; the source code is available, and you are free to change +it as you wish (*note GNU GENERAL PUBLIC LICENSE: Copying.). + + This section discusses the ways you might wish to change `gawk', and +any considerations you should bear in mind. + +* Menu: + +* Adding Code:: Adding code to the main body of `gawk'. +* New Ports:: Porting `gawk' to a new operating system. + + +File: gawk.info, Node: Adding Code, Next: New Ports, Prev: Additions, Up: Additions + +Adding New Features +------------------- + + You are free to add any new features you like to `gawk'. However, +if you want your changes to be incorporated into the `gawk' +distribution, there are several steps that you need to take in order to +make it possible for me to include to your changes. + + 1. Get the latest version. It is much easier for me to integrate + changes if they are relative to the most recent distributed + version of `gawk'. If your version of `gawk' is very old, I may + not be able to integrate them at all. *Note Getting the `gawk' + Distribution: Getting, for information on getting the latest + version of `gawk'. + + 2. See *note (Version)Top:: standards, GNU Coding Standards. This + document describes how GNU software should be written. If you + haven't read it, please do so, preferably _before_ starting to + modify `gawk'. (The `GNU Coding Standards' are available as part + of the Autoconf distribution, from the FSF.) + + 3. Use the `gawk' coding style. The C code for `gawk' follows the + instructions in the `GNU Coding Standards', with minor exceptions. + The code is formatted using the traditional "K&R" style, + particularly as regards the placement of braces and the use of + tabs. In brief, the coding rules for `gawk' are: + + * Use old style (non-prototype) function headers when defining + functions. + + * Put the name of the function at the beginning of its own line. + + * Put the return type of the function, even if it is `int', on + the line above the line with the name and arguments of the + function. + + * The declarations for the function arguments should not be + indented. + + * Put spaces around parentheses used in control structures + (`if', `while', `for', `do', `switch' and `return'). + + * Do not put spaces in front of parentheses used in function + calls. + + * Put spaces around all C operators, and after commas in + function calls. + + * Do not use the comma operator to produce multiple + side-effects, except in `for' loop initialization and + increment parts, and in macro bodies. + + * Use real tabs for indenting, not spaces. + + * Use the "K&R" brace layout style. + + * Use comparisons against `NULL' and `'\0'' in the conditions of + `if', `while' and `for' statements, and in the `case's of + `switch' statements, instead of just the plain pointer or + character value. + + * Use the `TRUE', `FALSE', and `NULL' symbolic constants, and + the character constant `'\0'' where appropriate, instead of + `1' and `0'. + + * Provide one-line descriptive comments for each function. + + * Do not use `#elif'. Many older Unix C compilers cannot handle + it. + + * Do not use the `alloca' function for allocating memory off + the stack. Its use causes more portability trouble than the + minor benefit of not having to free the storage. Instead, use + `malloc' and `free'. + + If I have to reformat your code to follow the coding style used in + `gawk', I may not bother. + + 4. Be prepared to sign the appropriate paperwork. In order for the + FSF to distribute your changes, you must either place those + changes in the public domain, and submit a signed statement to that + effect, or assign the copyright in your changes to the FSF. Both + of these actions are easy to do, and _many_ people have done so + already. If you have questions, please contact me (*note Reporting + Problems and Bugs: Bugs.), or `gnu@prep.ai.mit.edu'. + + 5. Update the documentation. Along with your new code, please supply + new sections and or chapters for this Info file. If at all + possible, please use real Texinfo, instead of just supplying + unformatted ASCII text (although even that is better than no + documentation at all). Conventions to be followed in `The GNU Awk + User's Guide' are provided after the `@bye' at the end of the + Texinfo source file. If possible, please update the man page as + well. + + You will also have to sign paperwork for your documentation + changes. + + 6. Submit changes as context diffs or unified diffs. Use `diff -c -r + -N' or `diff -u -r -N' to compare the original `gawk' source tree + with your version. (I find context diffs to be more readable, but + unified diffs are more compact.) I recommend using the GNU + version of `diff'. Send the output produced by either run of + `diff' to me when you submit your changes. *Note Reporting + Problems and Bugs: Bugs, for the electronic mail information. + + Using this format makes it easy for me to apply your changes to the + master version of the `gawk' source code (using `patch'). If I + have to apply the changes manually, using a text editor, I may not + do so, particularly if there are lots of changes. + + Although this sounds like a lot of work, please remember that while +you may write the new code, I have to maintain it and support it, and +if it isn't possible for me to do that with a minimum of extra work, +then I probably will not. + + +File: gawk.info, Node: New Ports, Prev: Adding Code, Up: Additions + +Porting `gawk' to a New Operating System +---------------------------------------- + + If you wish to port `gawk' to a new operating system, there are +several steps to follow. + + 1. Follow the guidelines in *Note Adding New Features: Adding Code, + concerning coding style, submission of diffs, and so on. + + 2. When doing a port, bear in mind that your code must co-exist + peacefully with the rest of `gawk', and the other ports. Avoid + gratuitous changes to the system-independent parts of the code. If + at all possible, avoid sprinkling `#ifdef's just for your port + throughout the code. + + If the changes needed for a particular system affect too much of + the code, I probably will not accept them. In such a case, you + will, of course, be able to distribute your changes on your own, + as long as you comply with the GPL (*note GNU GENERAL PUBLIC + LICENSE: Copying.). + + 3. A number of the files that come with `gawk' are maintained by other + people at the Free Software Foundation. Thus, you should not + change them unless it is for a very good reason. I.e. changes are + not out of the question, but changes to these files will be + scrutinized extra carefully. The files are `alloca.c', + `getopt.h', `getopt.c', `getopt1.c', `regex.h', `regex.c', `dfa.h', + `dfa.c', `install-sh', and `mkinstalldirs'. + + 4. Be willing to continue to maintain the port. Non-Unix operating + systems are supported by volunteers who maintain the code needed + to compile and run `gawk' on their systems. If no-one volunteers + to maintain a port, that port becomes unsupported, and it may be + necessary to remove it from the distribution. + + 5. Supply an appropriate `gawkmisc.???' file. Each port has its own + `gawkmisc.???' that implements certain operating system specific + functions. This is cleaner than a plethora of `#ifdef's scattered + throughout the code. The `gawkmisc.c' in the main source + directory includes the appropriate `gawkmisc.???' file from each + subdirectory. Be sure to update it as well. + + Each port's `gawkmisc.???' file has a suffix reminiscent of the + machine or operating system for the port. For example, + `pc/gawkmisc.pc' and `vms/gawkmisc.vms'. The use of separate + suffixes, instead of plain `gawkmisc.c', makes it possible to move + files from a port's subdirectory into the main subdirectory, + without accidentally destroying the real `gawkmisc.c' file. + (Currently, this is only an issue for the MS-DOS and OS/2 ports.) + + 6. Supply a `Makefile' and any other C source and header files that + are necessary for your operating system. All your code should be + in a separate subdirectory, with a name that is the same as, or + reminiscent of, either your operating system or the computer + system. If possible, try to structure things so that it is not + necessary to move files out of the subdirectory into the main + source directory. If that is not possible, then be sure to avoid + using names for your files that duplicate the names of files in + the main source directory. + + 7. Update the documentation. Please write a section (or sections) + for this Info file describing the installation and compilation + steps needed to install and/or compile `gawk' for your system. + + 8. Be prepared to sign the appropriate paperwork. In order for the + FSF to distribute your code, you must either place your code in + the public domain, and submit a signed statement to that effect, + or assign the copyright in your code to the FSF. Both of these + actions are easy to do, and _many_ people have done so already. If + you have questions, please contact me, or `gnu@prep.ai.mit.edu'. + + Following these steps will make it much easier to integrate your +changes into `gawk', and have them co-exist happily with the code for +other operating systems that is already there. + + In the code that you supply, and that you maintain, feel free to use +a coding style and brace layout that suits your taste. + + +File: gawk.info, Node: Future Extensions, Next: Improvements, Prev: Additions, Up: Notes + +Probable Future Extensions +========================== + + AWK is a language similar to PERL, only considerably more elegant. + Arnold Robbins + + Hey! + Larry Wall + + This section briefly lists extensions and possible improvements that +indicate the directions we are currently considering for `gawk'. The +file `FUTURES' in the `gawk' distributions lists these extensions as +well. + + This is a list of probable future changes that will be usable by the +`awk' language programmer. + +Localization + The GNU project is starting to support multiple languages. It + will at least be possible to make `gawk' print its warnings and + error messages in languages other than English. It may be + possible for `awk' programs to also use the multiple language + facilities, separate from `gawk' itself. + +Databases + It may be possible to map a GDBM/NDBM/SDBM file into an `awk' + array. + +A `PROCINFO' Array + The special files that provide process-related information (*note + Special File Names in `gawk': Special Files.) may be superseded + by a `PROCINFO' array that would provide the same information, in + an easier to access fashion. + +More `lint' warnings + There are more things that could be checked for portability. + +Control of subprocess environment + Changes made in `gawk' to the array `ENVIRON' may be propagated to + subprocesses run by `gawk'. + + This is a list of probable improvements that will make `gawk' +perform better. + +An Improved Version of `dfa' + The `dfa' pattern matcher from GNU `grep' has some problems. + Either a new version or a fixed one will deal with some important + regexp matching issues. + +Use of GNU `malloc' + The GNU version of `malloc' could potentially speed up `gawk', + since it relies heavily on the use of dynamic memory allocation. + +Use of the `rx' regexp library + The `rx' regular expression library could potentially speed up all + regexp operations that require knowing the exact location of + matches. This includes record termination, field and array + splitting, and the `sub', `gsub', `gensub' and `match' functions. + + +File: gawk.info, Node: Improvements, Prev: Future Extensions, Up: Notes + +Suggestions for Improvements +============================ + + Here are some projects that would-be `gawk' hackers might like to +take on. They vary in size from a few days to a few weeks of +programming, depending on which one you choose and how fast a +programmer you are. Please send any improvements you write to the +maintainers at the GNU project. *Note Adding New Features: Adding Code, +for guidelines to follow when adding new features to `gawk'. *Note +Reporting Problems and Bugs: Bugs, for information on contacting the +maintainers. + + 1. Compilation of `awk' programs: `gawk' uses a Bison (YACC-like) + parser to convert the script given it into a syntax tree; the + syntax tree is then executed by a simple recursive evaluator. + This method incurs a lot of overhead, since the recursive + evaluator performs many procedure calls to do even the simplest + things. + + It should be possible for `gawk' to convert the script's parse tree + into a C program which the user would then compile, using the + normal C compiler and a special `gawk' library to provide all the + needed functions (regexps, fields, associative arrays, type + coercion, and so on). + + An easier possibility might be for an intermediate phase of `awk' + to convert the parse tree into a linear byte code form like the + one used in GNU Emacs Lisp. The recursive evaluator would then be + replaced by a straight line byte code interpreter that would be + intermediate in speed between running a compiled program and doing + what `gawk' does now. + + 2. The programs in the test suite could use documenting in this + Info file. + + 3. See the `FUTURES' file for more ideas. Contact us if you would + seriously like to tackle any of the items listed there. + + +File: gawk.info, Node: Glossary, Next: Copying, Prev: Notes, Up: Top + +Glossary +******** + +Action + A series of `awk' statements attached to a rule. If the rule's + pattern matches an input record, `awk' executes the rule's action. + Actions are always enclosed in curly braces. *Note Overview of + Actions: Action Overview. + +Amazing `awk' Assembler + Henry Spencer at the University of Toronto wrote a retargetable + assembler completely as `awk' scripts. It is thousands of lines + long, including machine descriptions for several eight-bit + microcomputers. It is a good example of a program that would have + been better written in another language. + +Amazingly Workable Formatter (`awf') + Henry Spencer at the University of Toronto wrote a formatter that + accepts a large subset of the `nroff -ms' and `nroff -man' + formatting commands, using `awk' and `sh'. + +ANSI + The American National Standards Institute. This organization + produces many standards, among them the standards for the C and + C++ programming languages. + +Assignment + An `awk' expression that changes the value of some `awk' variable + or data object. An object that you can assign to is called an + "lvalue". The assigned values are called "rvalues". *Note + Assignment Expressions: Assignment Ops. + +`awk' Language + The language in which `awk' programs are written. + +`awk' Program + An `awk' program consists of a series of "patterns" and "actions", + collectively known as "rules". For each input record given to the + program, the program's rules are all processed in turn. `awk' + programs may also contain function definitions. + +`awk' Script + Another name for an `awk' program. + +Bash + The GNU version of the standard shell (the Bourne-Again shell). + See "Bourne Shell." + +BBS + See "Bulletin Board System." + +Boolean Expression + Named after the English mathematician Boole. See "Logical + Expression." + +Bourne Shell + The standard shell (`/bin/sh') on Unix and Unix-like systems, + originally written by Steven R. Bourne. Many shells (Bash, `ksh', + `pdksh', `zsh') are generally upwardly compatible with the Bourne + shell. + +Built-in Function + The `awk' language provides built-in functions that perform various + numerical, time stamp related, and string computations. Examples + are `sqrt' (for the square root of a number) and `substr' (for a + substring of a string). *Note Built-in Functions: Built-in. + +Built-in Variable + `ARGC', `ARGIND', `ARGV', `CONVFMT', `ENVIRON', `ERRNO', + `FIELDWIDTHS', `FILENAME', `FNR', `FS', `IGNORECASE', `NF', `NR', + `OFMT', `OFS', `ORS', `RLENGTH', `RSTART', `RS', `RT', and + `SUBSEP', are the variables that have special meaning to `awk'. + Changing some of them affects `awk''s running environment. + Several of these variables are specific to `gawk'. *Note Built-in + Variables::. + +Braces + See "Curly Braces." + +Bulletin Board System + A computer system allowing users to log in and read and/or leave + messages for other users of the system, much like leaving paper + notes on a bulletin board. + +C + The system programming language that most GNU software is written + in. The `awk' programming language has C-like syntax, and this + Info file points out similarities between `awk' and C when + appropriate. + +Character Set + The set of numeric codes used by a computer system to represent the + characters (letters, numbers, punctuation, etc.) of a particular + country or place. The most common character set in use today is + ASCII (American Standard Code for Information Interchange). Many + European countries use an extension of ASCII known as ISO-8859-1 + (ISO Latin-1). + +CHEM + A preprocessor for `pic' that reads descriptions of molecules and + produces `pic' input for drawing them. It was written in `awk' by + Brian Kernighan and Jon Bentley, and is available from + `netlib@research.att.com'. + +Compound Statement + A series of `awk' statements, enclosed in curly braces. Compound + statements may be nested. *Note Control Statements in Actions: + Statements. + +Concatenation + Concatenating two strings means sticking them together, one after + another, giving a new string. For example, the string `foo' + concatenated with the string `bar' gives the string `foobar'. + *Note String Concatenation: Concatenation. + +Conditional Expression + An expression using the `?:' ternary operator, such as `EXPR1 ? + EXPR2 : EXPR3'. The expression EXPR1 is evaluated; if the result + is true, the value of the whole expression is the value of EXPR2, + otherwise the value is EXPR3. In either case, only one of EXPR2 + and EXPR3 is evaluated. *Note Conditional Expressions: + Conditional Exp. + +Comparison Expression + A relation that is either true or false, such as `(a < b)'. + Comparison expressions are used in `if', `while', `do', and `for' + statements, and in patterns to select which input records to + process. *Note Variable Typing and Comparison Expressions: Typing + and Comparison. + +Curly Braces + The characters `{' and `}'. Curly braces are used in `awk' for + delimiting actions, compound statements, and function bodies. + +Dark Corner + An area in the language where specifications often were (or still + are) not clear, leading to unexpected or undesirable behavior. + Such areas are marked in this Info file with "(d.c.)" in the text, + and are indexed under the heading "dark corner." + +Data Objects + These are numbers and strings of characters. Numbers are + converted into strings and vice versa, as needed. *Note + Conversion of Strings and Numbers: Conversion. + +Double Precision + An internal representation of numbers that can have fractional + parts. Double precision numbers keep track of more digits than do + single precision numbers, but operations on them are more + expensive. This is the way `awk' stores numeric values. It is + the C type `double'. + +Dynamic Regular Expression + A dynamic regular expression is a regular expression written as an + ordinary expression. It could be a string constant, such as + `"foo"', but it may also be an expression whose value can vary. + *Note Using Dynamic Regexps: Computed Regexps. + +Environment + A collection of strings, of the form NAME`='VAL, that each program + has available to it. Users generally place values into the + environment in order to provide information to various programs. + Typical examples are the environment variables `HOME' and `PATH'. + +Empty String + See "Null String." + +Escape Sequences + A special sequence of characters used for describing non-printing + characters, such as `\n' for newline, or `\033' for the ASCII ESC + (escape) character. *Note Escape Sequences::. + +Field + When `awk' reads an input record, it splits the record into pieces + separated by whitespace (or by a separator regexp which you can + change by setting the built-in variable `FS'). Such pieces are + called fields. If the pieces are of fixed length, you can use the + built-in variable `FIELDWIDTHS' to describe their lengths. *Note + Specifying How Fields are Separated: Field Separators, and also see + *Note Reading Fixed-width Data: Constant Size. + +Floating Point Number + Often referred to in mathematical terms as a "rational" number, + this is just a number that can have a fractional part. See + "Double Precision" and "Single Precision." + +Format + Format strings are used to control the appearance of output in the + `printf' statement. Also, data conversions from numbers to strings + are controlled by the format string contained in the built-in + variable `CONVFMT'. *Note Format-Control Letters: Control Letters. + +Function + A specialized group of statements used to encapsulate general or + program-specific tasks. `awk' has a number of built-in functions, + and also allows you to define your own. *Note Built-in Functions: + Built-in, and *Note User-defined Functions: User-defined. + +FSF + See "Free Software Foundation." + +Free Software Foundation + A non-profit organization dedicated to the production and + distribution of freely distributable software. It was founded by + Richard M. Stallman, the author of the original Emacs editor. GNU + Emacs is the most widely used version of Emacs today. + +`gawk' + The GNU implementation of `awk'. + +General Public License + This document describes the terms under which `gawk' and its source + code may be distributed. (*note GNU GENERAL PUBLIC LICENSE: + Copying.) + +GNU + "GNU's not Unix". An on-going project of the Free Software + Foundation to create a complete, freely distributable, + POSIX-compliant computing environment. + +GPL + See "General Public License." + +Hexadecimal + Base 16 notation, where the digits are `0'-`9' and `A'-`F', with + `A' representing 10, `B' representing 11, and so on up to `F' for + 15. Hexadecimal numbers are written in C using a leading `0x', to + indicate their base. Thus, `0x12' is 18 (one times 16 plus 2). + +I/O + Abbreviation for "Input/Output," the act of moving data into and/or + out of a running program. + +Input Record + A single chunk of data read in by `awk'. Usually, an `awk' input + record consists of one line of text. *Note How Input is Split + into Records: Records. + +Integer + A whole number, i.e. a number that does not have a fractional part. + +Keyword + In the `awk' language, a keyword is a word that has special + meaning. Keywords are reserved and may not be used as variable + names. + + `gawk''s keywords are: `BEGIN', `END', `if', `else', `while', + `do...while', `for', `for...in', `break', `continue', `delete', + `next', `nextfile', `function', `func', and `exit'. + +Logical Expression + An expression using the operators for logic, AND, OR, and NOT, + written `&&', `||', and `!' in `awk'. Often called Boolean + expressions, after the mathematician who pioneered this kind of + mathematical logic. + +Lvalue + An expression that can appear on the left side of an assignment + operator. In most languages, lvalues can be variables or array + elements. In `awk', a field designator can also be used as an + lvalue. + +Null String + A string with no characters in it. It is represented explicitly in + `awk' programs by placing two double-quote characters next to each + other (`""'). It can appear in input data by having two successive + occurrences of the field separator appear next to each other. + +Number + A numeric valued data object. The `gawk' implementation uses + double precision floating point to represent numbers. Very old + `awk' implementations use single precision floating point. + +Octal + Base-eight notation, where the digits are `0'-`7'. Octal numbers + are written in C using a leading `0', to indicate their base. + Thus, `013' is 11 (one times 8 plus 3). + +Pattern + Patterns tell `awk' which input records are interesting to which + rules. + + A pattern is an arbitrary conditional expression against which + input is tested. If the condition is satisfied, the pattern is + said to "match" the input record. A typical pattern might compare + the input record against a regular expression. *Note Pattern + Elements: Pattern Overview. + +POSIX + The name for a series of standards being developed by the IEEE + that specify a Portable Operating System interface. The "IX" + denotes the Unix heritage of these standards. The main standard + of interest for `awk' users is `IEEE Standard for Information + Technology, Standard 1003.2-1992, Portable Operating System + Interface (POSIX) Part 2: Shell and Utilities'. Informally, this + standard is often referred to as simply "P1003.2." + +Private + Variables and/or functions that are meant for use exclusively by + library functions, and not for the main `awk' program. Special + care must be taken when naming such variables and functions. + *Note Naming Library Function Global Variables: Library Names. + +Range (of input lines) + A sequence of consecutive lines from the input file. A pattern + can specify ranges of input lines for `awk' to process, or it can + specify single lines. *Note Pattern Elements: Pattern Overview. + +Recursion + When a function calls itself, either directly or indirectly. If + this isn't clear, refer to the entry for "recursion." + +Redirection + Redirection means performing input from other than the standard + input stream, or output to other than the standard output stream. + + You can redirect the output of the `print' and `printf' statements + to a file or a system command, using the `>', `>>', and `|' + operators. You can redirect input to the `getline' statement using + the `<' and `|' operators. *Note Redirecting Output of `print' + and `printf': Redirection, and *Note Explicit Input with + `getline': Getline. + +Regexp + Short for "regular expression". A regexp is a pattern that + denotes a set of strings, possibly an infinite set. For example, + the regexp `R.*xp' matches any string starting with the letter `R' + and ending with the letters `xp'. In `awk', regexps are used in + patterns and in conditional expressions. Regexps may contain + escape sequences. *Note Regular Expressions: Regexp. + +Regular Expression + See "regexp." + +Regular Expression Constant + A regular expression constant is a regular expression written + within slashes, such as `/foo/'. This regular expression is chosen + when you write the `awk' program, and cannot be changed doing its + execution. *Note How to Use Regular Expressions: Regexp Usage. + +Rule + A segment of an `awk' program that specifies how to process single + input records. A rule consists of a "pattern" and an "action". + `awk' reads an input record; then, for each rule, if the input + record satisfies the rule's pattern, `awk' executes the rule's + action. Otherwise, the rule does nothing for that input record. + +Rvalue + A value that can appear on the right side of an assignment + operator. In `awk', essentially every expression has a value. + These values are rvalues. + +`sed' + See "Stream Editor." + +Short-Circuit + The nature of the `awk' logical operators `&&' and `||'. If the + value of the entire expression can be deduced from evaluating just + the left-hand side of these operators, the right-hand side will not + be evaluated (*note Boolean Expressions: Boolean Ops.). + +Side Effect + A side effect occurs when an expression has an effect aside from + merely producing a value. Assignment expressions, increment and + decrement expressions and function calls have side effects. *Note + Assignment Expressions: Assignment Ops. + +Single Precision + An internal representation of numbers that can have fractional + parts. Single precision numbers keep track of fewer digits than + do double precision numbers, but operations on them are less + expensive in terms of CPU time. This is the type used by some + very old versions of `awk' to store numeric values. It is the C + type `float'. + +Space + The character generated by hitting the space bar on the keyboard. + +Special File + A file name interpreted internally by `gawk', instead of being + handed directly to the underlying operating system. For example, + `/dev/stderr'. *Note Special File Names in `gawk': Special Files. + +Stream Editor + A program that reads records from an input stream and processes + them one or more at a time. This is in contrast with batch + programs, which may expect to read their input files in entirety + before starting to do anything, and with interactive programs, + which require input from the user. + +String + A datum consisting of a sequence of characters, such as `I am a + string'. Constant strings are written with double-quotes in the + `awk' language, and may contain escape sequences. *Note Escape + Sequences::. + +Tab + The character generated by hitting the `TAB' key on the keyboard. + It usually expands to up to eight spaces upon output. + +Unix + A computer operating system originally developed in the early + 1970's at AT&T Bell Laboratories. It initially became popular in + universities around the world, and later moved into commercial + evnironments as a software development system and network server + system. There are many commercial versions of Unix, as well as + several work-alike systems whose source code is freely available + (such as Linux, NetBSD, and FreeBSD). + +Whitespace + A sequence of space, tab, or newline characters occurring inside + an input record or a string. + + +File: gawk.info, Node: Copying, Next: Index, Prev: Glossary, Up: Top + +GNU GENERAL PUBLIC LICENSE +************************** + + Version 2, June 1991 + + Copyright (C) 1989, 1991 Free Software Foundation, Inc. + 59 Temple Place --- Suite 330, Boston, MA 02111-1307, USA + + Everyone is permitted to copy and distribute verbatim copies + of this license document, but changing it is not allowed. + +Preamble +======== + + The licenses for most software are designed to take away your +freedom to share and change it. By contrast, the GNU General Public +License is intended to guarantee your freedom to share and change free +software--to make sure the software is free for all its users. This +General Public License applies to most of the Free Software +Foundation's software and to any other program whose authors commit to +using it. (Some other Free Software Foundation software is covered by +the GNU Library General Public License instead.) You can apply it to +your programs, too. + + When we speak of free software, we are referring to freedom, not +price. Our General Public Licenses are designed to make sure that you +have the freedom to distribute copies of free software (and charge for +this service if you wish), that you receive source code or can get it +if you want it, that you can change the software or use pieces of it in +new free programs; and that you know you can do these things. + + To protect your rights, we need to make restrictions that forbid +anyone to deny you these rights or to ask you to surrender the rights. +These restrictions translate to certain responsibilities for you if you +distribute copies of the software, or if you modify it. + + For example, if you distribute copies of such a program, whether +gratis or for a fee, you must give the recipients all the rights that +you have. You must make sure that they, too, receive or can get the +source code. And you must show them these terms so they know their +rights. + + We protect your rights with two steps: (1) copyright the software, +and (2) offer you this license which gives you legal permission to copy, +distribute and/or modify the software. + + Also, for each author's protection and ours, we want to make certain +that everyone understands that there is no warranty for this free +software. If the software is modified by someone else and passed on, we +want its recipients to know that what they have is not the original, so +that any problems introduced by others will not reflect on the original +authors' reputations. + + Finally, any free program is threatened constantly by software +patents. We wish to avoid the danger that redistributors of a free +program will individually obtain patent licenses, in effect making the +program proprietary. To prevent this, we have made it clear that any +patent must be licensed for everyone's free use or not licensed at all. + + The precise terms and conditions for copying, distribution and +modification follow. + + TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION + + 0. This License applies to any program or other work which contains a + notice placed by the copyright holder saying it may be distributed + under the terms of this General Public License. The "Program", + below, refers to any such program or work, and a "work based on + the Program" means either the Program or any derivative work under + copyright law: that is to say, a work containing the Program or a + portion of it, either verbatim or with modifications and/or + translated into another language. (Hereinafter, translation is + included without limitation in the term "modification".) Each + licensee is addressed as "you". + + Activities other than copying, distribution and modification are + not covered by this License; they are outside its scope. The act + of running the Program is not restricted, and the output from the + Program is covered only if its contents constitute a work based on + the Program (independent of having been made by running the + Program). Whether that is true depends on what the Program does. + + 1. You may copy and distribute verbatim copies of the Program's + source code as you receive it, in any medium, provided that you + conspicuously and appropriately publish on each copy an appropriate + copyright notice and disclaimer of warranty; keep intact all the + notices that refer to this License and to the absence of any + warranty; and give any other recipients of the Program a copy of + this License along with the Program. + + You may charge a fee for the physical act of transferring a copy, + and you may at your option offer warranty protection in exchange + for a fee. + + 2. You may modify your copy or copies of the Program or any portion + of it, thus forming a work based on the Program, and copy and + distribute such modifications or work under the terms of Section 1 + above, provided that you also meet all of these conditions: + + a. You must cause the modified files to carry prominent notices + stating that you changed the files and the date of any change. + + b. You must cause any work that you distribute or publish, that + in whole or in part contains or is derived from the Program + or any part thereof, to be licensed as a whole at no charge + to all third parties under the terms of this License. + + c. If the modified program normally reads commands interactively + when run, you must cause it, when started running for such + interactive use in the most ordinary way, to print or display + an announcement including an appropriate copyright notice and + a notice that there is no warranty (or else, saying that you + provide a warranty) and that users may redistribute the + program under these conditions, and telling the user how to + view a copy of this License. (Exception: if the Program + itself is interactive but does not normally print such an + announcement, your work based on the Program is not required + to print an announcement.) + + These requirements apply to the modified work as a whole. If + identifiable sections of that work are not derived from the + Program, and can be reasonably considered independent and separate + works in themselves, then this License, and its terms, do not + apply to those sections when you distribute them as separate + works. But when you distribute the same sections as part of a + whole which is a work based on the Program, the distribution of + the whole must be on the terms of this License, whose permissions + for other licensees extend to the entire whole, and thus to each + and every part regardless of who wrote it. + + Thus, it is not the intent of this section to claim rights or + contest your rights to work written entirely by you; rather, the + intent is to exercise the right to control the distribution of + derivative or collective works based on the Program. + + In addition, mere aggregation of another work not based on the + Program with the Program (or with a work based on the Program) on + a volume of a storage or distribution medium does not bring the + other work under the scope of this License. + + 3. You may copy and distribute the Program (or a work based on it, + under Section 2) in object code or executable form under the terms + of Sections 1 and 2 above provided that you also do one of the + following: + + a. Accompany it with the complete corresponding machine-readable + source code, which must be distributed under the terms of + Sections 1 and 2 above on a medium customarily used for + software interchange; or, + + b. Accompany it with a written offer, valid for at least three + years, to give any third party, for a charge no more than your + cost of physically performing source distribution, a complete + machine-readable copy of the corresponding source code, to be + distributed under the terms of Sections 1 and 2 above on a + medium customarily used for software interchange; or, + + c. Accompany it with the information you received as to the offer + to distribute corresponding source code. (This alternative is + allowed only for non-commercial distribution and only if you + received the program in object code or executable form with + such an offer, in accord with Subsection b above.) + + The source code for a work means the preferred form of the work for + making modifications to it. For an executable work, complete + source code means all the source code for all modules it contains, + plus any associated interface definition files, plus the scripts + used to control compilation and installation of the executable. + However, as a special exception, the source code distributed need + not include anything that is normally distributed (in either + source or binary form) with the major components (compiler, + kernel, and so on) of the operating system on which the executable + runs, unless that component itself accompanies the executable. + + If distribution of executable or object code is made by offering + access to copy from a designated place, then offering equivalent + access to copy the source code from the same place counts as + distribution of the source code, even though third parties are not + compelled to copy the source along with the object code. + + 4. You may not copy, modify, sublicense, or distribute the Program + except as expressly provided under this License. Any attempt + otherwise to copy, modify, sublicense or distribute the Program is + void, and will automatically terminate your rights under this + License. However, parties who have received copies, or rights, + from you under this License will not have their licenses + terminated so long as such parties remain in full compliance. + + 5. You are not required to accept this License, since you have not + signed it. However, nothing else grants you permission to modify + or distribute the Program or its derivative works. These actions + are prohibited by law if you do not accept this License. + Therefore, by modifying or distributing the Program (or any work + based on the Program), you indicate your acceptance of this + License to do so, and all its terms and conditions for copying, + distributing or modifying the Program or works based on it. + + 6. Each time you redistribute the Program (or any work based on the + Program), the recipient automatically receives a license from the + original licensor to copy, distribute or modify the Program + subject to these terms and conditions. You may not impose any + further restrictions on the recipients' exercise of the rights + granted herein. You are not responsible for enforcing compliance + by third parties to this License. + + 7. If, as a consequence of a court judgment or allegation of patent + infringement or for any other reason (not limited to patent + issues), conditions are imposed on you (whether by court order, + agreement or otherwise) that contradict the conditions of this + License, they do not excuse you from the conditions of this + License. If you cannot distribute so as to satisfy simultaneously + your obligations under this License and any other pertinent + obligations, then as a consequence you may not distribute the + Program at all. For example, if a patent license would not permit + royalty-free redistribution of the Program by all those who + receive copies directly or indirectly through you, then the only + way you could satisfy both it and this License would be to refrain + entirely from distribution of the Program. + + If any portion of this section is held invalid or unenforceable + under any particular circumstance, the balance of the section is + intended to apply and the section as a whole is intended to apply + in other circumstances. + + It is not the purpose of this section to induce you to infringe any + patents or other property right claims or to contest validity of + any such claims; this section has the sole purpose of protecting + the integrity of the free software distribution system, which is + implemented by public license practices. Many people have made + generous contributions to the wide range of software distributed + through that system in reliance on consistent application of that + system; it is up to the author/donor to decide if he or she is + willing to distribute software through any other system and a + licensee cannot impose that choice. + + This section is intended to make thoroughly clear what is believed + to be a consequence of the rest of this License. + + 8. If the distribution and/or use of the Program is restricted in + certain countries either by patents or by copyrighted interfaces, + the original copyright holder who places the Program under this + License may add an explicit geographical distribution limitation + excluding those countries, so that distribution is permitted only + in or among countries not thus excluded. In such case, this + License incorporates the limitation as if written in the body of + this License. + + 9. The Free Software Foundation may publish revised and/or new + versions of the General Public License from time to time. Such + new versions will be similar in spirit to the present version, but + may differ in detail to address new problems or concerns. + + Each version is given a distinguishing version number. If the + Program specifies a version number of this License which applies + to it and "any later version", you have the option of following + the terms and conditions either of that version or of any later + version published by the Free Software Foundation. If the Program + does not specify a version number of this License, you may choose + any version ever published by the Free Software Foundation. + + 10. If you wish to incorporate parts of the Program into other free + programs whose distribution conditions are different, write to the + author to ask for permission. For software which is copyrighted + by the Free Software Foundation, write to the Free Software + Foundation; we sometimes make exceptions for this. Our decision + will be guided by the two goals of preserving the free status of + all derivatives of our free software and of promoting the sharing + and reuse of software generally. + + NO WARRANTY + + 11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO + WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE + LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT + HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT + WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT + NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND + FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE + QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE + PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY + SERVICING, REPAIR OR CORRECTION. + + 12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN + WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY + MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE + LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, + INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR + INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF + DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU + OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY + OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN + ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. + + END OF TERMS AND CONDITIONS + +How to Apply These Terms to Your New Programs +============================================= + + If you develop a new program, and you want it to be of the greatest +possible use to the public, the best way to achieve this is to make it +free software which everyone can redistribute and change under these +terms. + + To do so, attach the following notices to the program. It is safest +to attach them to the start of each source file to most effectively +convey the exclusion of warranty; and each file should have at least +the "copyright" line and a pointer to where the full notice is found. + + ONE LINE TO GIVE THE PROGRAM'S NAME AND AN IDEA OF WHAT IT DOES. + Copyright (C) 19YY NAME OF AUTHOR + + This program is free software; you can redistribute it and/or + modify it under the terms of the GNU General Public License + as published by the Free Software Foundation; either version 2 + of the License, or (at your option) any later version. + + This program is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU General Public License for more details. + + You should have received a copy of the GNU General Public License + along with this program; if not, write to the Free Software + Foundation, Inc., 59 Temple Place --- Suite 330, Boston, MA 02111-1307, USA. + + Also add information on how to contact you by electronic and paper +mail. + + If the program is interactive, make it output a short notice like +this when it starts in an interactive mode: + + Gnomovision version 69, Copyright (C) 19YY NAME OF AUTHOR + Gnomovision comes with ABSOLUTELY NO WARRANTY; for details + type `show w'. This is free software, and you are welcome + to redistribute it under certain conditions; type `show c' + for details. + + The hypothetical commands `show w' and `show c' should show the +appropriate parts of the General Public License. Of course, the +commands you use may be called something other than `show w' and `show +c'; they could even be mouse-clicks or menu items--whatever suits your +program. + + You should also get your employer (if you work as a programmer) or +your school, if any, to sign a "copyright disclaimer" for the program, +if necessary. Here is a sample; alter the names: + + Yoyodyne, Inc., hereby disclaims all copyright + interest in the program `Gnomovision' + (which makes passes at compilers) written + by James Hacker. + + SIGNATURE OF TY COON, 1 April 1989 + Ty Coon, President of Vice + + This General Public License does not permit incorporating your +program into proprietary programs. If your program is a subroutine +library, you may consider it more useful to permit linking proprietary +applications with the library. If this is what you want to do, use the +GNU Library General Public License instead of this License. + + +File: gawk.info, Node: Index, Prev: Copying, Up: Top + +Index +***** + +* Menu: + +* ! operator: Boolean Ops. +* != operator: Typing and Comparison. +* !~ operator <1>: Typing and Comparison. +* !~ operator <2>: Regexp Constants. +* !~ operator <3>: Computed Regexps. +* !~ operator <4>: Case-sensitivity. +* !~ operator: Regexp Usage. +* # (comment): Comments. +* #! (executable scripts): Executable Scripts. +* $ (field operator): Fields. +* && operator: Boolean Ops. +* --assign option: Options. +* --compat option: Options. +* --copyleft option: Options. +* --copyright option: Options. +* --field-separator option: Options. +* --file option: Options. +* --help option: Options. +* --lint option: Options. +* --lint-old option: Options. +* --posix option: Options. +* --source option: Options. +* --traditional option: Options. +* --usage option: Options. +* --version option: Options. +* -f option: Options. +* -F option <1>: Options. +* -F option: Command Line Field Separator. +* -f option: Long. +* -v option: Options. +* -W option: Options. +* /dev/fd: Special Files. +* /dev/pgrpid: Special Files. +* /dev/pid: Special Files. +* /dev/ppid: Special Files. +* /dev/stderr: Special Files. +* /dev/stdin: Special Files. +* /dev/stdout: Special Files. +* /dev/user <1>: Passwd Functions. +* /dev/user: Special Files. +* < operator: Typing and Comparison. +* <= operator: Typing and Comparison. +* == operator: Typing and Comparison. +* > operator: Typing and Comparison. +* >= operator: Typing and Comparison. +* \' regexp operator: GNU Regexp Operators. +* \< regexp operator: GNU Regexp Operators. +* \> regexp operator: GNU Regexp Operators. +* \` regexp operator: GNU Regexp Operators. +* \B regexp operator: GNU Regexp Operators. +* \W regexp operator: GNU Regexp Operators. +* \w regexp operator: GNU Regexp Operators. +* \y regexp operator: GNU Regexp Operators. +* _gr_init: Group Functions. +* _pw_init: Passwd Functions. +* _tm_addup: Mktime Function. +* _tm_isleap: Mktime Function. +* accessing fields: Fields. +* account information <1>: Group Functions. +* account information: Passwd Functions. +* acronym: History. +* action, curly braces: Action Overview. +* action, default: Very Simple. +* action, definition of: Action Overview. +* action, empty: Very Simple. +* action, separating statements: Action Overview. +* adding new features: Adding Code. +* addition: Arithmetic Ops. +* Aho, Alfred: History. +* AI programming, using gawk: Distribution contents. +* alarm.awk: Alarm Program. +* amiga: Amiga Installation. +* anchors in regexps: Regexp Operators. +* and operator: Boolean Ops. +* anonymous ftp <1>: Other Versions. +* anonymous ftp: Getting. +* applications of awk: When. +* ARGC: Auto-set. +* ARGIND <1>: Other Arguments. +* ARGIND: Auto-set. +* argument processing: Getopt Function. +* arguments in function call: Function Calls. +* arguments, command line: Invoking Gawk. +* ARGV <1>: Other Arguments. +* ARGV: Auto-set. +* arithmetic operators: Arithmetic Ops. +* array assignment: Assigning Elements. +* array reference: Reference to Elements. +* array subscripts, uninitialized variables: Uninitialized Subscripts. +* arrays: Array Intro. +* arrays, associative: Array Intro. +* arrays, definition of: Array Intro. +* arrays, deleting an element: Delete. +* arrays, deleting entire contents: Delete. +* arrays, multi-dimensional subscripts: Multi-dimensional. +* arrays, presence of elements: Reference to Elements. +* arrays, sparse: Array Intro. +* arrays, special for statement: Scanning an Array. +* arrays, the in operator: Reference to Elements. +* artificial intelligence, using gawk: Distribution contents. +* ASCII: Ordinal Functions. +* assert: Assert Function. +* assert, C version: Assert Function. +* assertions: Assert Function. +* assignment operators: Assignment Ops. +* assignment to fields: Changing Fields. +* associative arrays: Array Intro. +* atan2: Numeric Functions. +* atari: Atari Installation. +* automatic initialization: More Complex. +* awk language, POSIX version <1>: Definition Syntax. +* awk language, POSIX version <2>: String Functions. +* awk language, POSIX version <3>: User-modified. +* awk language, POSIX version <4>: Next Statement. +* awk language, POSIX version <5>: Continue Statement. +* awk language, POSIX version <6>: Break Statement. +* awk language, POSIX version <7>: Precedence. +* awk language, POSIX version <8>: Assignment Ops. +* awk language, POSIX version <9>: Arithmetic Ops. +* awk language, POSIX version <10>: Conversion. +* awk language, POSIX version <11>: Format Modifiers. +* awk language, POSIX version <12>: OFMT. +* awk language, POSIX version <13>: Field Splitting Summary. +* awk language, POSIX version <14>: Regexp Operators. +* awk language, POSIX version: Escape Sequences. +* awk language, V.4 version <1>: SVR4. +* awk language, V.4 version: Escape Sequences. +* AWKPATH environment variable: AWKPATH Variable. +* awksed: Simple Sed. +* backslash continuation <1>: Egrep Program. +* backslash continuation: Statements/Lines. +* backslash continuation and comments: Statements/Lines. +* backslash continuation in csh <1>: Statements/Lines. +* backslash continuation in csh: More Complex. +* basic function of awk: Getting Started. +* BBS-list file: Sample Data Files. +* BEGIN special pattern: BEGIN/END. +* beginfile: Filetrans Function. +* body of a loop: While Statement. +* book, using this: This Manual. +* boolean expressions: Boolean Ops. +* boolean operators: Boolean Ops. +* break statement: Break Statement. +* break, outside of loops: Break Statement. +* Brennan, Michael <1>: Other Versions. +* Brennan, Michael <2>: Simple Sed. +* Brennan, Michael: Delete. +* buffer matching operators: GNU Regexp Operators. +* buffering output: I/O Functions. +* buffering, interactive vs. non-interactive: I/O Functions. +* buffering, non-interactive vs. interactive: I/O Functions. +* buffers, flushing: I/O Functions. +* bugs, known in gawk: Known Bugs. +* built-in functions: Built-in. +* built-in variables: Built-in Variables. +* built-in variables, convey information: Auto-set. +* built-in variables, user modifiable: User-modified. +* call by reference: Function Caveats. +* call by value: Function Caveats. +* calling a function <1>: Function Caveats. +* calling a function: Function Calls. +* case conversion: String Functions. +* case sensitivity: Case-sensitivity. +* changing contents of a field: Changing Fields. +* changing the record separator: Records. +* character classes: Regexp Operators. +* character encodings: Ordinal Functions. +* character list: Regexp Operators. +* character list, complemented: Regexp Operators. +* character sets: Ordinal Functions. +* chr: Ordinal Functions. +* close <1>: I/O Functions. +* close: Close Files And Pipes. +* closing input files and pipes: Close Files And Pipes. +* closing output files and pipes: Close Files And Pipes. +* coding style used in gawk: Adding Code. +* collating elements: Regexp Operators. +* collating symbols: Regexp Operators. +* command line: Invoking Gawk. +* command line formats: Running gawk. +* command line, setting FS on: Command Line Field Separator. +* comments: Comments. +* comments and backslash continuation: Statements/Lines. +* common mistakes <1>: Typing and Comparison. +* common mistakes <2>: Print Examples. +* common mistakes <3>: Basic Field Splitting. +* common mistakes: Computed Regexps. +* comp.lang.awk: Bugs. +* comparison expressions: Typing and Comparison. +* comparisons, string vs. regexp: Typing and Comparison. +* compatibility mode <1>: POSIX/GNU. +* compatibility mode: Options. +* complemented character list: Regexp Operators. +* compound statement: Statements. +* computed regular expressions: Computed Regexps. +* concatenation: Concatenation. +* conditional expression: Conditional Exp. +* configuring gawk: Configuration Philosophy. +* constants, types of: Constants. +* continuation of lines: Statements/Lines. +* continue statement: Continue Statement. +* continue, outside of loops: Continue Statement. +* control statement: Statements. +* conversion of case: String Functions. +* conversion of strings and numbers: Conversion. +* conversions, during subscripting: Numeric Array Subscripts. +* converting dates to timestamps: Mktime Function. +* CONVFMT <1>: Numeric Array Subscripts. +* CONVFMT <2>: User-modified. +* CONVFMT: Conversion. +* cos: Numeric Functions. +* csh, backslash continuation <1>: Statements/Lines. +* csh, backslash continuation: More Complex. +* curly braces: Action Overview. +* custom.h configuration file: Configuration Philosophy. +* cut utility: Cut Program. +* cut.awk: Cut Program. +* d.c., see "dark corner": This Manual. +* dark corner <1>: Other Arguments. +* dark corner <2>: Invoking Gawk. +* dark corner <3>: String Functions. +* dark corner <4>: Uninitialized Subscripts. +* dark corner <5>: Auto-set. +* dark corner <6>: Exit Statement. +* dark corner <7>: Continue Statement. +* dark corner <8>: Break Statement. +* dark corner <9>: Using BEGIN/END. +* dark corner <10>: Truth Values. +* dark corner <11>: Conversion. +* dark corner <12>: Assignment Options. +* dark corner <13>: Using Constant Regexps. +* dark corner <14>: Format Modifiers. +* dark corner <15>: Control Letters. +* dark corner <16>: OFMT. +* dark corner <17>: Getline Summary. +* dark corner <18>: Plain Getline. +* dark corner <19>: Multiple Line. +* dark corner <20>: Field Splitting Summary. +* dark corner <21>: Single Character Fields. +* dark corner <22>: Records. +* dark corner <23>: Escape Sequences. +* dark corner: This Manual. +* data-driven languages: Getting Started. +* dates, converting to timestamps: Mktime Function. +* decrement operators: Increment Ops. +* default action: Very Simple. +* default pattern: Very Simple. +* defining functions: Definition Syntax. +* Deifik, Scott <1>: Bugs. +* Deifik, Scott: Acknowledgements. +* delete statement: Delete. +* deleting elements of arrays: Delete. +* deleting entire arrays: Delete. +* deprecated features: Obsolete. +* deprecated options: Obsolete. +* differences between gawk and awk <1>: AWKPATH Variable. +* differences between gawk and awk <2>: String Functions. +* differences between gawk and awk <3>: Calling Built-in. +* differences between gawk and awk <4>: Delete. +* differences between gawk and awk <5>: Nextfile Statement. +* differences between gawk and awk <6>: I/O And BEGIN/END. +* differences between gawk and awk <7>: Conditional Exp. +* differences between gawk and awk <8>: Arithmetic Ops. +* differences between gawk and awk <9>: Using Constant Regexps. +* differences between gawk and awk <10>: Scalar Constants. +* differences between gawk and awk <11>: Close Files And Pipes. +* differences between gawk and awk <12>: Special Files. +* differences between gawk and awk <13>: Redirection. +* differences between gawk and awk <14>: Getline Summary. +* differences between gawk and awk <15>: Getline Intro. +* differences between gawk and awk <16>: Single Character Fields. +* differences between gawk and awk <17>: Records. +* differences between gawk and awk: Case-sensitivity. +* directory search: AWKPATH Variable. +* division: Arithmetic Ops. +* documenting awk programs <1>: Library Names. +* documenting awk programs: Comments. +* dupword.awk: Dupword Program. +* dynamic regular expressions: Computed Regexps. +* EBCDIC: Ordinal Functions. +* egrep <1>: Regexp Operators. +* egrep: One-shot. +* egrep utility: Egrep Program. +* egrep.awk: Egrep Program. +* element assignment: Assigning Elements. +* element of array: Reference to Elements. +* empty action: Very Simple. +* empty pattern: Empty. +* empty program: Invoking Gawk. +* empty string <1>: Truth Values. +* empty string <2>: Conversion. +* empty string <3>: Regexp Field Splitting. +* empty string: Records. +* END special pattern: BEGIN/END. +* endfile: Filetrans Function. +* endgrent: Group Functions. +* endpwent: Passwd Functions. +* ENVIRON: Auto-set. +* environment variable, AWKPATH: AWKPATH Variable. +* environment variable, POSIXLY_CORRECT: Options. +* equivalence classes: Regexp Operators. +* ERRNO <1>: Auto-set. +* ERRNO <2>: Close Files And Pipes. +* ERRNO: Getline Intro. +* errors, common <1>: Typing and Comparison. +* errors, common <2>: Print Examples. +* errors, common <3>: Basic Field Splitting. +* errors, common: Computed Regexps. +* escape processing, sub et. al.: String Functions. +* escape sequence notation: Escape Sequences. +* evaluation, order of: Calling Built-in. +* examining fields: Fields. +* executable scripts: Executable Scripts. +* exit statement: Exit Statement. +* exp: Numeric Functions. +* explicit input: Getline. +* exponentiation: Arithmetic Ops. +* expression: Expressions. +* expression, assignment: Assignment Ops. +* expression, boolean: Boolean Ops. +* expression, comparison: Typing and Comparison. +* expression, conditional: Conditional Exp. +* expression, matching: Typing and Comparison. +* extract.awk: Extract Program. +* features, adding: Adding Code. +* fflush: I/O Functions. +* field operator $: Fields. +* field separator, choice of: Basic Field Splitting. +* field separator, FS: Basic Field Splitting. +* field separator, on command line: Command Line Field Separator. +* field, changing contents of: Changing Fields. +* fields: Fields. +* fields, separating: Basic Field Splitting. +* FIELDWIDTHS: User-modified. +* file descriptors: Special Files. +* file, awk program: Long. +* FILENAME <1>: Auto-set. +* FILENAME <2>: Getline Summary. +* FILENAME: Reading Files. +* FILENAME, being set by getline: Getline Summary. +* Fish, Fred: Bugs. +* flushing buffers: I/O Functions. +* FNR <1>: Auto-set. +* FNR: Records. +* for (x in ...): Scanning an Array. +* for statement: For Statement. +* format specifier: Control Letters. +* format string: Basic Printf. +* format, numeric output: OFMT. +* formatted output: Printf. +* formatted timestamps: Gettimeofday Function. +* Free Software Foundation <1>: Getting. +* Free Software Foundation: Manual History. +* FreeBSD: Manual History. +* Friedl, Jeffrey: Acknowledgements. +* FS <1>: User-modified. +* FS: Basic Field Splitting. +* ftp, anonymous <1>: Other Versions. +* ftp, anonymous: Getting. +* function call <1>: Function Caveats. +* function call: Function Calls. +* function definition: Definition Syntax. +* function, recursive: Definition Syntax. +* functions, undefined: Function Caveats. +* functions, user-defined: User-defined. +* gawk coding style: Adding Code. +* gensub: String Functions. +* getgrent: Group Functions. +* getgrent, C version: Group Functions. +* getgrgid: Group Functions. +* getgrnam: Group Functions. +* getgruser: Group Functions. +* getline: Getline. +* getline, return values: Getline Intro. +* getline, setting FILENAME: Getline Summary. +* getopt: Getopt Function. +* getopt, C version: Getopt Function. +* getpwent: Passwd Functions. +* getpwent, C version: Passwd Functions. +* getpwnam: Passwd Functions. +* getpwuid: Passwd Functions. +* gettimeofday: Gettimeofday Function. +* getting gawk: Getting. +* GNU Project: Manual History. +* grcat program: Group Functions. +* grcat.c: Group Functions. +* group file: Group Functions. +* group information: Group Functions. +* gsub: String Functions. +* gsub, third argument of: String Functions. +* Hankerson, Darrel <1>: Bugs. +* Hankerson, Darrel: Acknowledgements. +* historical features <1>: Historical Features. +* historical features <2>: String Functions. +* historical features <3>: Continue Statement. +* historical features <4>: Break Statement. +* historical features: Command Line Field Separator. +* history of awk: History. +* histsort.awk: History Sorting. +* how awk works: Two Rules. +* Hughes, Phil: Acknowledgements. +* I/O from BEGIN and END: I/O And BEGIN/END. +* id utility: Id Program. +* id.awk: Id Program. +* if-else statement: If Statement. +* igawk.sh: Igawk Program. +* IGNORECASE <1>: User-modified. +* IGNORECASE: Case-sensitivity. +* ignoring case: Case-sensitivity. +* implementation limits <1>: Redirection. +* implementation limits: Getline Summary. +* in operator: Typing and Comparison. +* increment operators: Increment Ops. +* index: String Functions. +* initialization, automatic: More Complex. +* input: Reading Files. +* input file, sample: Sample Data Files. +* input files, skipping: Nextfile Function. +* input pipeline: Getline/Pipe. +* input redirection: Getline/File. +* input, explicit: Getline. +* input, getline command: Getline. +* input, multiple line records: Multiple Line. +* input, standard: Read Terminal. +* installation, amiga: Amiga Installation. +* installation, atari: Atari Installation. +* installation, MS-DOS and OS/2: PC Installation. +* installation, unix: Quick Installation. +* installation, vms: VMS Installation. +* int: Numeric Functions. +* interaction, awk and other programs: I/O Functions. +* interactive buffering vs. non-interactive: I/O Functions. +* interval expressions: Regexp Operators. +* inventory-shipped file: Sample Data Files. +* invocation of gawk: Invoking Gawk. +* ISO 8601: Time Functions. +* ISO 8859-1 <1>: Glossary. +* ISO 8859-1: Case-sensitivity. +* ISO Latin-1 <1>: Glossary. +* ISO Latin-1: Case-sensitivity. +* Jaegermann, Michal <1>: Bugs. +* Jaegermann, Michal: Acknowledgements. +* join: Join Function. +* Kernighan, Brian <1>: Other Versions. +* Kernighan, Brian <2>: BTL. +* Kernighan, Brian <3>: Acknowledgements. +* Kernighan, Brian: History. +* known bugs: Known Bugs. +* labels.awk: Labels Program. +* language, awk: This Manual. +* language, data-driven: Getting Started. +* language, procedural: Getting Started. +* leftmost longest match <1>: Multiple Line. +* leftmost longest match: Leftmost Longest. +* length: String Functions. +* limitations <1>: Redirection. +* limitations: Getline Summary. +* line break: Statements/Lines. +* line continuation <1>: Conditional Exp. +* line continuation <2>: Boolean Ops. +* line continuation <3>: Print Examples. +* line continuation: Statements/Lines. +* Linux <1>: Atari Compiling. +* Linux: Manual History. +* locale, definition of: Time Functions. +* log: Numeric Functions. +* logical false: Truth Values. +* logical operations: Boolean Ops. +* logical true: Truth Values. +* login information: Passwd Functions. +* long options: Invoking Gawk. +* loop: While Statement. +* loops, exiting: Break Statement. +* lvalue: Assignment Ops. +* mark parity: Ordinal Functions. +* match: String Functions. +* matching ranges of lines: Ranges. +* matching, leftmost longest <1>: Multiple Line. +* matching, leftmost longest: Leftmost Longest. +* mawk: Other Versions. +* merging strings: Join Function. +* metacharacters: Regexp Operators. +* mistakes, common <1>: Typing and Comparison. +* mistakes, common <2>: Print Examples. +* mistakes, common <3>: Basic Field Splitting. +* mistakes, common: Computed Regexps. +* mktime: Mktime Function. +* modifiers (in format specifiers): Format Modifiers. +* multi-dimensional subscripts: Multi-dimensional. +* multiple line records: Multiple Line. +* multiple passes over data: Other Arguments. +* multiple statements on one line: Statements/Lines. +* multiplication: Arithmetic Ops. +* names, use of: Definition Syntax. +* namespace issues in awk: Library Names. +* namespaces: Definition Syntax. +* NetBSD: Manual History. +* new awk: History. +* new awk vs. old awk: Names. +* newline: Statements/Lines. +* next file statement: Nextfile Statement. +* next statement: Next Statement. +* next, inside a user-defined function: Next Statement. +* nextfile function: Nextfile Function. +* nextfile statement: Nextfile Statement. +* NF <1>: Auto-set. +* NF: Fields. +* non-interactive buffering vs. interactive: I/O Functions. +* not operator: Boolean Ops. +* NR <1>: Auto-set. +* NR: Records. +* null string <1>: Truth Values. +* null string <2>: Conversion. +* null string: Regexp Field Splitting. +* null string, as array subscript: Uninitialized Subscripts. +* number of fields, NF: Fields. +* number of records, NR, FNR: Records. +* numbers, used as subscripts: Numeric Array Subscripts. +* numeric character values: Ordinal Functions. +* numeric constant: Scalar Constants. +* numeric output format: OFMT. +* numeric string: Typing and Comparison. +* numeric value: Scalar Constants. +* obsolete features: Obsolete. +* obsolete options: Obsolete. +* OFMT <1>: User-modified. +* OFMT <2>: Conversion. +* OFMT: OFMT. +* OFS <1>: User-modified. +* OFS: Output Separators. +* old awk: History. +* old awk vs. new awk: Names. +* one-liners: One-liners. +* operations, logical: Boolean Ops. +* operator precedence: Precedence. +* operators, arithmetic: Arithmetic Ops. +* operators, assignment: Assignment Ops. +* operators, boolean: Boolean Ops. +* operators, decrement: Increment Ops. +* operators, increment: Increment Ops. +* operators, regexp matching: Regexp Usage. +* operators, relational: Typing and Comparison. +* operators, short-circuit: Boolean Ops. +* operators, string: Concatenation. +* operators, string-matching: Regexp Usage. +* options, command line: Invoking Gawk. +* options, long: Invoking Gawk. +* or operator: Boolean Ops. +* ord: Ordinal Functions. +* order of evaluation: Calling Built-in. +* ORS <1>: User-modified. +* ORS: Output Separators. +* output: Printing. +* output field separator, OFS: Output Separators. +* output format specifier, OFMT: OFMT. +* output record separator, ORS: Output Separators. +* output redirection: Redirection. +* output, buffering: I/O Functions. +* output, formatted: Printf. +* output, piping: Redirection. +* passes, multiple: Other Arguments. +* password file: Passwd Functions. +* path, search: AWKPATH Variable. +* pattern, BEGIN: BEGIN/END. +* pattern, default: Very Simple. +* pattern, definition of: Patterns and Actions. +* pattern, empty: Empty. +* pattern, END: BEGIN/END. +* pattern, range: Ranges. +* pattern, regular expressions: Regexp. +* patterns, types of: Kinds of Patterns. +* per file initialization and clean-up: Filetrans Function. +* PERL: Future Extensions. +* pipeline, input: Getline/Pipe. +* pipes for output: Redirection. +* portability issues <1>: Portability Notes. +* portability issues <2>: Definition Syntax. +* portability issues <3>: I/O Functions. +* portability issues <4>: String Functions. +* portability issues <5>: Delete. +* portability issues <6>: Close Files And Pipes. +* portability issues <7>: Escape Sequences. +* portability issues: Statements/Lines. +* porting gawk: New Ports. +* POSIX awk <1>: Definition Syntax. +* POSIX awk <2>: String Functions. +* POSIX awk <3>: User-modified. +* POSIX awk <4>: Next Statement. +* POSIX awk <5>: Continue Statement. +* POSIX awk <6>: Break Statement. +* POSIX awk <7>: Precedence. +* POSIX awk <8>: Assignment Ops. +* POSIX awk <9>: Arithmetic Ops. +* POSIX awk <10>: Conversion. +* POSIX awk <11>: Format Modifiers. +* POSIX awk <12>: OFMT. +* POSIX awk <13>: Field Splitting Summary. +* POSIX awk <14>: Regexp Operators. +* POSIX awk: Escape Sequences. +* POSIX mode: Options. +* POSIXLY_CORRECT environment variable: Options. +* precedence: Precedence. +* precedence, regexp operators: Regexp Operators. +* print statement: Print. +* printf statement, syntax of: Basic Printf. +* printf, format-control characters: Control Letters. +* printf, modifiers: Format Modifiers. +* printing: Printing. +* procedural languages: Getting Started. +* process information: Special Files. +* processing arguments: Getopt Function. +* program file: Long. +* program, awk: This Manual. +* program, definition of: Getting Started. +* program, self contained: Executable Scripts. +* programs, documenting <1>: Library Names. +* programs, documenting: Comments. +* pwcat program: Passwd Functions. +* pwcat.c: Passwd Functions. +* quotient: Arithmetic Ops. +* quoting, shell <1>: Long. +* quoting, shell: Read Terminal. +* Rakitzis, Byron: History Sorting. +* rand: Numeric Functions. +* random numbers, seed of: Numeric Functions. +* range pattern: Ranges. +* Rankin, Pat <1>: Bugs. +* Rankin, Pat <2>: Assignment Ops. +* Rankin, Pat: Acknowledgements. +* reading files: Reading Files. +* reading files, getline command: Getline. +* reading files, multiple line records: Multiple Line. +* record separator, RS: Records. +* record terminator, RT: Records. +* record, definition of: Records. +* records, multiple line: Multiple Line. +* recursive function: Definition Syntax. +* redirection of input: Getline/File. +* redirection of output: Redirection. +* reference to array: Reference to Elements. +* regexp: Regexp. +* regexp as expression: Typing and Comparison. +* regexp comparison vs. string comparison: Typing and Comparison. +* regexp constant: Regexp Usage. +* regexp constants, difference between slashes and quotes: Computed Regexps. +* regexp match/non-match operators <1>: Typing and Comparison. +* regexp match/non-match operators: Regexp Usage. +* regexp matching operators: Regexp Usage. +* regexp operators: Regexp Operators. +* regexp operators, GNU specific: GNU Regexp Operators. +* regexp operators, precedence of: Regexp Operators. +* regexp, anchors: Regexp Operators. +* regexp, dynamic: Computed Regexps. +* regexp, effect of command line options: GNU Regexp Operators. +* regular expression: Regexp. +* regular expression metacharacters: Regexp Operators. +* regular expressions as field separators: Basic Field Splitting. +* regular expressions as patterns: Regexp. +* regular expressions as record separators: Records. +* regular expressions, computed: Computed Regexps. +* relational operators: Typing and Comparison. +* remainder: Arithmetic Ops. +* removing elements of arrays: Delete. +* return statement: Return Statement. +* RFC-1036: Time Functions. +* RFC-822: Time Functions. +* RLENGTH <1>: String Functions. +* RLENGTH: Auto-set. +* Robbins, Miriam: Acknowledgements. +* Rommel, Kai Uwe <1>: Bugs. +* Rommel, Kai Uwe: Acknowledgements. +* round: Round Function. +* rounding: Round Function. +* RS <1>: User-modified. +* RS: Records. +* RSTART <1>: String Functions. +* RSTART: Auto-set. +* RT <1>: Auto-set. +* RT <2>: Multiple Line. +* RT: Records. +* rule, definition of: Getting Started. +* running awk programs: Running gawk. +* running long programs: Long. +* rvalue: Assignment Ops. +* sample input file: Sample Data Files. +* scanning an array: Scanning an Array. +* script, definition of: Getting Started. +* scripts, executable: Executable Scripts. +* scripts, shell: Executable Scripts. +* search path: AWKPATH Variable. +* search path, for source files: AWKPATH Variable. +* sed utility <1>: Igawk Program. +* sed utility <2>: Simple Sed. +* sed utility: Field Splitting Summary. +* seed for random numbers: Numeric Functions. +* self contained programs: Executable Scripts. +* shell quoting <1>: Long. +* shell quoting: Read Terminal. +* shell scripts: Executable Scripts. +* short-circuit operators: Boolean Ops. +* side effect: Assignment Ops. +* simple stream editor: Simple Sed. +* sin: Numeric Functions. +* single character fields: Single Character Fields. +* single quotes, why needed: One-shot. +* skipping input files: Nextfile Function. +* skipping lines between markers: Ranges. +* sparse arrays: Array Intro. +* split: String Functions. +* split utility: Split Program. +* split.awk: Split Program. +* sprintf: String Functions. +* sqrt: Numeric Functions. +* srand: Numeric Functions. +* Stallman, Richard <1>: Acknowledgements. +* Stallman, Richard: Manual History. +* standard error output: Special Files. +* standard input <1>: Special Files. +* standard input <2>: Reading Files. +* standard input: Read Terminal. +* standard output: Special Files. +* statement, compound: Statements. +* stream editor: Field Splitting Summary. +* stream editor, simple: Simple Sed. +* strftime: Time Functions. +* string comparison vs. regexp comparison: Typing and Comparison. +* string constants: Constants. +* string operators: Concatenation. +* string-matching operators: Regexp Usage. +* sub: String Functions. +* sub, third argument of: String Functions. +* subscripts in arrays: Multi-dimensional. +* SUBSEP <1>: Multi-dimensional. +* SUBSEP: User-modified. +* substr: String Functions. +* subtraction: Arithmetic Ops. +* system: I/O Functions. +* systime: Time Functions. +* Tcl: Library Names. +* tee utility: Tee Program. +* tee.awk: Tee Program. +* terminator, record: Records. +* time of day: Time Functions. +* timestamps: Time Functions. +* timestamps, converting from dates: Mktime Function. +* timestamps, formatted: Gettimeofday Function. +* tolower: String Functions. +* toupper: String Functions. +* translate.awk: Translate Program. +* Trueman, David: Acknowledgements. +* truth values: Truth Values. +* type conversion: Conversion. +* types of variables <1>: Typing and Comparison. +* types of variables: Assignment Ops. +* undefined functions: Function Caveats. +* undocumented features: Undocumented. +* uninitialized variables, as array subscripts: Uninitialized Subscripts. +* uniq utility: Uniq Program. +* uniq.awk: Uniq Program. +* use of comments: Comments. +* user information: Passwd Functions. +* user-defined functions: User-defined. +* user-defined variables: Using Variables. +* uses of awk: What Is Awk. +* using this book: This Manual. +* values of characters as numbers: Ordinal Functions. +* variable shadowing: Definition Syntax. +* variable typing: Typing and Comparison. +* variables, user-defined: Using Variables. +* Wall, Larry: Future Extensions. +* wc utility: Wc Program. +* wc.awk: Wc Program. +* Weinberger, Peter: History. +* when to use awk: When. +* while statement: While Statement. +* word boundaries, matching: GNU Regexp Operators. +* word, regexp definition of: GNU Regexp Operators. +* wordfreq.sh: Word Sorting. +* || operator: Boolean Ops. +* ~ operator <1>: Typing and Comparison. +* ~ operator <2>: Regexp Constants. +* ~ operator <3>: Computed Regexps. +* ~ operator <4>: Case-sensitivity. +* ~ operator: Regexp Usage. + + + +Tag Table: +Node: Top1197 +Node: Preface20700 +Ref: Preface-Footnote-121817 +Node: History22049 +Node: Manual History23407 +Node: Acknowledgements26997 +Node: What Is Awk30624 +Node: This Manual32278 +Node: Conventions34919 +Node: Sample Data Files36211 +Node: Getting Started39294 +Node: Names41602 +Ref: Names-Footnote-143099 +Node: Running gawk43171 +Node: One-shot44332 +Node: Read Terminal45719 +Node: Long47331 +Node: Executable Scripts48724 +Ref: Executable Scripts-Footnote-150374 +Ref: Executable Scripts-Footnote-250523 +Node: Comments50977 +Node: Very Simple52137 +Node: Two Rules54184 +Node: More Complex56363 +Node: Statements/Lines59479 +Node: Other Features63752 +Node: When64478 +Node: One-liners66412 +Node: Regexp69299 +Node: Regexp Usage70625 +Node: Escape Sequences72775 +Node: Regexp Operators78227 +Node: GNU Regexp Operators89260 +Node: Case-sensitivity92965 +Node: Leftmost Longest96080 +Node: Computed Regexps97615 +Node: Reading Files100272 +Node: Records102039 +Node: Fields108534 +Ref: Fields-Footnote-1111516 +Node: Non-Constant Fields111602 +Node: Changing Fields113888 +Node: Field Separators118295 +Node: Basic Field Splitting118997 +Node: Regexp Field Splitting122226 +Node: Single Character Fields124792 +Node: Command Line Field Separator125869 +Node: Field Splitting Summary129109 +Ref: Field Splitting Summary-Footnote-1131028 +Node: Constant Size131129 +Node: Multiple Line135166 +Node: Getline140574 +Node: Getline Intro141648 +Node: Plain Getline142611 +Node: Getline/Variable144875 +Node: Getline/File146017 +Node: Getline/Variable/File147327 +Node: Getline/Pipe149301 +Node: Getline/Variable/Pipe151391 +Node: Getline Summary152509 +Node: Printing154103 +Node: Print155171 +Node: Print Examples157271 +Node: Output Separators159882 +Node: OFMT161780 +Node: Printf163182 +Node: Basic Printf164086 +Node: Control Letters165620 +Node: Format Modifiers168308 +Node: Printf Examples172457 +Node: Redirection175236 +Node: Special Files179874 +Node: Close Files And Pipes185111 +Node: Expressions189172 +Node: Constants191378 +Node: Scalar Constants191857 +Ref: Scalar Constants-Footnote-1192717 +Node: Regexp Constants192861 +Node: Using Constant Regexps193323 +Node: Variables196524 +Node: Using Variables197178 +Node: Assignment Options198613 +Node: Conversion200557 +Node: Arithmetic Ops203738 +Node: Concatenation205872 +Node: Assignment Ops207227 +Node: Increment Ops212822 +Node: Truth Values215350 +Node: Typing and Comparison216398 +Node: Boolean Ops222298 +Node: Conditional Exp225991 +Node: Function Calls227668 +Node: Precedence230548 +Node: Patterns and Actions233936 +Node: Pattern Overview234362 +Node: Kinds of Patterns235137 +Node: Regexp Patterns236274 +Node: Expression Patterns236828 +Node: Ranges240480 +Node: BEGIN/END243199 +Node: Using BEGIN/END243668 +Node: I/O And BEGIN/END246630 +Node: Empty248646 +Node: Action Overview248945 +Node: Statements251516 +Node: If Statement253222 +Node: While Statement254725 +Node: Do Statement256756 +Node: For Statement257858 +Node: Break Statement261115 +Node: Continue Statement263386 +Node: Next Statement265382 +Node: Nextfile Statement267879 +Node: Exit Statement269793 +Node: Built-in Variables271803 +Node: User-modified272899 +Ref: User-modified-Footnote-1277688 +Node: Auto-set277750 +Ref: Auto-set-Footnote-1284073 +Node: ARGC and ARGV284279 +Node: Arrays286981 +Node: Array Intro288444 +Node: Reference to Elements292320 +Node: Assigning Elements294270 +Node: Array Example294772 +Node: Scanning an Array296491 +Node: Delete298821 +Node: Numeric Array Subscripts300881 +Node: Uninitialized Subscripts302787 +Node: Multi-dimensional304427 +Node: Multi-scanning307522 +Node: Built-in309165 +Node: Calling Built-in310154 +Node: Numeric Functions312125 +Ref: Numeric Functions-Footnote-1315673 +Node: String Functions315943 +Ref: String Functions-Footnote-1334729 +Ref: String Functions-Footnote-2334780 +Node: I/O Functions334873 +Ref: I/O Functions-Footnote-1340366 +Node: Time Functions340457 +Ref: Time Functions-Footnote-1348776 +Ref: Time Functions-Footnote-2348887 +Ref: Time Functions-Footnote-3349163 +Node: User-defined349307 +Node: Definition Syntax350019 +Node: Function Example354268 +Node: Function Caveats356598 +Node: Return Statement360469 +Node: Invoking Gawk363124 +Node: Options364359 +Ref: Options-Footnote-1373162 +Node: Other Arguments373187 +Node: AWKPATH Variable375833 +Ref: AWKPATH Variable-Footnote-1378281 +Node: Obsolete378581 +Node: Undocumented379247 +Node: Known Bugs379455 +Node: Library Functions380593 +Node: Portability Notes383012 +Node: Nextfile Function384296 +Ref: Nextfile Function-Footnote-1389001 +Node: Assert Function389171 +Node: Round Function392510 +Node: Ordinal Functions394155 +Ref: Ordinal Functions-Footnote-1397387 +Node: Join Function397606 +Node: Mktime Function399658 +Ref: Mktime Function-Footnote-1411149 +Node: Gettimeofday Function411232 +Node: Filetrans Function415244 +Node: Getopt Function418921 +Node: Passwd Functions430277 +Node: Group Functions438612 +Node: Library Names446509 +Node: Sample Programs450434 +Node: Clones450925 +Node: Cut Program452019 +Node: Egrep Program462048 +Node: Id Program469710 +Node: Split Program472981 +Node: Tee Program476359 +Node: Uniq Program479155 +Node: Wc Program486700 +Ref: Wc 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Operator Summary572964 +Node: Control Flow Summary574191 +Node: I/O Summary574748 +Node: Printf Summary577737 +Node: Special File Summary581075 +Node: Built-in Functions Summary582753 +Node: Time Functions Summary586753 +Node: String Constants Summary587644 +Node: Functions Summary588964 +Node: Historical Features590025 +Node: Installation591523 +Node: Gawk Distribution592738 +Node: Getting593241 +Node: Extracting596226 +Node: Distribution contents597613 +Node: Unix Installation602527 +Node: Quick Installation603036 +Node: Configuration Philosophy604554 +Node: VMS Installation606956 +Node: VMS Compilation607495 +Node: VMS Installation Details609099 +Node: VMS Running610741 +Node: VMS POSIX612331 +Node: PC Installation613611 +Node: Atari Installation617014 +Node: Atari Compiling618198 +Node: Atari Using620107 +Node: Amiga Installation622953 +Node: Bugs624071 +Node: Other Versions627040 +Node: Notes628614 +Node: Compatibility Mode629221 +Node: Additions630064 +Node: Adding Code630762 +Node: New Ports636102 +Node: Future Extensions640270 +Node: Improvements642519 +Node: Glossary644387 +Node: Copying661452 +Node: Index680644 + +End Tag Table |