ISBN 1-57831-000-8
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.

Cover art by Amy Wells Wood.

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.

Preface

This book 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 book, 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.

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.

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 (see section GNU GENERAL PUBLIC LICENSE). The GPL applies to the C language source code for gawk.

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. As of this writing (early 1997), the GNU operating system kernel (the HURD), has been released, but is still in an early stage of development.

Until the GNU operating system is more fully developed, you should consider using Linux, a freely distributable, Unix-like operating system for 80386, DEC Alpha, Sun SPARC 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. (There are three other freely available, Unix-like operating systems for 80386 and other systems, NetBSD, FreeBSD,and OpenBSD. All are based on the 4.4-Lite Berkeley Software Distribution, and they use recent versions of gawk for their versions of awk.)

This book you are reading now is actually free. The information in it is freely available to anyone, the machine readable source code for the book comes with gawk, and anyone may take this book to a copying machine and make as many copies of it as they like. (Take a moment to check the copying permissions on the Copyright page.)

If you paid money for this book, what you actually paid for was the book's nice printing and binding, and the publisher's associated costs to produce it. We have made an effort to keep these costs reasonable; most people would prefer a bound book to over 330 pages of photo-copied text that would then have to be held in a loose-leaf binder (not to mention the time and labor involved in doing the copying). The same is true of producing this book from the machine readable source; the retail price is only slightly more than the cost per page of printing it on a laser printer.

This book 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 Effective AWK Programming 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.

Effective AWK Programming will undoubtedly continue to evolve. An electronic version comes with the gawk distribution from the FSF. If you find an error in this book, please report it! See section Reporting Problems and Bugs, for information on submitting problem reports electronically, or write to me in care of the FSF.

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 Effective AWK Programming: 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 book 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 book 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
February, 1997

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

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 book is to explain both the awk language and how to run the awk utility.

The main purpose of the book 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.

This book has the difficult task of being both tutorial and reference. If you are a novice, feel free to skip over details that seem too complex. You should also ignore the many cross references; they are for the expert user, and for the on-line Info version of the document.

The term awk program refers to a program written by you in the awk programming language.

See section Getting Started with awk, 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 (see section Useful One Line Programs).

Many sample awk programs have been provided for you (see section A Library of awk Functions; also see section Practical awk Programs).

The entire awk language is summarized for quick reference in section 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 (see section 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 book 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 section A Library of awk Functions, and section Practical awk Programs, should be of interest.

Dark Corners

Who opened that window shade?!?
Count Dracula

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 book with "(d.c.)". They also appear in the index under the heading "dark corner."

Typographical Conventions

This book 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. Because of this, the typographical conventions are slightly different than in other books you may have read.

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

In the text, command names appear in this font, while code segments appear in the same font and quoted, `like this'. Some things will be emphasized like this, and if a point needs to be made strongly, it will be done like this. The first occurrence of a new term is usually its definition, and appears in the same font as the previous occurrence of "definition" in this sentence. File names are indicated like this: `/path/to/ourfile'.

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.

Data Files for the Examples

Many of the examples in this book 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

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. See section User-defined Functions.) 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 }
...

A Rose By Any Other Name

The awk language has evolved over the years. Full details are provided in section The Evolution of the awk Language. The language described in this book 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.(2)

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 book, chances are good that you have gawk!)

Throughout this book, 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.

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 section One-shot Throw-away awk Programs.)

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 ...

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.

section Useful One Line Programs, presents several short, self-contained programs.

As an interesting side point, the command

awk '/foo/' files ...

is essentially the same as

egrep foo files ...

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

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 (see section Running awk without Input Files). 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.

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(3) (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(4) 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.)

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.

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 (see section Regular Expressions). 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.

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, see section The next Statement, and also see section The 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.

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 (see section Overview of Actions). 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.

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 section Conditional Expressions.)

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 book. 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 book. 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.

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 section Built-in Variables, and section Built-in Functions.

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. (See section A More Complex Example.)

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 (see section 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.

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 book 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'.

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.

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.

Initially, the examples will be simple. As we explain more about how regular expressions work, we will present more complicated examples.

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.

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.

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. See section Additional Regexp Operators Only in gawk.

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 section Additional Regexp Operators Only in gawk, 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 (see section Regular Expression 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 (see section Command Line 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 (see section Using Dynamic Regexps), for the special operators listed in section Additional Regexp Operators Only in gawk.
3. A backslash before any other character means to treat that character literally.

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 above in section 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 (see section Command Line 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 (see section Command Line 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 (see section Command Line Options), character classes and interval expressions are not available in regular expressions.

The next section 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.

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 boundary). 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 (see section 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.

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; see section Built-in Functions for String Manipulation). 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 (see section 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 (see section Other Command Line Arguments; also see section Startup and Cleanup Actions). 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 (see section Command Line Options). Case is always significant in compatibility mode.

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, see section Built-in Functions for String Manipulation) 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. Understanding this principle is also important for regexp-based record and field splitting (see section How Input is Split into Records, and also see section Specifying How Fields are Separated).

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.

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 (see section 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 (see section Explicit Input with getline).

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, `=' (see section Assignment Expressions). 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 (see section The BEGIN and END Special Patterns). 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 (see section Data Files for the Examples), 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 (see section Other Command Line 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. See section Multiple-Line Records, 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 (see section Regular Expressions). 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 (see section Regular Expressions).

$ 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.

See section A Simple Stream Editor, 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 (see section Command Line 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.

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;(5) 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 (see section Non-constant Field Numbers). 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 (see section How to Use Regular Expressions); 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

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 section Operator Precedence (How Operators Nest).)

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 section Examining Fields, the number of fields in the current record is stored in the built-in variable NF (also see section 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.

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'. (See section Arithmetic Operators.) 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. See section Conversion of Strings and Numbers.

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; see section Examining Fields), and by a feature that has not been discussed yet, the output field separator, OFS, which is used to separate the fields (see section 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. (See section The if-else Statement, for more information about awk's if-else statements. See section Variable Typing and Comparison Expressions, 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

Specifying How Fields are Separated

This section is rather long; it describes one of the most fundamental operations in awk.

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, `=' (see section Assignment Expressions). 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 (see section The BEGIN and END Special Patterns). 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.

As you know, 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.

Using Regular Expressions to Separate Fields

The previous subsection 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; see section 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 (see section Regular Expressions).

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.

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
>                  }'
-| 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 (see section Command Line Options), if FS is the null string, then gawk will also behave this way.

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 (see section Escape Sequences), finally yielding a single `\' to be used for the field separator.

As a special case, in compatibility mode (see section Command Line 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 (see section Command Line 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 (see section Data Files for the Examples), 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

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(6) 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.

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 (see section Built-in Functions for String Manipulation), 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.

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 (see section How Much Text Matches?). 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 (see section Built-in Functions for String Manipulation).

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
-| 
...

See section Printing Mailing Labels, 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.

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.

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 book 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.

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; see section Examining Fields), NR (the number of records read so far; see section How Input is Split into 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. See section The next Statement.

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.

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.

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.

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. See section Closing Input and Output 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. See section An Easy Way to Use Library Functions, for a program that does handle nested `@include' statements.

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. See section Closing Input and Output 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.

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.

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.). (See section The BEGIN and END Special Patterns, also see section Built-in Variables that Convey Information.)

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.

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.

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 (see section Redirecting Output of print and printf).

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, see section Output Separators, and section Controlling Numeric Output with print.) For that, you need the printf statement (see section Using printf Statements for Fancier Printing).

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.

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 section 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 (see section The BEGIN and END Special Patterns) 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 (see section Using printf Statements for Fancier Printing); 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 (see section awk Statements Versus Lines).

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 (see section The BEGIN and END Special Patterns), 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 (see section Command Line 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.

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 (see section Built-in Functions for String Manipulation). 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 section Format-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.).

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.

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 (see section Redirecting Output of print and printf).

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.

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.).

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 (see section Command Line Options), gawk will warn about their use. If `--posix' is supplied, their use is a fatal error.

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 (see section The BEGIN and END Special Patterns) 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 (see section The print Statement).

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 (see section 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. See section Closing Input and Output 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.

As mentioned earlier (see section Summary of getline Variants), 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.

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 (see section Closing Input and Output 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 (see section Command Line 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. See section Probable Future Extensions.

Closing Input and Output Files and Pipes

If the same file name or the same shell command is used with getline (see section Explicit Input with 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.

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.

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.

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.(7) 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.

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).

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.) (see section Expressions as 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 (see section Built-in Functions for String Manipulation). 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 (see section User-defined Functions). 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.

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.

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. See section Assignment Expressions.

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. See section 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.

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 (see section Other Command Line 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. (See section Command Line 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 (see section Using ARGC and ARGV).

awk processes the values of command line assignments for escape sequences (d.c.) (see section Escape Sequences).

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 (see section Built-in Variables). Numbers are converted using the sprintf function (see section Built-in Functions for String Manipulation) 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! See section The print Statement, for more information on the print statement.

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.

String Concatenation

It seemed like a good idea at the time.
Brian Kernighan

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 `=').

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 (see section Variables); it can also be a field (see section Changing the Contents of a Field) or an array element (see section Arrays in awk). 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. See section Arrays in awk, and see section Numeric Built-in 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.

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.

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.).

Variable Typing and Comparison Expressions

The Guide is definitive. Reality is frequently inaccurate.
The Hitchhiker's Guide to the Galaxy

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:

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 (see section Conversion of Strings and Numbers).

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 (see section How to Use Regular Expressions; also see section Using Dynamic 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 `!~'! See section Using Regular Expression Constants, where this is discussed in more detail.

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 (see section Control Statements in Actions). 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 (see section awk Statements Versus 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.

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. See section Arrays in awk, 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 (see section awk Statements Versus Lines).

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. See section Built-in Functions, for a list of built-in functions and their descriptions. In addition, you can define your own functions for use in your program. See section User-defined Functions, 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. See section Built-in Functions, 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 (see section User-defined Functions).

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

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.)

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.

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.

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. (See section Regular Expressions.)

expression

A single expression. It matches when its value is non-zero (if a number) or non-null (if a string). (See section Expressions as 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. (See section Specifying Record Ranges with Patterns.)

BEGIN

END

Special patterns for you to supply start-up or clean-up actions for your awk program. (See section The BEGIN and END Special Patterns.)

empty

The empty pattern matches every input record. (See section The Empty Pattern.)

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" }

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 section Variable Typing and Comparison Expressions.

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 (see section Using Dynamic 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'.

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, see section The 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 look 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.

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.

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 (see section 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. See section Command Line Options, for more information on using library functions. See section A Library of awk 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.

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 (see section Explicit Input with 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.

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.

Overview of Actions

An awk program or script consists of a series of rules and function definitions, interspersed. (Functions are described later. See section User-defined Functions.)

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 (see section Expressions). Executing this kind of statement simply computes the value of the expression. This is useful when the expression has side effects (see section Assignment Expressions).
• 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 (see section Control Statements in Actions).
• 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 (see section Explicit Input with getline), the next statement (see section The next Statement), and the nextfile statement (see section The nextfile Statement).
• Output statements, print and printf. See section Printing Output.
• Deletion statements, for deleting array elements. See section The delete Statement.

The next chapter covers control statements in detail.

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.

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.

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. 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.

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.

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 (see section The 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]

See section Scanning All Elements of 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.

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. See section The 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 (see section The 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 (see section Command Line 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.).

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 (see section The 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 (see section Command Line 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.).

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 (see section Explicit Input with 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. See section Special File Names in gawk.

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 (see section User-defined Functions). 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. See section The BEGIN and END Special Patterns.

Caution: Some awk implementations generate a run-time error if you use the next statement inside a user-defined function (see section User-defined Functions). gawk does not have this problem.

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. See section 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. See section The BEGIN and END Special Patterns.

The nextfile statement is a gawk extension; it is not (currently) available in any other awk implementation. See section Implementing nextfile as a 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.

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 (see section The BEGIN and END Special Patterns).

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. See section Assertions, 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")
}

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.

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 (see section Conversion of Strings and Numbers). It works by being passed, in effect, as the first argument to the sprintf function (see section Built-in Functions for String Manipulation). 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. See section Reading Fixed-width Data, for more information. If gawk is in compatibility mode (see section Command Line 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 (see section Specifying How Fields are Separated). 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.(8) 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. The value of IGNORECASE does not affect array subscripting. See section Case-sensitivity in Matching. If gawk is in compatibility mode (see section Command Line 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 (see section Conversion of Strings and Numbers) for printing with the print statement. It works by being passed, in effect, as the first argument to the sprintf function (see section Built-in Functions for String Manipulation). 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 (see section 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". (See section 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. (See section How Input is Split into 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"] (see section Multi-dimensional Arrays).

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. See section Other Command Line 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. See section Using 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 (see section Command Line 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 (see section Command Line 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 (see section Reading Input Files). Inside a BEGIN rule, the value of FILENAME is "", since there are no input files being processed yet.(9) (d.c.)

FNR

FNR is the current record number in the current file. FNR is incremented each time a new record is read (see section Explicit Input with 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 (see section Examining Fields).

NR

This is the number of input records awk has processed since the beginning of the program's execution (see section How Input is Split into Records). NR is set each time a new record is read.

RLENGTH

RLENGTH is the length of the substring matched by the match function (see section Built-in Functions for String Manipulation). 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 (see section Built-in Functions for String Manipulation). 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 (see section Command Line 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 (see section Major Changes between V7 and 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.

Using ARGC and ARGV

In section Built-in Variables that Convey Information, 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 (see section The delete Statement).

All of these actions are typically done from the BEGIN rule, before actual processing of the input begins. See section Splitting a Large File Into Pieces, and see section Duplicating Output Into Multiple Files, 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]
    }
}

To actually get the options into the awk program, you have to end the awk options with `--', and then supply your options, like so:

awk -f myprog -- -v -d file1 file2 ...

This is not necessary in gawk: Unless `--posix' has been specified, gawk silently puts any unrecognized options into ARGV for the awk program to deal with.

As soon as it sees an unknown option, gawk stops looking for other options it might otherwise recognize. The above example with gawk would be:

gawk -f myprog -d -v file1 file2 ...

Since `-d' is not a valid gawk option, the following `-v' is passed on to the awk program.

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 (see section User-defined Functions). Thus, you cannot have a variable and an array with the same name in the same awk program.

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:

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 section Using Numbers to Subscript Arrays.)

The value of IGNORECASE has no effect upon array subscripting. You must use the exact same string value to retrieve an array element as you used to store it.

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. (See section Built-in Functions for String Manipulation.)

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 (see section The delete Statement). 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."

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.

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]
}

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. See section Built-in Functions for String Manipulation, 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"
}

See section Generating Word Usage Counts, 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.

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 (see section Command Line 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 (see section Built-in Functions for String Manipulation) 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.

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 (see section Conversion of Strings and Numbers).

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 (see section Conversion of Strings and Numbers), 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.

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 lines 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 (see section Command Line Options), gawk will warn about the use of the null string as a subscript.

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 (see section Conversion of Strings and Numbers) 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

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 (see section Scanning All Elements of an Array) with the split built-in function (see section Built-in Functions for String Manipulation). 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.

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. See section User-defined Functions.)

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.

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.(10) 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.

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 (see section Using printf Statements for Fancier Printing). 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 (see section How Much Text Matches?). 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 (see section Command Line 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 section 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:

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:

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.(11)

The POSIX standard is under revision.(12) 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.

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 book 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.

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.

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. See section Closing Input and Output 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 (see section Command Line 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.(13)

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.

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 (see section Built-in Functions for String Manipulation), 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).(14)

%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.(15)

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 (see section Installing gawk), 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.(16) (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
}' "$@"

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 (see section Function Calls), but it is up to you to define them--to tell awk what they should do.

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 (see section Command Line 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.

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 (see section The delete Statement). 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. (See section Functions for Dealing with Time Stamps, 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)
}

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 (see section Command Line Options), gawk will report about calls to undefined functions.

Some awk implementations generate a run-time error if you use the next statement (see section The next Statement) inside a user-defined function. gawk does not have this problem.

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.

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.

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 (see section Specifying How Fields are Separated).

-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 (see section Other Command Line 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. See section Extensions in gawk Not in POSIX awk, which summarizes the extensions. Also see section Downward Compatibility and Debugging.

-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 (see section Major Changes between V7 and 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 (see section 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 (see section Function Definition Syntax).
• The operators `**' and `**=' cannot be used in place of `^' and `^=' (see section Arithmetic Operators, and also see section Assignment Expressions).
• Specifying `-Ft' on the command line does not set the value of FS to be a single tab character (see section Specifying How Fields are Separated).
• The fflush built-in function is not supported (see section Built-in Functions for Input/Output).

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 (see section Regular Expression 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 (see section The AWKPATH Environment 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 (see section Reporting Problems and 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' (see section Specifying How Fields are Separated).

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 (see section The AWKPATH Environment 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,(17) you would add this line to the `.login' file in your home directory.

setenv POSIXLY_CORRECT true

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 (see section 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 (see section The BEGIN and END Special Patterns), 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.) (see section 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.

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'.(18) (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. See section A Library of awk Functions.

Path searching is not done if gawk is in compatibility mode. This is true for both `--traditional' and `--posix'. See section Command Line 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.

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 3.0.3 of gawk, there are no command line options or other deprecated features from the previous version of gawk. This section is thus essentially a place holder, in case some option becomes obsolete in a future version of gawk.

Undocumented Options and Features

Use the Source, Luke!
Obi-Wan

This section intentionally left blank.

Known Bugs in gawk

• The `-F' option for changing the value of FS (see section Command Line 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 (see section Special File Names in gawk).
• 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.

A Library of awk Functions

This chapter presents a library of useful awk functions. The sample programs presented later (see section Practical awk Programs) use these functions. The functions are presented here in a progression from simple to complex.

section Extracting Programs from Texinfo Source Files, 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 book. (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 book, please contact the author. See section Reporting Problems and 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 (see section GNU GENERAL PUBLIC LICENSE), or assign the copyright in it to the Free Software Foundation.

Simulating gawk-specific Features

The programs in this chapter and in section Practical awk 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 (see section The nextfile Statement), to skip any remaining input in the input file. section Implementing nextfile as a 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.

Implementing nextfile as a Function

The nextfile statement presented in section The 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 section Naming Library Function Global Variables.)

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.(19)

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).

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 (see section Startup and Cleanup Actions), most likely causing the program to hang, waiting for input.

Rounding Numbers

The way printf and sprintf (see section Using printf Statements for Fancier Printing) 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) }

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.(20) 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.

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 (see section Practical awk 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 (see section Built-in Functions for String Manipulation).

# 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.

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, see section Functions for Dealing with Time Stamps.)

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 section Extracting Programs from Texinfo Source Files, you will see how the Texinfo source file for this book 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 (see section The BEGIN and END Special Patterns) 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,(21) 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".