AWK

This article is about the programming language. For other uses, seeAWK (disambiguation).

AWK

Paradigm	Scripting,procedural,data-driven
Designed by	Alfred Aho,Peter Weinberger,andBrian Kernighan
First appeared	1977(1977)
Stable release	IEEE Std 1003.1-2008(POSIX) / 1985
Typing discipline	none; can handle strings, integers and floating-point numbers; regular expressions
OS	Cross-platform
Majorimplementations
awk, GNU Awk, mawk, nawk, MKS AWK, Thompson AWK (compiler), Awka (compiler)
Dialects
old awkoawk 1977,new awknawk 1985,GNU Awkgawk
Influenced by
C,sed,SNOBOL
Influenced
Tcl,AMPL,Perl,Korn Shell(ksh93,dtksh,tksh),Lua

AWK(/ɔːk/) is adomain-specific languagedesigned for text processing and typically used as adata extractionand reporting tool. Likesedandgrep,it is afilter,and is a standard feature of mostUnix-like operating systems.

The AWK language is adata-drivenscripting languageconsisting of a set of actions to be taken againststreamsof textual data – either run directly on files or used as part of apipeline– for purposes of extracting or transforming text, such as producing formatted reports. The language extensively uses thestringdatatype,associative arrays(that is, arrays indexed by key strings), andregular expressions.While AWK has a limited intendedapplication domainand was especially designed to supportone-liner programs,the language isTuring-complete,and even the early Bell Labs users of AWK often wrote well-structured large AWK programs.

AWK was created atBell Labsin the 1970s,and its name is derived from the surnames of its authors:Alfred Aho(author ofegrep),Peter Weinberger(who worked on tiny relational databases), andBrian Kernighan.The acronym is pronounced the same as the name of the bird speciesauk,which is illustrated on the cover ofThe AWK Programming Language.When written in all lowercase letters, asawk,it refers to theUnixorPlan 9program that runs scripts written in the AWK programming language.

History

According to Brian Kernighan, one of the goals of AWK was to have a tool that would easily manipulate both numbers and strings. AWK was also inspired byMarc Rochkind's programming language that was used to search for patterns in input data, and was implemented usingyacc.

As one of the early tools to appear inVersion 7 Unix,AWK added computational features to a Unixpipelinebesides theBourne shell,the only scripting language available in a standard Unix environment. It is one of the mandatory utilities of theSingle UNIX Specification,and is required by theLinux Standard Basespecification.

In 1983, AWK was one of several UNIX tools available for Charles River Data Systems'UNOSoperating system underBell Laboratorieslicense.

AWK was significantly revised and expanded in 1985–88, resulting in theGNU AWKimplementation written by Paul Rubin,Jay Fenlason,andRichard Stallman,released in 1988.GNU AWK may be the most widely deployed versionbecause it is included with GNU-based Linux packages. GNU AWK has been maintained solely byArnold Robbinssince 1994.Brian Kernighan'snawk(New AWK) source was first released in 1993 unpublicized, and publicly since the late 1990s; many BSD systems use it to avoid the GPL license.

AWK was preceded bysed(1974). Both were designed for text processing. They share the line-oriented, data-driven paradigm, and are particularly suited to writingone-liner programs,due to the implicitmain loopand current line variables. The power and terseness of early AWK programs – notably the powerful regular expression handling and conciseness due to implicit variables, which facilitate one-liners – together with the limitations of AWK at the time, were important inspirations for thePerllanguage (1987). In the 1990s, Perl became very popular, competing with AWK in the niche of Unix text-processing languages.

Structure of AWK programs

AWK reads the input a line at a time. A line is scanned for each pattern in the program, and for each pattern that matches, the associated action is executed.

— Alfred V. Aho

An AWK program is a series of pattern action pairs, written as:

condition{action}
condition{action}
...

whereconditionis typically an expression andactionis a series of commands. The input is split into records, where by default records are separated by newline characters so that the input is split into lines. The program tests each record against each of the conditions in turn, and executes theactionfor each expression that is true. Either the condition or the action may be omitted. The condition defaults to matching every record. The default action is to print the record. This is the same pattern-action structure as sed.

In addition to a simple AWK expression, such asfoo == 1or/^foo/,the condition can beBEGINorENDcausing the action to be executed before or after all records have been read, orpattern1, pattern2which matches the range of records starting with a record that matchespattern1up to and including the record that matchespattern2before again trying to match againstpattern1on subsequent lines.

In addition to normal arithmetic and logical operators, AWK expressions include the tilde operator,~,which matches aregular expressionagainst a string. As handysyntactic sugar,/regexp/without using the tilde operator matches against the current record; this syntax derives fromsed,which in turn inherited it from theededitor, where/is used for searching. This syntax of using slashes asdelimitersfor regular expressions was subsequently adopted byPerlandECMAScript,and is now common. The tilde operator was also adopted by Perl.

Commands

AWK commands are the statements that are substituted foractionin the examples above. AWK commands can include function calls, variable assignments, calculations, or any combination thereof. AWK contains built-in support for many functions; many more are provided by the various flavors of AWK. Also, some flavors support the inclusion ofdynamically linked libraries,which can also provide more functions.

Theprintcommand

Theprintcommand is used to output text. The output text is always terminated with a predefined string called the output record separator (ORS) whose default value is a newline. The simplest form of this command is:

print

This displays the contents of the current record. In AWK, records are broken down intofields,and these can be displayed separately:

print $1

Displays the first field of the current record

print $1, $3

Displays the first and third fields of the current record, separated by a predefined string called the output field separator (OFS) whose default value is a single space character

Although these fields ($X) may bear resemblance to variables (the $ symbol indicates variables in the usual Unix shells and inPerl), they actually refer to the fields of the current record. A special case,$0,refers to the entire record. In fact, the commands "print"and"print $0"are identical in functionality.

Theprintcommand can also display the results of calculations and/or function calls:

/regex_pattern/{
# Actions to perform in the event the record (line) matches the above regex_pattern
print3+2
printfoobar(3)
printfoobar(variable)
printsin(3-2)
}

Output may be sent to a file:

/regex_pattern/{
# Actions to perform in the event the record (line) matches the above regex_pattern
print"expression">"file name"
}

or through apipe:

/regex_pattern/{
# Actions to perform in the event the record (line) matches the above regex_pattern
print"expression"|"command"
}

Built-in variables

Awk's built-in variables include the field variables: $1, $2, $3, and so on ($0 represents the entire record). They hold the text or values in the individual text-fields in a record.

Other variables include:

• NR:Number of Records. Keeps a current count of the number of input records read so far from all data files. It starts at zero, but is never automatically reset to zero.
• FNR:File Number of Records. Keeps a current count of the number of input records read so farin the current file.This variable is automatically reset to zero each time a new file is started.
• NF:Number of Fields. Contains the number of fields in the current input record. The last field in the input record can be designated by $NF, the 2nd-to-last field by $(NF-1), the 3rd-to-last field by $(NF-2), etc.
• FILENAME:Contains the name of the current input-file.
• FS:Field Separator. Contains the "field separator" used to divide fields in the input record. The default, "white space", allows any sequence of space and tab characters. FS can be reassigned with another character or character sequence to change the field separator.
• RS:Record Separator. Stores the current "record separator" character. Since, by default, an input line is the input record, the default record separator character is a "newline".
• OFS:Output Field Separator. Stores the "output field separator", which separates the fields when Awk prints them. The default is a "space" character.
• ORS:Output Record Separator. Stores the "output record separator", which separates the output records when Awk prints them. The default is a "newline" character.
• OFMT:Output Format. Stores the format for numeric output. The default format is "%.6g".

Variables and syntax

Variable names can use any of the characters [A-Za-z0-9_], with the exception of language keywords. The operators+ - * /represent addition, subtraction, multiplication, and division, respectively. For stringconcatenation,simply place two variables (or string constants) next to each other. It is optional to use a space in between if string constants are involved, but two variable names placed adjacent to each other require a space in between. Double quotesdelimitstring constants. Statements need not end with semicolons. Finally, comments can be added to programs by using#as the first character on a line, or behind a command or sequence of commands.

User-defined functions

In a format similar toC,function definitions consist of the keywordfunction,the function name, argument names and the function body. Here is an example of a function.

functionadd_three(number){
returnnumber+3
}

This statement can be invoked as follows:

(pattern){
printadd_three(36)# Outputs '''39'''
}

Functions can have variables that are in the local scope. The names of these are added to the end of the argument list, though values for these should be omitted when calling the function. It is convention to add somewhitespacein the argument list before the local variables, to indicate where the parameters end and the local variables begin.

Examples

Hello, World!

Here is the customary"Hello, World!" programwritten in AWK:

BEGIN{
print"Hello, world!"
exit
}

Print lines longer than 80 characters

Print all lines longer than 80 characters. The default action is to print the current line.

length($0)>80

Count words

Count words in the input and print the number of lines, words, and characters (likewc):

{
words+=NF
chars+=length+1# add one to account for the newline character at the end of each record (line)
}
END{printNR,words,chars}

As there is no pattern for the first line of the program, every line of input matches by default, so the increment actions are executed for every line.words += NFis shorthand forwords = words + NF.

Sum last word

{s+=$NF}
END{prints+0}

sis incremented by the numeric value of$NF,which is the last word on the line as defined by AWK's field separator (by default, white-space).NFis the number of fields in the current line, e.g. 4. Since$4is the value of the fourth field,$NFis the value of the last field in the line regardless of how many fields this line has, or whether it has more or fewer fields than surrounding lines. $ is actually a unary operator with the highestoperator precedence.(If the line has no fields, thenNFis 0,$0is the whole line, which in this case is empty apart from possible white-space, and so has the numeric value 0.)

At the end of the input theENDpattern matches, sosis printed. However, since there may have been no lines of input at all, in which case no value has ever been assigned tos,it will by default be an empty string. Adding zero to a variable is an AWK idiom for coercing it from a string to a numeric value. (Concatenating an empty string is to coerce from a number to a string, e.g.s "".Note, there's no operator to concatenate strings, they're just placed adjacently.) With the coercion the program prints "0" on an empty input, without it, an empty line is printed.

Match a range of input lines

NR%4==1,NR%4==3{printf"%6d %s\n",NR,$0}

The action statement prints each line numbered. The printf function emulates the standard Cprintfand works similarly to the print command described above. The pattern to match, however, works as follows:NRis the number of records, typically lines of input, AWK has so far read, i.e. the current line number, starting at 1 for the first line of input.%is themodulooperator.NR % 4 == 1is true for the 1st, 5th, 9th, etc., lines of input. Likewise,NR % 4 == 3is true for the 3rd, 7th, 11th, etc., lines of input. The range pattern is false until the first part matches, on line 1, and then remains true up to and including when the second part matches, on line 3. It then stays false until the first part matches again on line 5.

Thus, the program prints lines 1,2,3, skips line 4, and then 5,6,7, and so on. For each line, it prints the line number (on a 6 character-wide field) and then the line contents. For example, when executed on this input:

Rome
Florence
Milan
Naples
Turin
Venice

The previous program prints:

1 Rome
2 Florence
3 Milan
5 Turin
6 Venice

Printing the initial or the final part of a file

As a special case, when the first part of a range pattern is constantly true, e.g.1,the range will start at the beginning of the input. Similarly, if the second part is constantly false, e.g.0,the range will continue until the end of input. For example,

/^--cut here--$/,0

prints lines of input from the first line matching the regular expression^--cut here--$,that is, a line containing only the phrase "--cut here--", to the end.

Calculate word frequencies

Word frequencyusingassociative arrays:

BEGIN{
FS="[^a-zA-Z]+"
}
{
for(i=1;i<=NF;i++)
words[tolower($i)]++
}
END{
for(iinwords)
printi,words[i]
}

The BEGIN block sets the field separator to any sequence of non- Alpha betic characters. Separators can be regular expressions. After that, we get to a bare action, which performs the action on every input line. In this case, for every field on the line, we add one to the number of times that word, first converted to lowercase, appears. Finally, in the END block, we print the words with their frequencies. The line

for (i in words)

creates a loop that goes through the arraywords,settingito eachsubscriptof the array. This is different from most languages, where such a loop goes through eachvaluein the array. The loop thus prints out each word followed by its frequency count.tolowerwas an addition to the One True awk (see below) made after the book was published.

Match pattern from command line

This program can be represented in several ways. The first one uses theBourne shellto make a shell script that does everything. It is the shortest of these methods:

#!/bin/sh

pattern="$1"
shift
awk'/'"$pattern"'/ { print FILENAME ":" $0 }'"$@"

The$patternin the awk command is not protected by single quotes so that the shell does expand the variable but it needs to be put in double quotes to properly handle patterns containing spaces. A pattern by itself in the usual way checks to see if the whole line ($0) matches.FILENAMEcontains the current filename. awk has no explicit concatenation operator; two adjacent strings concatenate them.$0expands to the original unchanged input line.

There are alternate ways of writing this. This shell script accesses the environment directly from within awk:

#!/bin/sh

exportpattern="$1"
shift
awk'$0 ~ ENVIRON[ "pattern" ] { print FILENAME ":" $0 }'"$@"

This is a shell script that usesENVIRON,an array introduced in a newer version of the One True awk after the book was published. The subscript ofENVIRONis the name of an environment variable; its result is the variable's value. This is like thegetenvfunction in various standard libraries andPOSIX.The shell script makes an environment variablepatterncontaining the first argument, then drops that argument and has awk look for the pattern in each file.

~checks to see if its left operand matches its right operand;!~is its inverse. A regular expression is just a string and can be stored in variables.

The next way uses command-line variable assignment, in which an argument to awk can be seen as an assignment to a variable:

#!/bin/sh

pattern="$1"
shift
awk'$0 ~ pattern { print FILENAME ":" $0 }'"pattern=$pattern""$@"

Or You can use the-v var=valuecommand line option (e.g.awk -v pattern= "$pattern"...).

Finally, this is written in pure awk, without help from a shell or without the need to know too much about the implementation of the awk script (as the variable assignment on command line one does), but is a bit lengthy:

BEGIN{
pattern=ARGV[1]
for(i=1;i<ARGC;i++)# remove first argument
ARGV[i]=ARGV[i+1]
ARGC--
if(ARGC==1){# the pattern was the only thing, so force read from standard input (used by book)
ARGC=2
ARGV[1]="-"
}
}
$0~pattern{printFILENAME":"$0}

TheBEGINis necessary not only to extract the first argument, but also to prevent it from being interpreted as a filename after theBEGINblock ends.ARGC,the number of arguments, is always guaranteed to be ≥1, asARGV[0]is the name of the command that executed the script, most often the string"awk".ARGV[ARGC]is the empty string,"".#initiates a comment that expands to the end of the line.

Note theifblock. awk only checks to see if it should read from standard input before it runs the command. This means that

awk 'prog'

only works because the fact that there are no filenames is only checked beforeprogis run! If you explicitly setARGCto 1 so that there are no arguments, awk will simply quit because it feels there are no more input files. Therefore, you need to explicitly say to read from standard input with the special filename-.

Self-contained AWK scripts

On Unix-like operating systems self-contained AWK scripts can be constructed using theshebangsyntax.

For example, a script that sends the content of a given file to standard output may be built by creating a file namedprint.awkwith the following content:

#!/usr/bin/awk -f
{print$0}

It can be invoked with:./print.awk <filename>

The-ftells awk that the argument that follows is the file to read the AWK program from, which is the same flag that is used in sed. Since they are often used for one-liners, both these programs default to executing a program given as a command-line argument, rather than a separate file.

Versions and implementations

AWK was originally written in 1977 and distributed withVersion 7 Unix.

In 1985 its authors started expanding the language, most significantly by adding user-defined functions. The language is described in the bookThe AWK Programming Language,published 1988, and its implementation was made available in releases ofUNIX System V.To avoid confusion with the incompatible older version, this version was sometimes called "new awk" ornawk.This implementation was released under afree software licensein 1996 and is still maintained by Brian Kernighan (see external links below).

Old versions of Unix, such asUNIX/32V,includedawkcc,which converted AWK to C. Kernighan wrote a program to turn awk into C++; its state is not known.

• BWK awk,also known asnawk,refers to the version byBrian Kernighan.It has been dubbed the "One True AWK" because of the use of the term in association with the book that originally described the language and the fact that Kernighan was one of the original authors of AWK.FreeBSD refers to this version asone-true-awk.This version also has features not in the book, such astolowerandENVIRONthat are explained above; see the FIXES file in the source archive for details. This version is used by, for example,Android,FreeBSD,NetBSD,OpenBSD,macOS,andillumos.Brian Kernighan and Arnold Robbins are the main contributors to a source repository fornawk:github/onetrueawk/awk.
• gawk(GNUawk) is another free-software implementation and the only implementation that makes serious progress implementinginternationalization and localizationand TCP/IP networking. It was written before the original implementation became freely available. It includes its own debugger, and itsprofilerenables the user to make measured performance enhancements to a script. It also enables the user to extend functionality with shared libraries. SomeLinux distributionsincludegawkas their default AWK implementation.As of version 5.2 (September 2022)gawkincludes a persistent memory feature that can remember script-defined variables and functions from one invocation of a script to the next and pass data between unrelated scripts, as described in the Persistent-MemorygawkUser Manual:www.gnu.org/software/gawk/manual/pm-gawk/.
  • gawk-csv.TheCSVextension ofgawkprovides facilities for handling input and output CSV formatted data.
• mawkis a very fast AWK implementation by Mike Brennan based on abytecodeinterpreter.
• libmawkis a fork of mawk, allowing applications to embed multiple parallel instances of awk interpreters.
• awka(whose front end is written atop themawkprogram) is another translator of AWK scripts into C code. When compiled, statically including the author's libawka.a, the resulting executables are considerably sped up and, according to the author's tests, compare very well with other versions of AWK,Perl,orTcl.Small scripts will turn into programs of 160–170 kB.
• tawk(Thompson AWK) is an AWKcompilerforSolaris,DOS,OS/2,andWindows,previously sold by Thompson Automation Software (which has ceased its activities).
• Jawkis a project to implement AWK inJava,hosted on SourceForge.Extensions to the language are added to provide access to Java features within AWK scripts (i.e., Java threads, sockets, collections, etc.).
• xgawkis a fork ofgawkthat extendsgawkwith dynamically loadable libraries. The XMLgawk extension was integrated into the official GNU Awk release 4.1.0.
• QSEAWKis an embedded AWK interpreter implementation included in the QSE library that provides embeddingapplication programming interface(API) forCandC++.
• libfawkis a very small, function-only, reentrant, embeddable interpreter written in C
• BusyBoxincludes an AWK implementation written by Dmitry Zakharov. This is a very small implementation suitable for embedded systems.
• CLAWKby Michael Parker provides an AWK implementation inCommon Lisp,based upon the regular expression library of the same author.
• goawkis an AWK implementation in Go with a few convenience extensions by Ben Hoyt, hosted onGithub.

gawk manual has a list of more Awk implementations.

Books

• Aho, Alfred V.;Kernighan, Brian W.;Weinberger, Peter J.(1988-01-01).The AWK Programming Language.New York, NY:Addison-Wesley.ISBN0-201-07981-X.Retrieved2017-01-22.
• Aho, Alfred V.;Kernighan, Brian W.;Weinberger, Peter J.(2023-09-06).The AWK Programming Language, Second Edition.Hoboken, New Jersey:Addison-Wesley Professional.ISBN978-0-13-826972-2.Archivedfrom the original on 2023-10-27.Retrieved2023-11-03.
• Robbins, Arnold (2001-05-15).Effective awk Programming(3rd ed.). Sebastopol, CA:O'Reilly Media.ISBN0-596-00070-7.Retrieved2009-04-16.
• Dougherty, Dale;Robbins, Arnold (1997-03-01).sed & awk(2nd ed.). Sebastopol, CA: O'Reilly Media.ISBN1-56592-225-5.Retrieved2009-04-16.
• Robbins, Arnold (2000).Effective Awk Programming: A User's Guide for Gnu Awk(1.0.3 ed.). Bloomington, IN:iUniverse.ISBN0-595-10034-1.Archivedfrom the original on 12 April 2009.Retrieved2009-04-16.

See also

• Data transformation
• Event-driven programming
• List of Unix commands
• sed

Further reading

• Andy Oram (May 19, 2021)."Awk: The Power and Promise of a 40-Year-Old Language".Fosslife.RetrievedJune 9,2021.
• Hamilton, Naomi (May 30, 2008)."The A-Z of Programming Languages: AWK".Computerworld.Retrieved2008-12-12.– Interview with Alfred V. Aho on AWK
• Robbins, Daniel(2000-12-01)."Awk by example, Part 1: An intro to the great language with the strange name".Common threads.IBM DeveloperWorks.Retrieved2009-04-16.
• Robbins, Daniel (2001-01-01)."Awk by example, Part 2: Records, loops, and arrays".Common threads.IBM DeveloperWorks.Retrieved2009-04-16.
• Robbins, Daniel (2001-04-01)."Awk by example, Part 3: String functions and... checkbooks?".Common threads.IBM DeveloperWorks.Archivedfrom the original on 19 May 2009.Retrieved2009-04-16.
• AWK – Become an expert in 60 minutes
• awk:pattern scanning and processing language – Shell and Utilities Reference,The Single UNIX Specification,Version 4 fromThe Open Group
• gawk(1)–LinuxUserManual– User Commands

External links

• The Amazing Awk AssemblerbyHenry Spencer.
• AWKatCurlie
• awklang.orgThe site for things related to the awk language