Provided by: gawk_4.0.1+dfsg-2.1ubuntu2_amd64 
NAME
gawk - pattern scanning and processing language
SYNOPSIS
gawk [ POSIX or GNU style options ] -f program-file [ -- ] file ...
gawk [ POSIX or GNU style options ] [ -- ] program-text file ...
pgawk [ POSIX or GNU style options ] -f program-file [ -- ] file ...
pgawk [ POSIX or GNU style options ] [ -- ] program-text file ...
dgawk [ POSIX or GNU style options ] -f program-file [ -- ] file ...
DESCRIPTION
Gawk is the GNU Project's implementation of the AWK programming language. It conforms to the definition
of the language in the POSIX 1003.1 Standard. This version in turn is based on the description in The
AWK Programming Language, by Aho, Kernighan, and Weinberger. Gawk provides the additional features found
in the current version of UNIX awk and a number of GNU-specific extensions.
The command line consists of options to gawk itself, the AWK program text (if not supplied via the -f or
--file options), and values to be made available in the ARGC and ARGV pre-defined AWK variables.
Pgawk is the profiling version of gawk. It is identical in every way to gawk, except that programs run
more slowly, and it automatically produces an execution profile in the file awkprof.out when done. See
the --profile option, below.
Dgawk is an awk debugger. Instead of running the program directly, it loads the AWK source code and then
prompts for debugging commands. Unlike gawk and pgawk, dgawk only processes AWK program source provided
with the -f option. The debugger is documented in GAWK: Effective AWK Programming.
OPTION FORMAT
Gawk options may be either traditional POSIX-style one letter options, or GNU-style long options. POSIX
options start with a single “-”, while long options start with “--”. Long options are provided for both
GNU-specific features and for POSIX-mandated features.
Gawk- specific options are typically used in long-option form. Arguments to long options are either
joined with the option by an = sign, with no intervening spaces, or they may be provided in the next
command line argument. Long options may be abbreviated, as long as the abbreviation remains unique.
Additionally, each long option has a corresponding short option, so that the option's functionality may
be used from within #! executable scripts.
OPTIONS
Gawk accepts the following options. Standard options are listed first, followed by options for gawk
extensions, listed alphabetically by short option.
-f program-file
--file program-file
Read the AWK program source from the file program-file, instead of from the first command line
argument. Multiple -f (or --file) options may be used.
-F fs
--field-separator fs
Use fs for the input field separator (the value of the FS predefined variable).
-v var=val
--assign var=val
Assign the value val to the variable var, before execution of the program begins. Such variable
values are available to the BEGIN block of an AWK program.
-b
--characters-as-bytes
Treat all input data as single-byte characters. In other words, don't pay any attention to the
locale information when attempting to process strings as multibyte characters. The --posix option
overrides this one.
-c
--traditional
Run in compatibility mode. In compatibility mode, gawk behaves identically to UNIX awk; none of
the GNU-specific extensions are recognized. See GNU EXTENSIONS, below, for more information.
-C
--copyright
Print the short version of the GNU copyright information message on the standard output and exit
successfully.
-d[file]
--dump-variables[=file]
Print a sorted list of global variables, their types and final values to file. If no file is
provided, gawk uses a file named awkvars.out in the current directory.
Having a list of all the global variables is a good way to look for typographical errors in your
programs. You would also use this option if you have a large program with a lot of functions, and
you want to be sure that your functions don't inadvertently use global variables that you meant to
be local. (This is a particularly easy mistake to make with simple variable names like i, j, and
so on.)
-e program-text
--source program-text
Use program-text as AWK program source code. This option allows the easy intermixing of library
functions (used via the -f and --file options) with source code entered on the command line. It
is intended primarily for medium to large AWK programs used in shell scripts.
-E file
--exec file
Similar to -f, however, this is option is the last one processed. This should be used with #!
scripts, particularly for CGI applications, to avoid passing in options or source code (!) on the
command line from a URL. This option disables command-line variable assignments.
-g
--gen-pot
Scan and parse the AWK program, and generate a GNU .pot (Portable Object Template) format file on
standard output with entries for all localizable strings in the program. The program itself is
not executed. See the GNU gettext distribution for more information on .pot files.
-h
--help Print a relatively short summary of the available options on the standard output. (Per the GNU
Coding Standards, these options cause an immediate, successful exit.)
-L [value]
--lint[=value]
Provide warnings about constructs that are dubious or non-portable to other AWK implementations.
With an optional argument of fatal, lint warnings become fatal errors. This may be drastic, but
its use will certainly encourage the development of cleaner AWK programs. With an optional
argument of invalid, only warnings about things that are actually invalid are issued. (This is not
fully implemented yet.)
-n
--non-decimal-data
Recognize octal and hexadecimal values in input data. Use this option with great caution!
-N
--use-lc-numeric
This forces gawk to use the locale's decimal point character when parsing input data. Although
the POSIX standard requires this behavior, and gawk does so when --posix is in effect, the default
is to follow traditional behavior and use a period as the decimal point, even in locales where the
period is not the decimal point character. This option overrides the default behavior, without
the full draconian strictness of the --posix option.
-O
--optimize
Enable optimizations upon the internal representation of the program. Currently, this includes
just simple constant-folding. The gawk maintainer hopes to add additional optimizations over time.
-p[prof_file]
--profile[=prof_file]
Send profiling data to prof_file. The default is awkprof.out. When run with gawk, the profile is
just a “pretty printed” version of the program. When run with pgawk, the profile contains
execution counts of each statement in the program in the left margin and function call counts for
each user-defined function.
-P
--posix
This turns on compatibility mode, with the following additional restrictions:
• \x escape sequences are not recognized.
• Only space and tab act as field separators when FS is set to a single space, newline does not.
• You cannot continue lines after ? and :.
• The synonym func for the keyword function is not recognized.
• The operators ** and **= cannot be used in place of ^ and ^=.
• The fflush() function is not available.
-r
--re-interval
Enable the use of interval expressions in regular expression matching (see Regular Expressions,
below). Interval expressions were not traditionally available in the AWK language. The POSIX
standard added them, to make awk and egrep consistent with each other. They are enabled by
default, but this option remains for use with --traditional.
-R
--command file
Dgawk only. Read stored debugger commands from file.
-S
--sandbox
Runs gawk in sandbox mode, disabling the system() function, input redirection with getline, output
redirection with print and printf, and loading dynamic extensions. Command execution (through
pipelines) is also disabled. This effectively blocks a script from accessing local resources
(except for the files specified on the command line).
-t
--lint-old
Provide warnings about constructs that are not portable to the original version of Unix awk.
-V
--version
Print version information for this particular copy of gawk on the standard output. This is useful
mainly for knowing if the current copy of gawk on your system is up to date with respect to
whatever the Free Software Foundation is distributing. This is also useful when reporting bugs.
(Per the GNU Coding Standards, these options cause an immediate, successful exit.)
-- Signal the end of options. This is useful to allow further arguments to the AWK program itself to
start with a “-”. This provides consistency with the argument parsing convention used by most
other POSIX programs.
In compatibility mode, any other options are flagged as invalid, but are otherwise ignored. In normal
operation, as long as program text has been supplied, unknown options are passed on to the AWK program in
the ARGV array for processing. This is particularly useful for running AWK programs via the “#!”
executable interpreter mechanism.
AWK PROGRAM EXECUTION
An AWK program consists of a sequence of pattern-action statements and optional function definitions.
@include "filename" pattern { action statements }
function name(parameter list) { statements }
Gawk first reads the program source from the program-file(s) if specified, from arguments to --source, or
from the first non-option argument on the command line. The -f and --source options may be used multiple
times on the command line. Gawk reads the program text as if all the program-files and command line
source texts had been concatenated together. This is useful for building libraries of AWK functions,
without having to include them in each new AWK program that uses them. It also provides the ability to
mix library functions with command line programs.
In addition, lines beginning with @include may be used to include other source files into your program,
making library use even easier.
The environment variable AWKPATH specifies a search path to use when finding source files named with the
-f option. If this variable does not exist, the default path is ".:/usr/local/share/awk". (The actual
directory may vary, depending upon how gawk was built and installed.) If a file name given to the -f
option contains a “/” character, no path search is performed.
Gawk executes AWK programs in the following order. First, all variable assignments specified via the -v
option are performed. Next, gawk compiles the program into an internal form. Then, gawk executes the
code in the BEGIN block(s) (if any), and then proceeds to read each file named in the ARGV array (up to
ARGV[ARGC]). If there are no files named on the command line, gawk reads the standard input.
If a filename on the command line has the form var=val it is treated as a variable assignment. The
variable var will be assigned the value val. (This happens after any BEGIN block(s) have been run.)
Command line variable assignment is most useful for dynamically assigning values to the variables AWK
uses to control how input is broken into fields and records. It is also useful for controlling state if
multiple passes are needed over a single data file.
If the value of a particular element of ARGV is empty (""), gawk skips over it.
For each input file, if a BEGINFILE rule exists, gawk executes the associated code before processing the
contents of the file. Similarly, gawk executes the code associated with ENDFILE after processing the
file.
For each record in the input, gawk tests to see if it matches any pattern in the AWK program. For each
pattern that the record matches, the associated action is executed. The patterns are tested in the order
they occur in the program.
Finally, after all the input is exhausted, gawk executes the code in the END block(s) (if any).
Command Line Directories
According to POSIX, files named on the awk command line must be text files. The behavior is
``undefined'' if they are not. Most versions of awk treat a directory on the command line as a fatal
error.
Starting with version 4.0 of gawk, a directory on the command line produces a warning, but is otherwise
skipped. If either of the --posix or --traditional options is given, then gawk reverts to treating
directories on the command line as a fatal error.
VARIABLES, RECORDS AND FIELDS
AWK variables are dynamic; they come into existence when they are first used. Their values are either
floating-point numbers or strings, or both, depending upon how they are used. AWK also has one
dimensional arrays; arrays with multiple dimensions may be simulated. Several pre-defined variables are
set as a program runs; these are described as needed and summarized below.
Records
Normally, records are separated by newline characters. You can control how records are separated by
assigning values to the built-in variable RS. If RS is any single character, that character separates
records. Otherwise, RS is a regular expression. Text in the input that matches this regular expression
separates the record. However, in compatibility mode, only the first character of its string value is
used for separating records. If RS is set to the null string, then records are separated by blank lines.
When RS is set to the null string, the newline character always acts as a field separator, in addition to
whatever value FS may have.
Fields
As each input record is read, gawk splits the record into fields, using the value of the FS variable as
the field separator. If FS is a single character, fields are separated by that character. If FS is the
null string, then each individual character becomes a separate field. Otherwise, FS is expected to be a
full regular expression. In the special case that FS is a single space, fields are separated by runs of
spaces and/or tabs and/or newlines. (But see the section POSIX COMPATIBILITY, below). NOTE: The value
of IGNORECASE (see below) also affects how fields are split when FS is a regular expression, and how
records are separated when RS is a regular expression.
If the FIELDWIDTHS variable is set to a space separated list of numbers, each field is expected to have
fixed width, and gawk splits up the record using the specified widths. The value of FS is ignored.
Assigning a new value to FS or FPAT overrides the use of FIELDWIDTHS.
Similarly, if the FPAT variable is set to a string representing a regular expression, each field is made
up of text that matches that regular expression. In this case, the regular expression describes the
fields themselves, instead of the text that separates the fields. Assigning a new value to FS or
FIELDWIDTHS overrides the use of FPAT.
Each field in the input record may be referenced by its position, $1, $2, and so on. $0 is the whole
record. Fields need not be referenced by constants:
n = 5
print $n
prints the fifth field in the input record.
The variable NF is set to the total number of fields in the input record.
References to non-existent fields (i.e. fields after $NF) produce the null-string. However, assigning to
a non-existent field (e.g., $(NF+2) = 5) increases the value of NF, creates any intervening fields with
the null string as their value, and causes the value of $0 to be recomputed, with the fields being
separated by the value of OFS. References to negative numbered fields cause a fatal error. Decrementing
NF causes the values of fields past the new value to be lost, and the value of $0 to be recomputed, with
the fields being separated by the value of OFS.
Assigning a value to an existing field causes the whole record to be rebuilt when $0 is referenced.
Similarly, assigning a value to $0 causes the record to be resplit, creating new values for the fields.
Built-in Variables
Gawk's built-in variables are:
ARGC The number of command line arguments (does not include options to gawk, or the program
source).
ARGIND The index in ARGV of the current file being processed.
ARGV Array of command line arguments. The array is indexed from 0 to ARGC - 1. Dynamically
changing the contents of ARGV can control the files used for data.
BINMODE On non-POSIX systems, specifies use of “binary” mode for all file I/O. Numeric values of 1,
2, or 3, specify that input files, output files, or all files, respectively, should use
binary I/O. String values of "r", or "w" specify that input files, or output files,
respectively, should use binary I/O. String values of "rw" or "wr" specify that all files
should use binary I/O. Any other string value is treated as "rw", but generates a warning
message.
CONVFMT The conversion format for numbers, "%.6g", by default.
ENVIRON An array containing the values of the current environment. The array is indexed by the
environment variables, each element being the value of that variable (e.g., ENVIRON["HOME"]
might be /home/arnold). Changing this array does not affect the environment seen by programs
which gawk spawns via redirection or the system() function.
ERRNO If a system error occurs either doing a redirection for getline, during a read for getline,
or during a close(), then ERRNO will contain a string describing the error. The value is
subject to translation in non-English locales.
FIELDWIDTHS A whitespace separated list of field widths. When set, gawk parses the input into fields of
fixed width, instead of using the value of the FS variable as the field separator. See
Fields, above.
FILENAME The name of the current input file. If no files are specified on the command line, the value
of FILENAME is “-”. However, FILENAME is undefined inside the BEGIN block (unless set by
getline).
FNR The input record number in the current input file.
FPAT A regular expression describing the contents of the fields in a record. When set, gawk
parses the input into fields, where the fields match the regular expression, instead of using
the value of the FS variable as the field separator. See Fields, above.
FS The input field separator, a space by default. See Fields, above.
IGNORECASE Controls the case-sensitivity of all regular expression and string operations. If IGNORECASE
has a non-zero value, then string comparisons and pattern matching in rules, field splitting
with FS and FPAT, record separating with RS, regular expression matching with ~ and !~, and
the gensub(), gsub(), index(), match(), patsplit(), split(), and sub() built-in functions all
ignore case when doing regular expression operations. NOTE: Array subscripting is not
affected. However, the asort() and asorti() functions are affected.
Thus, if IGNORECASE is not equal to zero, /aB/ matches all of the strings "ab", "aB", "Ab",
and "AB". As with all AWK variables, the initial value of IGNORECASE is zero, so all regular
expression and string operations are normally case-sensitive.
LINT Provides dynamic control of the --lint option from within an AWK program. When true, gawk
prints lint warnings. When false, it does not. When assigned the string value "fatal", lint
warnings become fatal errors, exactly like --lint=fatal. Any other true value just prints
warnings.
NF The number of fields in the current input record.
NR The total number of input records seen so far.
OFMT The output format for numbers, "%.6g", by default.
OFS The output field separator, a space by default.
ORS The output record separator, by default a newline.
PROCINFO The elements of this array provide access to information about the running AWK program. On
some systems, there may be elements in the array, "group1" through "groupn" for some n, which
is the number of supplementary groups that the process has. Use the in operator to test for
these elements. The following elements are guaranteed to be available:
PROCINFO["egid"] the value of the getegid(2) system call.
PROCINFO["strftime"]
The default time format string for strftime().
PROCINFO["euid"] the value of the geteuid(2) system call.
PROCINFO["FS"] "FS" if field splitting with FS is in effect, "FPAT" if field splitting
with FPAT is in effect, or "FIELDWIDTHS" if field splitting with
FIELDWIDTHS is in effect.
PROCINFO["gid"] the value of the getgid(2) system call.
PROCINFO["pgrpid"] the process group ID of the current process.
PROCINFO["pid"] the process ID of the current process.
PROCINFO["ppid"] the parent process ID of the current process.
PROCINFO["uid"] the value of the getuid(2) system call.
PROCINFO["sorted_in"]
If this element exists in PROCINFO, then its value controls the order in
which array elements are traversed in for loops. Supported values are
"@ind_str_asc", "@ind_num_asc", "@val_type_asc", "@val_str_asc",
"@val_num_asc", "@ind_str_desc", "@ind_num_desc", "@val_type_desc",
"@val_str_desc", "@val_num_desc", and "@unsorted". The value can also be
the name of any comparison function defined as follows:
function cmp_func(i1, v1, i2, v2)
where i1 and i2 are the indices, and v1 and v2 are the corresponding values of the two
elements being compared. It should return a number less than, equal to, or greater than 0,
depending on how the elements of the array are to be ordered.
PROCINFO["version"]
the version of gawk.
RS The input record separator, by default a newline.
RT The record terminator. Gawk sets RT to the input text that matched the character or regular
expression specified by RS.
RSTART The index of the first character matched by match(); 0 if no match. (This implies that
character indices start at one.)
RLENGTH The length of the string matched by match(); -1 if no match.
SUBSEP The character used to separate multiple subscripts in array elements, by default "\034".
TEXTDOMAIN The text domain of the AWK program; used to find the localized translations for the program's
strings.
Arrays
Arrays are subscripted with an expression between square brackets ([ and ]). If the expression is an
expression list (expr, expr ...) then the array subscript is a string consisting of the concatenation of
the (string) value of each expression, separated by the value of the SUBSEP variable. This facility is
used to simulate multiply dimensioned arrays. For example:
i = "A"; j = "B"; k = "C"
x[i, j, k] = "hello, world\n"
assigns the string "hello, world\n" to the element of the array x which is indexed by the string
"A\034B\034C". All arrays in AWK are associative, i.e. indexed by string values.
The special operator in may be used to test if an array has an index consisting of a particular value:
if (val in array)
print array[val]
If the array has multiple subscripts, use (i, j) in array.
The in construct may also be used in a for loop to iterate over all the elements of an array.
An element may be deleted from an array using the delete statement. The delete statement may also be
used to delete the entire contents of an array, just by specifying the array name without a subscript.
gawk supports true multidimensional arrays. It does not require that such arrays be ``rectangular'' as in
C or C++. For example:
a[1] = 5
a[2][1] = 6
a[2][2] = 7
Variable Typing And Conversion
Variables and fields may be (floating point) numbers, or strings, or both. How the value of a variable
is interpreted depends upon its context. If used in a numeric expression, it will be treated as a
number; if used as a string it will be treated as a string.
To force a variable to be treated as a number, add 0 to it; to force it to be treated as a string,
concatenate it with the null string.
When a string must be converted to a number, the conversion is accomplished using strtod(3). A number is
converted to a string by using the value of CONVFMT as a format string for sprintf(3), with the numeric
value of the variable as the argument. However, even though all numbers in AWK are floating-point,
integral values are always converted as integers. Thus, given
CONVFMT = "%2.2f"
a = 12
b = a ""
the variable b has a string value of "12" and not "12.00".
NOTE: When operating in POSIX mode (such as with the --posix command line option), beware that locale
settings may interfere with the way decimal numbers are treated: the decimal separator of the numbers you
are feeding to gawk must conform to what your locale would expect, be it a comma (,) or a period (.).
Gawk performs comparisons as follows: If two variables are numeric, they are compared numerically. If
one value is numeric and the other has a string value that is a “numeric string,” then comparisons are
also done numerically. Otherwise, the numeric value is converted to a string and a string comparison is
performed. Two strings are compared, of course, as strings.
Note that string constants, such as "57", are not numeric strings, they are string constants. The idea
of “numeric string” only applies to fields, getline input, FILENAME, ARGV elements, ENVIRON elements and
the elements of an array created by split() or patsplit() that are numeric strings. The basic idea is
that user input, and only user input, that looks numeric, should be treated that way.
Uninitialized variables have the numeric value 0 and the string value "" (the null, or empty, string).
Octal and Hexadecimal Constants
You may use C-style octal and hexadecimal constants in your AWK program source code. For example, the
octal value 011 is equal to decimal 9, and the hexadecimal value 0x11 is equal to decimal 17.
String Constants
String constants in AWK are sequences of characters enclosed between double quotes (like "value").
Within strings, certain escape sequences are recognized, as in C. These are:
\\ A literal backslash.
\a The “alert” character; usually the ASCII BEL character.
\b backspace.
\f form-feed.
\n newline.
\r carriage return.
\t horizontal tab.
\v vertical tab.
\xhex digits
The character represented by the string of hexadecimal digits following the \x. As in ANSI C, all
following hexadecimal digits are considered part of the escape sequence. (This feature should tell
us something about language design by committee.) E.g., "\x1B" is the ASCII ESC (escape) character.
\ddd The character represented by the 1-, 2-, or 3-digit sequence of octal digits. E.g., "\033" is the
ASCII ESC (escape) character.
\c The literal character c.
The escape sequences may also be used inside constant regular expressions (e.g., /[ \t\f\n\r\v]/ matches
whitespace characters).
In compatibility mode, the characters represented by octal and hexadecimal escape sequences are treated
literally when used in regular expression constants. Thus, /a\52b/ is equivalent to /a\*b/.
PATTERNS AND ACTIONS
AWK is a line-oriented language. The pattern comes first, and then the action. Action statements are
enclosed in { and }. Either the pattern may be missing, or the action may be missing, but, of course,
not both. If the pattern is missing, the action is executed for every single record of input. A missing
action is equivalent to
{ print }
which prints the entire record.
Comments begin with the # character, and continue until the end of the line. Blank lines may be used to
separate statements. Normally, a statement ends with a newline, however, this is not the case for lines
ending in a comma, {, ?, :, &&, or ||. Lines ending in do or else also have their statements
automatically continued on the following line. In other cases, a line can be continued by ending it with
a “\”, in which case the newline is ignored.
Multiple statements may be put on one line by separating them with a “;”. This applies to both the
statements within the action part of a pattern-action pair (the usual case), and to the pattern-action
statements themselves.
Patterns
AWK patterns may be one of the following:
BEGIN
END
BEGINFILE
ENDFILE
/regular expression/
relational expression
pattern && pattern
pattern || pattern
pattern ? pattern : pattern
(pattern)
! pattern
pattern1, pattern2
BEGIN and END are two special kinds of patterns which are not tested against the input. The action parts
of all BEGIN patterns are merged as if all the statements had been written in a single BEGIN block. They
are executed before any of the input is read. Similarly, all the END blocks are merged, and executed
when all the input is exhausted (or when an exit statement is executed). BEGIN and END patterns cannot
be combined with other patterns in pattern expressions. BEGIN and END patterns cannot have missing
action parts.
BEGINFILE and ENDFILE are additional special patterns whose bodies are executed before reading the first
record of each command line input file and after reading the last record of each file. Inside the
BEGINFILE rule, the value of ERRNO will be the empty string if the file could be opened successfully.
Otherwise, there is some problem with the file and the code should use nextfile to skip it. If that is
not done, gawk produces its usual fatal error for files that cannot be opened.
For /regular expression/ patterns, the associated statement is executed for each input record that
matches the regular expression. Regular expressions are the same as those in egrep(1), and are
summarized below.
A relational expression may use any of the operators defined below in the section on actions. These
generally test whether certain fields match certain regular expressions.
The &&, ||, and ! operators are logical AND, logical OR, and logical NOT, respectively, as in C. They
do short-circuit evaluation, also as in C, and are used for combining more primitive pattern expressions.
As in most languages, parentheses may be used to change the order of evaluation.
The ?: operator is like the same operator in C. If the first pattern is true then the pattern used for
testing is the second pattern, otherwise it is the third. Only one of the second and third patterns is
evaluated.
The pattern1, pattern2 form of an expression is called a range pattern. It matches all input records
starting with a record that matches pattern1, and continuing until a record that matches pattern2,
inclusive. It does not combine with any other sort of pattern expression.
Regular Expressions
Regular expressions are the extended kind found in egrep. They are composed of characters as follows:
c matches the non-metacharacter c.
\c matches the literal character c.
. matches any character including newline.
^ matches the beginning of a string.
$ matches the end of a string.
[abc...] character list, matches any of the characters abc....
[^abc...] negated character list, matches any character except abc....
r1|r2 alternation: matches either r1 or r2.
r1r2 concatenation: matches r1, and then r2.
r+ matches one or more r's.
r* matches zero or more r's.
r? matches zero or one r's.
(r) grouping: matches r.
r{n}
r{n,}
r{n,m} One or two numbers inside braces denote an interval expression. If there is one number in the
braces, the preceding regular expression r is repeated n times. If there are two numbers
separated by a comma, r is repeated n to m times. If there is one number followed by a comma,
then r is repeated at least n times.
\y matches the empty string at either the beginning or the end of a word.
\B matches the empty string within a word.
\< matches the empty string at the beginning of a word.
\> matches the empty string at the end of a word.
\s matches any whitespace character.
\S matches any nonwhitespace character.
\w matches any word-constituent character (letter, digit, or underscore).
\W matches any character that is not word-constituent.
\` matches the empty string at the beginning of a buffer (string).
\' matches the empty string at the end of a buffer.
The escape sequences that are valid in string constants (see below) are also valid in regular
expressions.
Character classes are a 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 regular expression inside the brackets of a character list.
Character classes consist of [:, a keyword denoting the class, and :]. The character classes defined by
the POSIX standard are:
[:alnum:] Alphanumeric characters.
[:alpha:] Alphabetic characters.
[:blank:] Space or tab characters.
[:cntrl:] Control characters.
[:digit:] Numeric characters.
[:graph:] Characters that are both printable and visible. (A space is printable, but not visible, while
an a is both.)
[:lower:] Lowercase 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:] Uppercase alphabetic characters.
[:xdigit:] Characters that are hexadecimal digits.
For example, before the POSIX standard, to match alphanumeric characters, you would have had to write
/[A-Za-z0-9]/. If your character set had other alphabetic characters in it, this would not match them,
and if your character set collated differently from ASCII, this might not even match the ASCII
alphanumeric characters. With the POSIX character classes, you can write /[[:alnum:]]/, and this matches
the alphabetic and numeric characters in your character set, no matter what it is.
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 “`” 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 regular expression that matches this collating
element, while [ch] is a regular expression 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,”
“´,” and “`.” In this case, [[=e=]] is a regular expression that matches any of e, e´, or e`.
These features are very valuable in non-English speaking locales. 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.
The \y, \B, \<, \>, \s, \S, \w, \W, \`, and \' operators are specific to gawk; they are extensions based
on facilities in the GNU regular expression libraries.
The various command line options control how gawk interprets characters in regular expressions.
No options
In the default case, gawk provide all the facilities of POSIX regular expressions and the GNU
regular expression operators described above.
--posix
Only POSIX regular expressions are supported, the GNU operators are not special. (E.g., \w
matches a literal w).
--traditional
Traditional Unix awk regular expressions are matched. The GNU operators are not special, and
interval expressions are not available. Characters described by octal and hexadecimal escape
sequences are treated literally, even if they represent regular expression metacharacters.
--re-interval
Allow interval expressions in regular expressions, even if --traditional has been provided.
Actions
Action statements are enclosed in braces, { and }. Action statements consist of the usual assignment,
conditional, and looping statements found in most languages. The operators, control statements, and
input/output statements available are patterned after those in C.
Operators
The operators in AWK, in order of decreasing precedence, are
(...) Grouping
$ Field reference.
++ -- Increment and decrement, both prefix and postfix.
^ Exponentiation (** may also be used, and **= for the assignment operator).
+ - ! Unary plus, unary minus, and logical negation.
* / % Multiplication, division, and modulus.
+ - Addition and subtraction.
space String concatenation.
| |& Piped I/O for getline, print, and printf.
< > <= >= != ==
The regular relational operators.
~ !~ Regular expression match, negated match. NOTE: Do not use a constant regular expression
(/foo/) on the left-hand side of a ~ or !~. Only use one on the right-hand side. The
expression /foo/ ~ exp has the same meaning as (($0 ~ /foo/) ~ exp). This is usually not
what was intended.
in Array membership.
&& Logical AND.
|| Logical OR.
?: The C conditional expression. This has the form expr1 ? expr2 : expr3. If expr1 is true,
the value of the expression is expr2, otherwise it is expr3. Only one of expr2 and expr3 is
evaluated.
= += -= *= /= %= ^=
Assignment. Both absolute assignment (var = value) and operator-assignment (the other forms)
are supported.
Control Statements
The control statements are as follows:
if (condition) statement [ else statement ]
while (condition) statement
do statement while (condition)
for (expr1; expr2; expr3) statement
for (var in array) statement
break
continue
delete array[index]
delete array
exit [ expression ]
{ statements }
switch (expression) {
case value|regex : statement
...
[ default: statement ]
}
I/O Statements
The input/output statements are as follows:
close(file [, how]) Close file, pipe or co-process. The optional how should only be used when closing
one end of a two-way pipe to a co-process. It must be a string value, either "to"
or "from".
getline Set $0 from next input record; set NF, NR, FNR.
getline <file Set $0 from next record of file; set NF.
getline var Set var from next input record; set NR, FNR.
getline var <file Set var from next record of file.
command | getline [var]
Run command piping the output either into $0 or var, as above.
command |& getline [var]
Run command as a co-process piping the output either into $0 or var, as above. Co-
processes are a gawk extension. (command can also be a socket. See the subsection
Special File Names, below.)
next Stop processing the current input record. The next input record is read and
processing starts over with the first pattern in the AWK program. If the end of
the input data is reached, the END block(s), if any, are executed.
nextfile Stop processing the current input file. The next input record read comes from the
next input file. FILENAME and ARGIND are updated, FNR is reset to 1, and
processing starts over with the first pattern in the AWK program. If the end of the
input data is reached, the END block(s), if any, are executed.
print Print the current record. The output record is terminated with the value of the
ORS variable.
print expr-list Print expressions. Each expression is separated by the value of the OFS variable.
The output record is terminated with the value of the ORS variable.
print expr-list >file Print expressions on file. Each expression is separated by the value of the OFS
variable. The output record is terminated with the value of the ORS variable.
printf fmt, expr-list Format and print. See The printf Statement, below.
printf fmt, expr-list >file
Format and print on file.
system(cmd-line) Execute the command cmd-line, and return the exit status. (This may not be
available on non-POSIX systems.)
fflush([file]) Flush any buffers associated with the open output file or pipe file. If file is
missing, then flush standard output. If file is the null string, then flush all
open output files and pipes.
Additional output redirections are allowed for print and printf.
print ... >> file
Appends output to the file.
print ... | command
Writes on a pipe.
print ... |& command
Sends data to a co-process or socket. (See also the subsection Special File Names, below.)
The getline command returns 1 on success, 0 on end of file, and -1 on an error. Upon an error, ERRNO
contains a string describing the problem.
NOTE: Failure in opening a two-way socket will result in a non-fatal error being returned to the calling
function. If using a pipe, co-process, or socket to getline, or from print or printf within a loop, you
must use close() to create new instances of the command or socket. AWK does not automatically close
pipes, sockets, or co-processes when they return EOF.
The printf Statement
The AWK versions of the printf statement and sprintf() function (see below) accept the following
conversion specification formats:
%c A single character. If the argument used for %c is numeric, it is treated as a character and
printed. Otherwise, the argument is assumed to be a string, and the only first character of that
string is printed.
%d, %i A decimal number (the integer part).
%e, %E A floating point number of the form [-]d.dddddde[+-]dd. The %E format uses E instead of e.
%f, %F A floating point number of the form [-]ddd.dddddd. If the system library supports it, %F is
available as well. This is like %f, but uses capital letters for special “not a number” and
“infinity” values. If %F is not available, gawk uses %f.
%g, %G Use %e or %f conversion, whichever is shorter, with nonsignificant zeros suppressed. The %G
format uses %E instead of %e.
%o An unsigned octal number (also an integer).
%u An unsigned decimal number (again, an integer).
%s A character string.
%x, %X An unsigned hexadecimal number (an integer). The %X format uses ABCDEF instead of abcdef.
%% A single % character; no argument is converted.
Optional, additional parameters may lie between the % and the control letter:
count$ Use the count'th argument at this point in the formatting. This is called a positional specifier
and is intended primarily for use in translated versions of format strings, not in the original
text of an AWK program. It is a gawk extension.
- The expression should be left-justified within its field.
space For numeric conversions, prefix positive values with a space, and negative values with a minus
sign.
+ The plus sign, used before the width modifier (see below), says to always supply a sign for
numeric conversions, even if the data to be formatted is positive. The + overrides the space
modifier.
# Use an “alternate form” for certain control letters. For %o, supply a leading zero. For %x, and
%X, supply a leading 0x or 0X for a nonzero result. For %e, %E, %f and %F, the result always
contains 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 zeroes instead of
spaces. This applies only to the numeric output formats. This flag only has an effect when the
field width is wider than the value to be printed.
width The field should be padded to this width. The field is normally padded with spaces. If the 0
flag has been used, it is padded with zeroes.
.prec A number that specifies the precision to use when printing. For the %e, %E, %f 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, %i, %o,
%u, %x, and %X formats, it specifies the minimum number of digits to print. For %s, it specifies
the maximum number of characters from the string that should be printed.
The dynamic width and prec capabilities of the ANSI C printf() routines are supported. A * in place of
either the width or prec specifications causes their values to be taken from the argument list to printf
or sprintf(). To use a positional specifier with a dynamic width or precision, supply the count$ after
the * in the format string. For example, "%3$*2$.*1$s".
Special File Names
When doing I/O redirection from either print or printf into a file, or via getline from a file, gawk
recognizes certain special filenames internally. These filenames allow access to open file descriptors
inherited from gawk's parent process (usually the shell). These file names may also be used on the
command line to name data files. The filenames are:
/dev/stdin The standard input.
/dev/stdout The standard output.
/dev/stderr The standard error output.
/dev/fd/n The file associated with the open file descriptor n.
These are particularly useful for error messages. For example:
print "You blew it!" > "/dev/stderr"
whereas you would otherwise have to use
print "You blew it!" | "cat 1>&2"
The following special filenames may be used with the |& co-process operator for creating TCP/IP network
connections:
/inet/tcp/lport/rhost/rport
/inet4/tcp/lport/rhost/rport
/inet6/tcp/lport/rhost/rport
Files for a TCP/IP connection on local port lport to remote host rhost on remote port rport. Use
a port of 0 to have the system pick a port. Use /inet4 to force an IPv4 connection, and /inet6 to
force an IPv6 connection. Plain /inet uses the system default (most likely IPv4).
/inet/udp/lport/rhost/rport
/inet4/udp/lport/rhost/rport
/inet6/udp/lport/rhost/rport
Similar, but use UDP/IP instead of TCP/IP.
Numeric Functions
AWK has the following built-in arithmetic functions:
atan2(y, x) Return the arctangent of y/x in radians.
cos(expr) Return the cosine of expr, which is in radians.
exp(expr) The exponential function.
int(expr) Truncate to integer.
log(expr) The natural logarithm function.
rand() Return a random number N, between 0 and 1, such that 0 ≤ N < 1.
sin(expr) Return the sine of expr, which is in radians.
sqrt(expr) The square root function.
srand([expr]) Use expr as the new seed for the random number generator. If no expr is provided, use the
time of day. The return value is the previous seed for the random number generator.
String Functions
Gawk has the following built-in string functions:
asort(s [, d [, how] ]) Return the number of elements in the source array s. Sort the contents of s
using gawk's normal rules for comparing values, and replace the indices of the
sorted values s with sequential integers starting with 1. If the optional
destination array d is specified, then first duplicate s into d, and then sort d,
leaving the indices of the source array s unchanged. The optional string how
controls the direction and the comparison mode. Valid values for how are any of
the strings valid for PROCINFO["sorted_in"]. It can also be the name of a user-
defined comparison function as described in PROCINFO["sorted_in"].
asorti(s [, d [, how] ])
Return the number of elements in the source array s. The behavior is the same as
that of asort(), except that the array indices are used for sorting, not the
array values. When done, the array is indexed numerically, and the values are
those of the original indices. The original values are lost; thus provide a
second array if you wish to preserve the original. The purpose of the optional
string how is the same as described in asort() above.
gensub(r, s, h [, t]) Search the target string t for matches of the regular expression r. If h is a
string beginning with g or G, then replace all matches of r with s. Otherwise, h
is a number indicating which match of r to replace. If t is not supplied, use $0
instead. Within the replacement text s, the sequence \n, where n is a digit from
1 to 9, may be used to indicate just the text that matched the n'th parenthesized
subexpression. The sequence \0 represents the entire matched text, as does the
character &. Unlike sub() and gsub(), the modified string is returned as the
result of the function, and the original target string is not changed.
gsub(r, s [, t]) For each substring matching the regular expression r in the string t, substitute
the string s, and return the number of substitutions. If t is not supplied, use
$0. An & in the replacement text is replaced with the text that was actually
matched. Use \& to get a literal &. (This must be typed as "\\&"; see GAWK:
Effective AWK Programming for a fuller discussion of the rules for &'s and
backslashes in the replacement text of sub(), gsub(), and gensub().)
index(s, t) Return the index of the string t in the string s, or 0 if t is not present.
(This implies that character indices start at one.)
length([s]) Return the length of the string s, or the length of $0 if s is not supplied. As
a non-standard extension, with an array argument, length() returns the number of
elements in the array.
match(s, r [, a]) Return the position in s where the regular expression r occurs, or 0 if r is not
present, and set the values of RSTART and RLENGTH. Note that the argument order
is the same as for the ~ operator: str ~ re. If array a is provided, a is
cleared and then elements 1 through n are filled with the portions of s that
match the corresponding parenthesized subexpression in r. The 0'th element of a
contains the portion of s matched by the entire regular expression r. Subscripts
a[n, "start"], and a[n, "length"] provide the starting index in the string and
length respectively, of each matching substring.
patsplit(s, a [, r [, seps] ])
Split the string s into the array a and the separators array seps on the regular
expression r, and return the number of fields. Element values are the portions
of s that matched r. The value of seps[i] is the separator that appeared in
front of a[i+1]. If r is omitted, FPAT is used instead. The arrays a and seps
are cleared first. Splitting behaves identically to field splitting with FPAT,
described above.
split(s, a [, r [, seps] ])
Split the string s into the array a and the separators array seps on the regular
expression r, and return the number of fields. If r is omitted, FS is used
instead. The arrays a and seps are cleared first. seps[i] is the field
separator matched by r between a[i] and a[i+1]. If r is a single space, then
leading whitespace in s goes into the extra array element seps[0] and trailing
whitespace goes into the extra array element seps[n], where n is the return value
of split(s, a, r, seps). Splitting behaves identically to field splitting,
described above.
sprintf(fmt, expr-list) Prints expr-list according to fmt, and returns the resulting string.
strtonum(str) Examine str, and return its numeric value. If str begins with a leading 0,
strtonum() assumes that str is an octal number. If str begins with a leading 0x
or 0X, strtonum() assumes that str is a hexadecimal number. Otherwise, decimal
is assumed.
sub(r, s [, t]) Just like gsub(), but replace only the first matching substring.
substr(s, i [, n]) Return the at most n-character substring of s starting at i. If n is omitted,
use the rest of s.
tolower(str) Return a copy of the string str, with all the uppercase characters in str
translated to their corresponding lowercase counterparts. Non-alphabetic
characters are left unchanged.
toupper(str) Return a copy of the string str, with all the lowercase characters in str
translated to their corresponding uppercase counterparts. Non-alphabetic
characters are left unchanged.
Gawk is multibyte aware. This means that index(), length(), substr() and match() all work in terms of
characters, not bytes.
Time Functions
Since one of the primary uses of AWK programs is processing log files that contain time stamp
information, gawk provides the following functions for obtaining time stamps and formatting them.
mktime(datespec)
Turn datespec into a time stamp of the same form as returned by systime(), and return the
result. The datespec is a string of the form YYYY MM DD HH MM SS[ DST]. The contents of the
string are six or seven numbers representing respectively the full year including century, the
month from 1 to 12, the day of the month from 1 to 31, the hour of the day from 0 to 23, the
minute from 0 to 59, the second from 0 to 60, and an optional daylight saving flag. The values
of these numbers need not be within the ranges specified; for example, an hour of -1 means 1
hour before midnight. The origin-zero Gregorian calendar is assumed, with year 0 preceding
year 1 and year -1 preceding year 0. The time is assumed to be in the local timezone. If the
daylight saving flag is positive, the time is assumed to be daylight saving time; if zero, the
time is assumed to be standard time; and if negative (the default), mktime() attempts to
determine whether daylight saving time is in effect for the specified time. If datespec does
not contain enough elements or if the resulting time is out of range, mktime() returns -1.
strftime([format [, timestamp[, utc-flag]]])
Format timestamp according to the specification in format. If utc-flag is present and is non-
zero or non-null, the result is in UTC, otherwise the result is in local time. The timestamp
should be of the same form as returned by systime(). If timestamp is missing, the current time
of day is used. If format is missing, a default format equivalent to the output of date(1) is
used. The default format is available in PROCINFO["strftime"]. See the specification for the
strftime() function in ANSI C for the format conversions that are guaranteed to be available.
systime() Return the current time of day as the number of seconds since the Epoch (1970-01-01 00:00:00
UTC on POSIX systems).
Bit Manipulations Functions
Gawk supplies the following bit manipulation functions. They work by converting double-precision
floating point values to uintmax_t integers, doing the operation, and then converting the result back to
floating point. The functions are:
and(v1, v2) Return the bitwise AND of the values provided by v1 and v2.
compl(val) Return the bitwise complement of val.
lshift(val, count) Return the value of val, shifted left by count bits.
or(v1, v2) Return the bitwise OR of the values provided by v1 and v2.
rshift(val, count) Return the value of val, shifted right by count bits.
xor(v1, v2) Return the bitwise XOR of the values provided by v1 and v2.
Type Function
The following function is for use with multidimensional arrays.
isarray(x)
Return true if x is an array, false otherwise.
Internationalization Functions
The following functions may be used from within your AWK program for translating strings at run-time.
For full details, see GAWK: Effective AWK Programming.
bindtextdomain(directory [, domain])
Specify the directory where gawk looks for the .mo files, in case they will not or cannot be
placed in the ``standard'' locations (e.g., during testing). It returns the directory where
domain is ``bound.''
The default domain is the value of TEXTDOMAIN. If directory is the null string (""), then
bindtextdomain() returns the current binding for the given domain.
dcgettext(string [, domain [, category]])
Return the translation of string in text domain domain for locale category category. The default
value for domain is the current value of TEXTDOMAIN. The default value for category is
"LC_MESSAGES".
If you supply a value for category, it must be a string equal to one of the known locale
categories described in GAWK: Effective AWK Programming. You must also supply a text domain. Use
TEXTDOMAIN if you want to use the current domain.
dcngettext(string1 , string2 , number [, domain [, category]])
Return the plural form used for number of the translation of string1 and string2 in text domain
domain for locale category category. The default value for domain is the current value of
TEXTDOMAIN. The default value for category is "LC_MESSAGES".
If you supply a value for category, it must be a string equal to one of the known locale
categories described in GAWK: Effective AWK Programming. You must also supply a text domain. Use
TEXTDOMAIN if you want to use the current domain.
USER-DEFINED FUNCTIONS
Functions in AWK are defined as follows:
function name(parameter list) { statements }
Functions are executed when they are called from within expressions in either patterns or actions.
Actual parameters supplied in the function call are used to instantiate the formal parameters declared in
the function. Arrays are passed by reference, other variables are passed by value.
Since functions were not originally part of the AWK language, the provision for local variables is rather
clumsy: They are declared as extra parameters in the parameter list. The convention is to separate local
variables from real parameters by extra spaces in the parameter list. For example:
function f(p, q, a, b) # a and b are local
{
...
}
/abc/ { ... ; f(1, 2) ; ... }
The left parenthesis in a function call is required to immediately follow the function name, without any
intervening whitespace. This avoids a syntactic ambiguity with the concatenation operator. This
restriction does not apply to the built-in functions listed above.
Functions may call each other and may be recursive. Function parameters used as local variables are
initialized to the null string and the number zero upon function invocation.
Use return expr to return a value from a function. The return value is undefined if no value is
provided, or if the function returns by “falling off” the end.
As a gawk extension, functions may be called indirectly. To do this, assign the name of the function to
be called, as a string, to a variable. Then use the variable as if it were the name of a function,
prefixed with an @ sign, like so:
function myfunc()
{
print "myfunc called"
...
}
{ ...
the_func = "myfunc"
@the_func() # call through the_func to myfunc
...
}
If --lint has been provided, gawk warns about calls to undefined functions at parse time, instead of at
run time. Calling an undefined function at run time is a fatal error.
The word func may be used in place of function.
DYNAMICALLY LOADING NEW FUNCTIONS
You can dynamically add new built-in functions to the running gawk interpreter. The full details are
beyond the scope of this manual page; see GAWK: Effective AWK Programming for the details.
extension(object, function)
Dynamically link the shared object file named by object, and invoke function in that object, to
perform initialization. These should both be provided as strings. Return the value returned by
function.
Using this feature at the C level is not pretty, but it is unlikely to go away. Additional mechanisms may
be added at some point.
SIGNALS
pgawk accepts two signals. SIGUSR1 causes it to dump a profile and function call stack to the profile
file, which is either awkprof.out, or whatever file was named with the --profile option. It then
continues to run. SIGHUP causes pgawk to dump the profile and function call stack and then exit.
INTERNATIONALIZATION
String constants are sequences of characters enclosed in double quotes. In non-English speaking
environments, it is possible to mark strings in the AWK program as requiring translation to the local
natural language. Such strings are marked in the AWK program with a leading underscore (“_”). For
example,
gawk 'BEGIN { print "hello, world" }'
always prints hello, world. But,
gawk 'BEGIN { print _"hello, world" }'
might print bonjour, monde in France.
There are several steps involved in producing and running a localizable AWK program.
1. Add a BEGIN action to assign a value to the TEXTDOMAIN variable to set the text domain to a name
associated with your program:
BEGIN { TEXTDOMAIN = "myprog" }
This allows gawk to find the .mo file associated with your program. Without this step, gawk uses the
messages text domain, which likely does not contain translations for your program.
2. Mark all strings that should be translated with leading underscores.
3. If necessary, use the dcgettext() and/or bindtextdomain() functions in your program, as appropriate.
4. Run gawk --gen-pot -f myprog.awk > myprog.pot to generate a .po file for your program.
5. Provide appropriate translations, and build and install the corresponding .mo files.
The internationalization features are described in full detail in GAWK: Effective AWK Programming.
POSIX COMPATIBILITY
A primary goal for gawk is compatibility with the POSIX standard, as well as with the latest version of
UNIX awk. To this end, gawk incorporates the following user visible features which are not described in
the AWK book, but are part of the Bell Laboratories version of awk, and are in the POSIX standard.
The book indicates that command line variable assignment happens when awk would otherwise open the
argument as a file, which is after the BEGIN block is executed. However, in earlier implementations,
when such an assignment appeared before any file names, the assignment would happen before the BEGIN
block was run. Applications came to depend on this “feature.” When awk was changed to match its
documentation, the -v option for assigning variables before program execution was added to accommodate
applications that depended upon the old behavior. (This feature was agreed upon by both the Bell
Laboratories and the GNU developers.)
When processing arguments, gawk uses the special option “--” to signal the end of arguments. In
compatibility mode, it warns about but otherwise ignores undefined options. In normal operation, such
arguments are passed on to the AWK program for it to process.
The AWK book does not define the return value of srand(). The POSIX standard has it return the seed it
was using, to allow keeping track of random number sequences. Therefore srand() in gawk also returns its
current seed.
Other new features are: The use of multiple -f options (from MKS awk); the ENVIRON array; the \a, and \v
escape sequences (done originally in gawk and fed back into the Bell Laboratories version); the tolower()
and toupper() built-in functions (from the Bell Laboratories version); and the ANSI C conversion
specifications in printf (done first in the Bell Laboratories version).
HISTORICAL FEATURES
There is one feature of historical AWK implementations that gawk supports: It is possible to call the
length() built-in function not only with no argument, but even without parentheses! Thus,
a = length # Holy Algol 60, Batman!
is the same as either of
a = length()
a = length($0)
Using this feature is poor practice, and gawk issues a warning about its use if --lint is specified on
the command line.
GNU EXTENSIONS
Gawk has a number of extensions to POSIX awk. They are described in this section. All the extensions
described here can be disabled by invoking gawk with the --traditional or --posix options.
The following features of gawk are not available in POSIX awk.
• No path search is performed for files named via the -f option. Therefore the AWKPATH environment
variable is not special.
• There is no facility for doing file inclusion (gawk's @include mechanism).
• The \x escape sequence. (Disabled with --posix.)
• The fflush() function. (Disabled with --posix.)
• The ability to continue lines after ? and :. (Disabled with --posix.)
• Octal and hexadecimal constants in AWK programs.
• The ARGIND, BINMODE, ERRNO, LINT, RT and TEXTDOMAIN variables are not special.
• The IGNORECASE variable and its side-effects are not available.
• The FIELDWIDTHS variable and fixed-width field splitting.
• The FPAT variable and field splitting based on field values.
• The PROCINFO array is not available.
• The use of RS as a regular expression.
• The special file names available for I/O redirection are not recognized.
• The |& operator for creating co-processes.
• The BEGINFILE and ENDFILE special patterns are not available.
• The ability to split out individual characters using the null string as the value of FS, and as the
third argument to split().
• An optional fourth argument to split() to receive the separator texts.
• The optional second argument to the close() function.
• The optional third argument to the match() function.
• The ability to use positional specifiers with printf and sprintf().
• The ability to pass an array to length().
• The use of delete array to delete the entire contents of an array.
• The use of nextfile to abandon processing of the current input file.
• The and(), asort(), asorti(), bindtextdomain(), compl(), dcgettext(), dcngettext(), gensub(), lshift(),
mktime(), or(), patsplit(), rshift(), strftime(), strtonum(), systime() and xor() functions.
• Localizable strings.
• Adding new built-in functions dynamically with the extension() function.
The AWK book does not define the return value of the close() function. Gawk's close() returns the value
from fclose(3), or pclose(3), when closing an output file or pipe, respectively. It returns the
process's exit status when closing an input pipe. The return value is -1 if the named file, pipe or co-
process was not opened with a redirection.
When gawk is invoked with the --traditional option, if the fs argument to the -F option is “t”, then FS
is set to the tab character. Note that typing gawk -F\t ... simply causes the shell to quote the “t,”
and does not pass “\t” to the -F option. Since this is a rather ugly special case, it is not the default
behavior. This behavior also does not occur if --posix has been specified. To really get a tab
character as the field separator, it is best to use single quotes: gawk -F'\t' ....
ENVIRONMENT VARIABLES
The AWKPATH environment variable can be used to provide a list of directories that gawk searches when
looking for files named via the -f and --file options.
For socket communication, two special environment variables can be used to control the number of retries
(GAWK_SOCK_RETRIES), and the interval between retries (GAWK_MSEC_SLEEP). The interval is in
milliseconds. On systems that do not support usleep(3), the value is rounded up to an integral number of
seconds.
If POSIXLY_CORRECT exists in the environment, then gawk behaves exactly as if --posix had been specified
on the command line. If --lint has been specified, gawk issues a warning message to this effect.
EXIT STATUS
If the exit statement is used with a value, then gawk exits with the numeric value given to it.
Otherwise, if there were no problems during execution, gawk exits with the value of the C constant
EXIT_SUCCESS. This is usually zero.
If an error occurs, gawk exits with the value of the C constant EXIT_FAILURE. This is usually one.
If gawk exits because of a fatal error, the exit status is 2. On non-POSIX systems, this value may be
mapped to EXIT_FAILURE.
VERSION INFORMATION
This man page documents gawk, version 4.0.
AUTHORS
The original version of UNIX awk was designed and implemented by Alfred Aho, Peter Weinberger, and Brian
Kernighan of Bell Laboratories. Brian Kernighan continues to maintain and enhance it.
Paul Rubin and Jay Fenlason, of the Free Software Foundation, wrote gawk, to be compatible with the
original version of awk distributed in Seventh Edition UNIX. John Woods contributed a number of bug
fixes. David Trueman, with contributions from Arnold Robbins, made gawk compatible with the new version
of UNIX awk. Arnold Robbins is the current maintainer.
The initial DOS port was done by Conrad Kwok and Scott Garfinkle. Scott Deifik maintains the port to MS-
DOS using DJGPP. Eli Zaretskii maintains the port to MS-Windows using MinGW. Pat Rankin did the port to
VMS, and Michal Jaegermann did the port to the Atari ST. The port to OS/2 was done by Kai Uwe Rommel,
with contributions and help from Darrel Hankerson. Andreas Buening now maintains the OS/2 port. The
late Fred Fish supplied support for the Amiga, and Martin Brown provided the BeOS port. Stephen Davies
provided the original Tandem port, and Matthew Woehlke provided changes for Tandem's POSIX-compliant
systems. Dave Pitts provided the port to z/OS.
See the README file in the gawk distribution for up-to-date information about maintainers and which ports
are currently supported.
BUG REPORTS
If you find a bug in gawk, please send electronic mail to bug-gawk@gnu.org. Please include your
operating system and its revision, the version of gawk (from gawk --version), which C compiler you used
to compile it, and a test program and data that are as small as possible for reproducing the problem.
Before sending a bug report, please do the following things. First, verify that you have the latest
version of gawk. Many bugs (usually subtle ones) are fixed at each release, and if yours is out of date,
the problem may already have been solved. Second, please see if setting the environment variable LC_ALL
to LC_ALL=C causes things to behave as you expect. If so, it's a locale issue, and may or may not really
be a bug. Finally, please read this man page and the reference manual carefully to be sure that what you
think is a bug really is, instead of just a quirk in the language.
Whatever you do, do NOT post a bug report in comp.lang.awk. While the gawk developers occasionally read
this newsgroup, posting bug reports there is an unreliable way to report bugs. Instead, please use the
electronic mail addresses given above.
If you're using a GNU/Linux or BSD-based system, you may wish to submit a bug report to the vendor of
your distribution. That's fine, but please send a copy to the official email address as well, since
there's no guarantee that the bug report will be forwarded to the gawk maintainer.
BUGS
The -F option is not necessary given the command line variable assignment feature; it remains only for
backwards compatibility.
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.
SEE ALSO
egrep(1), getpid(2), getppid(2), getpgrp(2), getuid(2), geteuid(2), getgid(2), getegid(2), getgroups(2),
usleep(3)
The AWK Programming Language, Alfred V. Aho, Brian W. Kernighan, Peter J. Weinberger, Addison-Wesley,
1988. ISBN 0-201-07981-X.
GAWK: Effective AWK Programming, Edition 4.0, shipped with the gawk source. The current version of this
document is available online at http://www.gnu.org/software/gawk/manual.
EXAMPLES
Print and sort the login names of all users:
BEGIN { FS = ":" }
{ print $1 | "sort" }
Count lines in a file:
{ nlines++ }
END { print nlines }
Precede each line by its number in the file:
{ print FNR, $0 }
Concatenate and line number (a variation on a theme):
{ print NR, $0 }
Run an external command for particular lines of data:
tail -f access_log |
awk '/myhome.html/ { system("nmap " $1 ">> logdir/myhome.html") }'
ACKNOWLEDGEMENTS
Brian Kernighan of Bell Laboratories provided valuable assistance during testing and debugging. We thank
him.
COPYING PERMISSIONS
Copyright © 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002, 2003, 2004, 2005,
2007, 2009, 2010, 2011 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this manual page 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 page 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 page 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.