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GAWK(1) Utility Commands GAWK(1)
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 ...
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 Wein-
berger. Gawk provides the additional features found in the current version of Brian
Kernighan's awk and numerous GNU-specific extensions.
The command line consists of options to gawk itself, the AWK program text (if not supplied
via the -f or --include options), and values to be made available in the ARGC and ARGV
pre-defined AWK variables.
When gawk is invoked with the --profile option, it starts gathering profiling statistics
from the execution of the program. Gawk runs more slowly in this mode, and automatically
produces an execution profile in the file awkprof.out when done. See the --profile op-
tion, below.
Gawk also has an integrated debugger. An interactive debugging session can be started by
supplying the --debug option to the command line. In this mode of execution, gawk loads
the AWK source code and then prompts for debugging commands. Gawk can only debug AWK pro-
gram source provided with the -f and --include options. 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, every 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 op-
tions 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. Files read
with -f are treated as if they begin with an implicit @namespace "awk" statement.
-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 rule of an AWK program.
-b
--characters-as-bytes
Treat all input data as single-byte characters. In other words, don't pay any at-
tention 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
Brian Kernighan's 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 pro-
gram with a lot of functions, and you want to be sure that your functions don't in-
advertently use global variables that you meant to be local. (This is a particu-
larly easy mistake to make with simple variable names like i, j, and so on.)
-D[file]
--debug[=file]
Enable debugging of AWK programs. By default, the debugger reads commands interac-
tively from the keyboard (standard input). The optional file argument specifies a
file with a list of commands for the debugger to execute non-interactively.
-e program-text
--source program-text
Use program-text as AWK program source code. This option allows the easy intermix-
ing of library functions (used via the -f and --include options) with source code
entered on the command line. It is intended primarily for medium to large AWK pro-
grams used in shell scripts. Each argument supplied via -e is treated as if it be-
gins with an implicit @namespace "awk" statement.
-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 op-
tions or source code (!) on the command line from a URL. This option disables com-
mand-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 pro-
gram. 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.)
-i include-file
--include include-file
Load an awk source library. This searches for the library using the AWKPATH envi-
ronment variable. If the initial search fails, another attempt will be made after
appending the .awk suffix. The file will be loaded only once (i.e., duplicates are
eliminated), and the code does not constitute the main program source. Files read
with --include are treated as if they begin with an implicit @namespace "awk"
statement.
-l lib
--load lib
Load a gawk extension from the shared library lib. This searches for the library
using the AWKLIBPATH environment variable. If the initial search fails, another
attempt will be made after appending the default shared library suffix for the
platform. The library initialization routine is expected to be named dl_load().
-L [value]
--lint[=value]
Provide warnings about constructs that are dubious or non-portable to other AWK im-
plementations. With an optional argument of fatal, lint warnings become fatal er-
rors. 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.)
With an optional argument of no-ext, warnings about gawk extensions are disabled.
-M
--bignum
Force arbitrary precision arithmetic on numbers. This option has no effect if gawk
is not compiled to use the GNU MPFR and GMP libraries. (In such a case, gawk is-
sues a warning.)
-n
--non-decimal-data
Recognize octal and hexadecimal values in input data. Use this option with great
caution!
-N
--use-lc-numeric
Force 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[file]
--pretty-print[=file]
Output a pretty printed version of the program to file. If no file is provided,
gawk uses a file named awkprof.out in the current directory. This option implies
--no-optimize.
-O
--optimize
Enable gawk's default optimizations upon the internal representation of the pro-
gram. Currently, this just includes simple constant folding. This option is on by
default.
-p[prof-file]
--profile[=prof-file]
Start a profiling session, and send the profiling data to prof-file. The default
is awkprof.out. The profile contains execution counts of each statement in the
program in the left margin and function call counts for each user-defined function.
This option implies --no-optimize.
-P
--posix
This turns on compatibility mode, with the following additional restrictions:
o \x escape sequences are not recognized.
o You cannot continue lines after ? and :.
o The synonym func for the keyword function is not recognized.
o The operators ** and **= cannot be used in place of ^ and ^=.
-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 together
with --traditional.
-s
--no-optimize
Disable gawk's default optimizations upon the internal representation of the pro-
gram.
-S
--sandbox
Run 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 com-
mand 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 op-
tions 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 ig-
nored. 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.
For POSIX compatibility, the -W option may be used, followed by the name of a long option.
AWK PROGRAM EXECUTION
An AWK program consists of a sequence of optional directives, pattern-action statements,
and optional function definitions.
@include "filename"
@load "filename"
@namespace "name"
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 to-
gether. 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. This is equivalent to using the --include
option.
Lines beginning with @load may be used to load extension functions into your program.
This is equivalent to using the --load option.
The environment variable AWKPATH specifies a search path to use when finding source files
named with the -f and --include options. 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.
The environment variable AWKLIBPATH specifies a search path to use when finding source
files named with the --load option. If this variable does not exist, the default path is
"/usr/local/lib/gawk". (The actual directory may vary, depending upon how gawk was built
and installed.)
Gawk executes AWK programs in the following order. First, all variable assignments speci-
fied via the -v option are performed. Next, gawk compiles the program into an internal
form. Then, gawk executes the code in the BEGIN rule(s) (if any), and then proceeds to
read each file named in the ARGV array (up to ARGV[ARGC-1]). 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 assign-
ment. The variable var will be assigned the value val. (This happens after any BEGIN
rule(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 pro-
gram. For each pattern that the record matches, gawk executes the associated action. 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 rule(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 val-
ues are either floating-point numbers or strings, or both, depending upon how they are
used. Additionally, gawk allows variables to have regular-expression type. AWK also has
one dimensional arrays; arrays with multiple dimensions may be simulated. Gawk provides
true arrays of arrays; see Arrays, below. Several pre-defined variables are set as a pro-
gram 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 in-
put 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 empty 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 sepa-
rate 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. 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 ex-
pression.
If the FIELDWIDTHS variable is set to a space-separated list of numbers, each field is ex-
pected to have fixed width, and gawk splits up the record using the specified widths.
Each field width may optionally be preceded by a colon-separated value specifying the num-
ber of characters to skip before the field starts. 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, including leading and trailing whitespace. Fields need not be refer-
enced 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. How-
ever, 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 values, 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. Nu-
meric 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 mes-
sage.
CONVFMT The conversion format for numbers, "%.6g", by default.
ENVIRON An array containing the values of the current environment. The array is in-
dexed by the environment variables, each element being the value of that vari-
able (e.g., ENVIRON["HOME"] might be "/home/arnold").
In POSIX mode, changing this array does not affect the environment seen by
programs which gawk spawns via redirection or the system() function. Other-
wise, gawk updates its real environment so that programs it spawns see the
changes.
ERRNO If a system error occurs either doing a redirection for getline, during a read
for getline, or during a close(), then ERRNO is set to a string describing the
error. The value is subject to translation in non-English locales. If the
string in ERRNO corresponds to a system error in the errno(3) variable, then
the numeric value can be found in PROCINFO["errno"]. For non-system errors,
PROCINFO["errno"] will be zero.
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. Each field width may optionally be preceded by a colon-
separated value specifying the number of characters to skip before the field
starts. 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 rule (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 ex-
pression, instead of using the value of FS as the field separator. See
Fields, above.
FS The input field separator, a space by default. See Fields, above.
FUNCTAB An array whose indices and corresponding values are the names of all the user-
defined or extension functions in the program. NOTE: You may not use the
delete statement with the FUNCTAB array.
IGNORECASE Controls the case-sensitivity of all regular expression and string operations.
If IGNORECASE has a non-zero value, then string comparisons and pattern match-
ing 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 af-
fected. 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 nor-
mally 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. The values al-
lowed for the --lint option may also be assigned to LINT, with the same ef-
fects. 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.
PREC The working precision of arbitrary precision floating-point numbers, 53 by de-
fault.
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 follow-
ing elements are guaranteed to be available:
PROCINFO["argv"] The command line arguments as received by gawk at the C-
language level. The subscripts start from zero.
PROCINFO["egid"] The value of the getegid(2) system call.
PROCINFO["errno"] The value of errno(3) when ERRNO is set to the associated
error message.
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, "FIELDWIDTHS" if
field splitting with FIELDWIDTHS is in effect, or "API"
if API input parser field splitting is in effect.
PROCINFO["gid"] The value of the getgid(2) system call.
PROCINFO["identifiers"]
A subarray, indexed by the names of all identifiers used
in the text of the AWK program. The values indicate what
gawk knows about the identifiers after it has finished
parsing the program; they are not updated while the pro-
gram runs. For each identifier, the value of the element
is one of the following:
"array" The identifier is an array.
"builtin" The identifier is a built-in function.
"extension" The identifier is an extension function
loaded via @load or --load.
"scalar" The identifier is a scalar.
"untyped" The identifier is untyped (could be used as a
scalar or array, gawk doesn't know yet).
"user" The identifier is a user-defined function.
PROCINFO["pgrpid"] The value of the getpgrp(2) system call.
PROCINFO["pid"] The value of the getpid(2) system call.
PROCINFO["platform"] A string indicating the platform for which gawk was com-
piled. It is one of:
"djgpp", "mingw"
Microsoft Windows, using either DJGPP, or MinGW,
respectively.
"os2" OS/2.
"posix"
GNU/Linux, Cygwin, Mac OS X, and legacy Unix sys-
tems.
"vms" OpenVMS or Vax/VMS.
PROCINFO["ppid"] The value of the getppid(2) system call.
PROCINFO["strftime"] The default time format string for strftime(). Changing
its value affects how strftime() formats time values when
called with no arguments.
PROCINFO["uid"] The value of the getuid(2) system call.
PROCINFO["version"] The version of gawk.
The following elements are present if loading dynamic extensions is available:
PROCINFO["api_major"]
The major version of the extension API.
PROCINFO["api_minor"]
The minor version of the extension API.
The following elements are available if MPFR support is compiled into gawk:
PROCINFO["gmp_version"]
The version of the GNU GMP library used for arbitrary precision number
support in gawk.
PROCINFO["mpfr_version"]
The version of the GNU MPFR library used for arbitrary precision number
support in gawk.
PROCINFO["prec_max"]
The maximum precision supported by the GNU MPFR library for arbitrary
precision floating-point numbers.
PROCINFO["prec_min"]
The minimum precision allowed by the GNU MPFR library for arbitrary
precision floating-point numbers.
The following elements may set by a program to change gawk's behavior:
PROCINFO["NONFATAL"]
If this exists, then I/O errors for all redirections become nonfatal.
PROCINFO["name", "NONFATAL"]
Make I/O errors for name be nonfatal.
PROCINFO["command", "pty"]
Use a pseudo-tty for two-way communication with command instead of set-
ting up two one-way pipes.
PROCINFO["input", "READ_TIMEOUT"]
The timeout in milliseconds for reading data from input, where input is
a redirection string or a filename. A value of zero or less than zero
means no timeout.
PROCINFO["input", "RETRY"]
If an I/O error that may be retried occurs when reading data from in-
put, and this array entry exists, then getline returns -2 instead of
following the default behavior of returning -1 and configuring input to
return no further data. An I/O error that may be retried is one where
errno(3) has the value EAGAIN, EWOULDBLOCK, EINTR, or ETIMEDOUT. This
may be useful in conjunction with PROCINFO["input", "READ_TIMEOUT"] or
in situations where a file descriptor has been configured to behave in
a non-blocking fashion.
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 (as a string) of any comparison function defined as fol-
lows:
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.
ROUNDMODE The rounding mode to use for arbitrary precision arithmetic on numbers, by de-
fault "N" (IEEE-754 roundTiesToEven mode). The accepted values are:
"A" or "a"
for rounding away from zero. These are only available if your version
of the GNU MPFR library supports rounding away from zero.
"D" or "d" for roundTowardNegative.
"N" or "n" for roundTiesToEven.
"U" or "u" for roundTowardPositive.
"Z" or "z" for roundTowardZero.
RS The input record separator, by default a newline.
RT The record terminator. Gawk sets RT to the input text that matched the char-
acter or regular expression specified by RS.
RSTART The index of the first character matched by match(); 0 if no match. (This im-
plies that character indices start at one.)
RLENGTH The length of the string matched by match(); -1 if no match.
SUBSEP The string used to separate multiple subscripts in array elements, by default
"\034".
SYMTAB An array whose indices are the names of all currently defined global variables
and arrays in the program. The array may be used for indirect access to read
or write the value of a variable:
foo = 5
SYMTAB["foo"] = 4
print foo # prints 4
The typeof() function may be used to test if an element in SYMTAB is an array.
You may not use the delete statement with the SYMTAB array, nor assign to ele-
ments with an index that is not a variable name.
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 ex-
pression is an expression list (expr, expr ...) then the array subscript is a string con-
sisting 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 ar-
rays. 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 par-
ticular 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 ar-
ray. However, the (i, j) in array construct only works in tests, not in for loops.
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 ``rec-
tangular'' as in C or C++. For example:
a[1] = 5
a[2][1] = 6
a[2][2] = 7
NOTE: You may need to tell gawk that an array element is really a subarray in order to use
it where gawk expects an array (such as in the second argument to split()). You can do
this by creating an element in the subarray and then deleting it with the delete state-
ment.
Namespaces
Gawk provides a simple namespace facility to help work around the fact that all variables
in AWK are global.
A qualified name consists of a two simple identifiers joined by a double colon (::). The
left-hand identifier represents the namespace and the right-hand identifier is the vari-
able within it. All simple (non-qualified) names are considered to be in the ``current''
namespace; the default namespace is awk. However, simple identifiers consisting solely of
uppercase letters are forced into the awk namespace, even if the current namespace is dif-
ferent.
You change the current namespace with an @namespace "name" directive.
The standard predefined builtin function names may not be used as namespace names. The
names of additional functions provided by gawk may be used as namespace names or as simple
identifiers in other namespaces. For more details, see GAWK: Effective AWK Programming.
Variable Typing And Conversion
Variables and fields may be (floating point) numbers, or strings, or both. They may also
be regular expressions. How the value of a variable is interpreted depends upon its con-
text. 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 zero to it; to force it to be treated
as a string, concatenate it with the null string.
Uninitialized variables have the numeric value zero and the string value "" (the null, or
empty, string).
When a string must be converted to a number, the conversion is accomplished using str-
tod(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 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 nu-
merically. 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 con-
verted 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 con-
stants. 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 pat-
split() that are numeric strings. The basic idea is that user input, and only user input,
that looks numeric, should be treated that way.
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. Up
to two following hexadecimal digits are considered part of the escape sequence.
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.
In compatibility mode, the characters represented by octal and hexadecimal escape se-
quences are treated literally when used in regular expression constants. Thus, /a\52b/ is
equivalent to /a\*b/.
Regexp Constants
A regular expression constant is a sequence of characters enclosed between forward slashes
(like /value/). Regular expression matching is described more fully below; see Regular
Expressions.
The escape sequences described earlier may also be used inside constant regular expres-
sions (e.g., /[ \t\f\n\r\v]/ matches whitespace characters).
Gawk provides strongly typed regular expression constants. These are written with a lead-
ing @ symbol (like so: @/value/). Such constants may be assigned to scalars (variables,
array elements) and passed to user-defined functions. Variables that have been so assigned
have regular expression type.
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 executes 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. Empty 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. However, a "\" after a # is not special.
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 writ-
ten in a single BEGIN rule. They are executed before any of the input is read. Simi-
larly, all the END rules 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 pat-
terns in pattern expressions. BEGIN and END patterns cannot have missing action parts.
BEGINFILE and ENDFILE are additional special patterns whose actions 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 is the empty string if the
file was 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 ac-
tions. 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 ex-
pression.
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...] A character list: matches any of the characters abc.... You may include a
range of characters by separating them with a dash. To include a literal dash
in the list, put it first or last.
[^abc...] A 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 String Constants) 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 char-
acter 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 charac-
ter 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 charac-
ters, 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 repre-
sented 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 ex-
pression that matches any of e, ', or `.
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 expres-
sions.
No options
In the default case, gawk provides 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 reg-
ular 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 you want.
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 coprocess. The optional how should only be used
when closing one end of a two-way pipe to a coprocess. It must be a
string value, either "to" or "from".
getline Set $0 from the next input record; set NF, NR, FNR, RT.
getline <file Set $0 from the next record of file; set NF, RT.
getline var Set var from the next input record; set NR, FNR, RT.
getline var <file Set var from the next record of file; set RT.
command | getline [var]
Run command, piping the output either into $0 or var, as above, and
RT.
command |& getline [var]
Run command as a coprocess piping the output either into $0 or var,
as above, and RT. Coprocesses are a gawk extension. (The command
can also be a socket. See the subsection Special File Names, be-
low.)
next Stop processing the current input record. Read the next input
record and start processing over with the first pattern in the AWK
program. Upon reaching the end of the input data, execute any END
rule(s).
nextfile Stop processing the current input file. The next input record read
comes from the next input file. Update FILENAME and ARGIND, reset
FNR to 1, and start processing over with the first pattern in the
AWK program. Upon reaching the end of the input data, execute any
ENDFILE and END rule(s).
print Print the current record. The output record is terminated with the
value of ORS.
print expr-list Print expressions. Each expression is separated by the value of
OFS. The output record is terminated with the value of ORS.
print expr-list >file Print expressions on file. Each expression is separated by the
value of OFS. The output record is terminated with the value of
ORS.
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.) See GAWK: Effective AWK
Programming for the full details on the exit status.
fflush([file]) Flush any buffers associated with the open output file or pipe file.
If file is missing or if it is the null string, then flush all open
output files and pipes.
Additional output redirections are allowed for print and printf.
print ... >> file
Append output to the file.
print ... | command
Write on a pipe.
print ... |& command
Send data to a coprocess or socket. (See also the subsection Special File Names,
below.)
The getline command returns 1 on success, zero on end of file, and -1 on an error. If the
errno(3) value indicates that the I/O operation may be retried, and PROCINFO["input",
"RETRY"] is set, then -2 is returned instead of -1, and further calls to getline may be
attempted. Upon an error, ERRNO is set to a string describing the problem.
NOTE: Failure in opening a two-way socket results in a non-fatal error being returned to
the calling function. If using a pipe, coprocess, 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 coprocesses 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:
%a, %A A floating point number of the form [-]0xh.hhhhp+-dd (C99 hexadecimal floating
point format). For %A, uppercase letters are used instead of lowercase ones.
%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 in-
stead 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 sup-
pressed. 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 posi-
tional 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, indicating that output should be padded with ze-
roes 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.
' A single quote character instructs gawk to insert the locale's thousands-separator
character into decimal numbers, and to also use the locale's decimal point charac-
ter with floating point formats. This requires correct locale support in the C li-
brary and in the definition of the current locale.
width The field should be padded to this width. The field is normally padded with spa-
ces. With the 0 flag, 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 the %s format, it specifies the maximum
number of characters from the string that should be printed.
The dynamic width and prec capabilities of the ISO 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:
- The standard input.
/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 |& coprocess 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). Usable only with the |& two-way I/O operator.
/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 zero and one, such that 0 <= N < 1.
sin(expr) Return the sine of expr, which is in radians.
sqrt(expr) Return the square root of expr.
srand([expr]) Use expr as the new seed for the random number generator. If no expr is
provided, use the time of day. Return the previous seed for the random num-
ber 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 inte-
gers starting with 1. If the optional destination array d is spec-
ified, first duplicate s into d, and then sort d, leaving the in-
dices 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 de-
scribed 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 in-
dices. 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 previously for asort().
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 substitu-
tions. 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: Ef-
fective AWK Programming for a fuller discussion of the rules for
ampersands 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 zero if t is
not present. (This implies that character indices start at one.)
It is a fatal error to use a regexp constant for t.
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
zero 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 zero'th
element of a contains the portion of s matched by the entire regu-
lar 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. El-
ement values are the portions of s that matched r. The value of
seps[i] is the possibly null separator that appeared after a[i].
The value of seps[0] is the possibly null leading separator. 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 be-
tween 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. In par-
ticular, if r is a single-character string, that string acts as
the separator, even if it happens to be a regular expression
metacharacter.
sprintf(fmt, expr-list) Print expr-list according to fmt, and return the resulting string.
strtonum(str) Examine str, and return its numeric value. If str begins with a
leading 0, treat it as an octal number. If str begins with a
leading 0x or 0X, treat it as a hexadecimal number. Otherwise,
assume it is a decimal number.
sub(r, s [, t]) Just like gsub(), but replace only the first matching substring.
Return either zero or one.
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 [, utc-flag])
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 respec-
tively 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. If utc-flag is
present and is non-zero or non-null, the time is assumed to be in the UTC time
zone; otherwise, the time is assumed to be in the local time zone. If the DST
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 de-
fault), 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 sys-
time(). 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 de-
fault format is available in PROCINFO["strftime"]. See the specification for
the strftime() function in ISO 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 con-
verting the result back to floating point.
NOTE: Passing negative operands to any of these functions causes a fatal error.
The functions are:
and(v1, v2 [, ...]) Return the bitwise AND of the values provided in the argument list.
There must be at least two.
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 in the argument list.
There must be at least two.
rshift(val, count) Return the value of val, shifted right by count bits.
xor(v1, v2 [, ...]) Return the bitwise XOR of the values provided in the argument list.
There must be at least two.
Type Functions
The following functions provide type related information about their arguments.
isarray(x) Return true if x is an array, false otherwise. This function is mainly for use
with the elements of multidimensional arrays and with function parameters.
typeof(x) Return a string indicating the type of x. The string will be one of "array",
"number", "regexp", "string", "strnum", "unassigned", or "undefined".
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 .gmo 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 cate-
gory. The default value for domain is the current value of TEXTDOMAIN. The de-
fault 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 sup-
ply 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 sup-
ply 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 execute when they are called from within expressions in either patterns or ac-
tions. 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 con-
catenation 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
...
}
As of version 4.1.2, this works with user-defined functions, built-in functions, and ex-
tension functions.
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, although this is deprecated.
DYNAMICALLY LOADING NEW FUNCTIONS
You can dynamically add new functions written in C or C++ to the running gawk interpreter
with the @load statement. The full details are beyond the scope of this manual page; see
GAWK: Effective AWK Programming.
SIGNALS
The gawk profiler 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 gawk to dump the pro-
file 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 .gmo 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 .pot file for your pro-
gram.
5. Provide appropriate translations, and build and install the corresponding .gmo files.
The internationalization features are described in full detail in GAWK: Effective AWK Pro-
gramming.
POSIX COMPATIBILITY
A primary goal for gawk is compatibility with the POSIX standard, as well as with the lat-
est version of Brian Kernighan's awk. To this end, gawk incorporates the following user
visible features which are not described in the AWK book, but are part of the Brian
Kernighan's 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 rule is executed. However, in ear-
lier implementations, when such an assignment appeared before any file names, the assign-
ment would happen before the BEGIN rule 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 devel-
opers and the GNU developers.)
When processing arguments, gawk uses the special option "--" to signal the end of argu-
ments. 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 re-
turn the seed it was using, to allow keeping track of random number sequences. Therefore
srand() in gawk also returns its current seed.
Other 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 Laborato-
ries version); the tolower() and toupper() built-in functions (from the Bell Laboratories
version); and the ISO C conversion specifications in printf (done first in the Bell Labo-
ratories 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 paren-
theses! 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 too-large number of extensions to POSIX awk. They are described in this sec-
tion. All the extensions described here can be disabled by invoking gawk with the --tra-
ditional or --posix options.
The following features of gawk are not available in POSIX awk.
o No path search is performed for files named via the -f option. Therefore the AWKPATH
environment variable is not special.
o There is no facility for doing file inclusion (gawk's @include mechanism).
o There is no facility for dynamically adding new functions written in C (gawk's @load
mechanism).
o The \x escape sequence.
o The ability to continue lines after ? and :.
o Octal and hexadecimal constants in AWK programs.
o The ARGIND, BINMODE, ERRNO, LINT, PREC, ROUNDMODE, RT and TEXTDOMAIN variables are not
special.
o The IGNORECASE variable and its side-effects are not available.
o The FIELDWIDTHS variable and fixed-width field splitting.
o The FPAT variable and field splitting based on field values.
o The FUNCTAB, SYMTAB, and PROCINFO arrays are not available.
o The use of RS as a regular expression.
o The special file names available for I/O redirection are not recognized.
o The |& operator for creating coprocesses.
o The BEGINFILE and ENDFILE special patterns are not available.
o The ability to split out individual characters using the null string as the value of FS,
and as the third argument to split().
o An optional fourth argument to split() to receive the separator texts.
o The optional second argument to the close() function.
o The optional third argument to the match() function.
o The ability to use positional specifiers with printf and sprintf().
o The ability to pass an array to length().
o The and(), asort(), asorti(), bindtextdomain(), compl(), dcgettext(), dcngettext(), gen-
sub(), lshift(), mktime(), or(), patsplit(), rshift(), strftime(), strtonum(), systime()
and xor() functions.
o Localizable strings.
o Non-fatal I/O.
o Retryable I/O.
The AWK book does not define the return value of the close() function. Gawk's close() re-
turns the value from fclose(3), or pclose(3), when closing an output file or pipe, respec-
tively. It returns the process's exit status when closing an input pipe. The return
value is -1 if the named file, pipe or coprocess 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 oc-
cur 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, --file, -i and --include options, and
the @include directive. If the initial search fails, the path is searched again after ap-
pending .awk to the filename.
The AWKLIBPATH environment variable can be used to provide a list of directories that gawk
searches when looking for files named via the -l and --load options.
The GAWK_READ_TIMEOUT environment variable can be used to specify a timeout in millisec-
onds for reading input from a terminal, pipe or two-way communication including sockets.
For connection to a remote host via socket, GAWK_SOCK_RETRIES controls the number of re-
tries, and GAWK_MSEC_SLEEP the interval between retries. 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 5.1.
AUTHORS
The original version of UNIX awk was designed and implemented by Alfred Aho, Peter Wein-
berger, 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 contrib-
uted 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 main-
tainer.
See GAWK: Effective AWK Programming for a full list of the contributors to gawk and its
documentation.
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 AT gnu.org. Please in-
clude 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 ex-
pect. 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 oc-
casionally read this newsgroup, posting bug reports there is an unreliable way to report
bugs. Similarly, do NOT use a web forum (such as Stack Overflow) for reporting bugs. In-
stead, please use the electronic mail addresses given above. Really.
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 re-
mains only for backwards compatibility.
SEE ALSO
egrep(1), sed(1), getpid(2), getppid(2), getpgrp(2), getuid(2), geteuid(2), getgid(2),
getegid(2), getgroups(2), printf(3), strftime(3), usleep(3)
The AWK Programming Language, Alfred V. Aho, Brian W. Kernighan, Peter J. Weinberger, Ad-
dison-Wesley, 1988. ISBN 0-201-07981-X.
GAWK: Effective AWK Programming, Edition 5.1, shipped with the gawk source. The current
version of this document is available online at https://www.gnu.org/software/gawk/manual.
The GNU gettext documentation, available online at https://www.gnu.org/software/gettext.
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 provided valuable assistance during testing and debugging. We thank him.
COPYING PERMISSIONS
Copyright (C) 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
2003, 2004, 2005, 2007, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019,
2020, 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.
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