nawk Man page

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 lan‐
guage. 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 Brian
Kernighan’s 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.

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 option, 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 program source pro‐
vided with the -f option. The debugger is documented in GAWK: Effec‐
tive 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. Argu‐
ments 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 abbre‐
viation 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 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 prede‐
fined 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 attention to the locale information when attempt‐
ing 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 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.)

-D[file] –debug[=file] Enable debugging of AWK programs. By default, the debugger
reads commands interactively from the keyboard (standard input).
The optional file argument specifies a file with a list of com‐
mands 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 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 pro‐
cessed. 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 (Porta‐
ble 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.)

-i include-file
–include include-file
Load an awk source library. This searches for the library using
the AWKPATH environment 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 elimi‐
nated), and the code does not constitute the main program
source.

-l lib
–load lib
Load a 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 initializa‐
tion routine is expected to be named dl_load().

-L [value] –lint[=value] Provide warnings about constructs that are dubious or non-porta‐
ble 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 warn‐
ings about things that are actually invalid are issued. (This is
not fully implemented yet.)

-M
–bignum
Force arbitrary precision arithmetic on numbers. This option has
no effect if gawk is not compiled to use the GNU MPFR and MP
libraries.

-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[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 cur‐
rent directory.

-O
–optimize
Enable optimizations upon the internal representation of the
program. Currently, this includes simple constant-folding, and
tail call elimination for recursive functions. The gawk main‐
tainer hopes to add additional optimizations over time.

-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 execu‐
tion 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 ^=.

-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.

-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 Stan‐
dards, these options cause an immediate, successful exit.)

— Signal the end of options. This is useful to allow further argu‐
ments to the AWK program itself to start with a “-”. This pro‐
vides 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.

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 pattern-action statements and
optional function definitions.

@include “filename”
@load “filename”
pattern { action statements }
function name(parameter list) { statements }

Gawk first reads the program source from the program-file(s) if speci‐
fied, 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. This
is equivalent to using the -i option.

Lines beginning with @load may be used to load shared libraries into
your program. This is equivalent to using the -l option.

The environment variable AWKPATH specifies a search path to use when
finding source files named with the -f and -i options. If this vari‐
able 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 “/” char‐
acter, no path search is performed.

The environment variable AWKLIBPATH specifies a search path to use when
finding source files named with the -l 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 specified via the -v option are performed. Next, gawk com‐
piles 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]). 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 rule(s) have been run.) Command
line variable assignment is most useful for dynamically assigning val‐
ues to the variables AWK uses to control how input is broken into
fields and records. It is also useful for controlling state if multi‐
ple 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 pat‐
tern in the AWK program. 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 pro‐
duces a warning, but is otherwise skipped. If either of the –posix or
–traditional options is given, then gawk reverts to treating directo‐
ries 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. Gawk pro‐
vides true arrays of arrays; see Arrays, below. 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 compatibil‐
ity mode, only the first character of its string value is used for sep‐
arating 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 new‐
line character always acts as a field separator, in addition to what‐
ever 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. Oth‐
erwise, 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 num‐
bers, 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 regu‐
lar 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 values, and causes the value of $0 to be recom‐
puted, 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” spec‐
ify 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., ENVI‐
RON[“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 separa‐
tor. See Fields, above.

FILENAME The name of the current input file. If no files are speci‐
fied 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 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.

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 matching in rules,
field splitting with FS and FPAT, record separating with
RS, regular expression matching with ~ and !~, and the gen‐
sub(), 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.

PREC The working precision of arbitrary precision floating-point
numbers, 53 by default.

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 system
call.

PROCINFO[“strftime”] The default time format string for
strftime().

PROCINFO[“euid”] The value of the geteuid 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[“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 program 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 -l.

“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[“gid”] The value of the getgid 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 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[“input”, “READ_TIMEOUT”] The timeout in milliseconds for reading
data from input, where input is a redi‐
rection string or a filename. A value
of zero or less than zero means no
timeout.

PROCINFO[“mpfr_version”] The version of the GNU MPFR library
used for arbitrary precision number
support in gawk. This entry is not
present if MPFR support is not compiled
into gawk.

PROCINFO[“gmp_version”] The version of the GNU MP library used
for arbitrary precision number support
in gawk. This entry is not present if
MPFR support is not compiled into gawk.

PROCINFO[“prec_max”] The maximum precision supported by the
GNU MPFR library for arbitrary preci‐
sion floating-point numbers. This
entry is not present if MPFR support is
not compiled into gawk.

PROCINFO[“prec_min”] The minimum precision allowed by the
GNU MPFR library for arbitrary preci‐
sion floating-point numbers. This
entry is not present if MPFR support is
not compiled into gawk.

PROCINFO[“api_major”] The major version of the extension API.
This entry is not present if loading
dynamic extensions is not available.

PROCINFO[“api_minor”] The minor version of the extension API.
This entry is not present if loading
dynamic extensions is not available.

PROCINFO[“version”] the version of gawk.

ROUNDMODE The rounding mode to use for arbitrary precision arithmetic
on numbers, by default “N” (IEEE-754 roundTiesToEven mode).
The accepted values are “N” or “n” for roundTiesToEven, “U”
or “u” for roundTowardPositive, “D” or “d” for roundToward‐
Negative, “Z” or “z” for roundTowardZero, and if your ver‐
sion of GNU MPFR library supports it, “A” or “a” for
roundTiesToAway.

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”.

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 isarray() 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.

TEXTDOMAIN The text domain of the AWK program; used to find the local‐
ized 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 associa‐
tive, 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. 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 “rectangular” 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 sub‐
array 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 statement.

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 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 0 to it; to force it
to be treated as a string, concatenate it with the null string.

Uninitialized variables have the numeric value 0 and the string value
“” (the null, or empty, string).

When a string must be converted to a number, the conversion is accom‐
plished using strtod. A number is converted to a string by using
the value of CONVFMT as a format string for sprintf, with the
numeric value of the variable as the argument. However, even though
all numbers in AWK are floating-point, integral values are always con‐
verted 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 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.

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 fol‐
lowing the \x. As in ISO 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 expres‐
sions (e.g., /[ \t\f\n\r\v]/ matches whitespace characters).

In compatibility mode, the characters represented by octal and hexadec‐
imal 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 state‐
ments 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 rule. They
are executed before any of the input is read. Similarly, 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 com‐
bined with other patterns in pattern expressions. BEGIN and END pat‐
terns 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 BEGIN‐
FILE rule, the value of ERRNO will be 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, 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, other‐
wise 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 pat‐
tern1, 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…] A character list: matches any of the characters abc…. You
may include a range of characters by separating them with a
dash.

[^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 expres‐
sion. 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 charac‐
ters 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 key‐
word 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 char‐
acters.)

[:punct:] Punctuation characters (characters that are not letter, dig‐
its, 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 charac‐
ters, 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 col‐
lating, 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 ele‐
ment, 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 expres‐
sion 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 cur‐
rently 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 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 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 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 expres‐
sion 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,
RT.

getline 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.)

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
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 is set to a string describing the prob‐
lem.

NOTE: Failure in opening a two-way socket results 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 sys‐
tem 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 nonsignifi‐
cant 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 origi‐
nal 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.

‘ 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 character with floating point
formats. This requires correct locale support in the C library
and in the definition of the current locale.

width The field should be padded to this width. The field is normally
padded with spaces. 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 sig‐
nificant 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 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 pre‐
cision, supply the count$ after the * in the format string. For exam‐
ple, “%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 |& 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) 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 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 sequen‐ tial integers starting with 1. If the optional destination array d is specified, first dupli‐ cate 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 val‐ ues 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 begin‐ ning 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 sup‐ plied, 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 subex‐ pression. 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 expres‐ sion 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 discus‐ sion of the rules for &'s and backslashes in the replacement text of sub(), gsub(), and gen‐ sub().) 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.) 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 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 sub‐ string. 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 identi‐ cally 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) 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. Oth‐ erwise, assume it is a decimal number. 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 func‐ tions 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 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 pre‐ ceding 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 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 is used. The default format is avail‐
able 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 inte‐
gers, 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 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 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 .gmo files, in
case they will not or cannot be placed in the “standard” loca‐
tions (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 pro‐
vision 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 fol‐
low the function name, without any intervening whitespace. This avoids
a syntactic ambiguity with the concatenation operator. This restric‐
tion does not apply to the built-in functions listed above.

Functions may call each other and may be recursive. Function parame‐
ters 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 “fall‐
ing 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 vari‐
able. Then use the variable as if it were the name of a function, pre‐
fixed 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 extension functions.

If –lint has been provided, gawk warns about calls to undefined func‐
tions 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 depre‐
cated.

DYNAMICALLY LOADING NEW FUNCTIONS
You can dynamically add new built-in functions 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 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 .gmo file associated with your pro‐
gram. 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 under‐
scores.

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 program.

5. Provide appropriate translations, and build and install the corre‐
sponding .gmo 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 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 ver‐
sion 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 earlier implementations, when such
an assignment appeared before any file names, the assignment would hap‐
pen 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 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 other‐
wise 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 ISO C conversion specifications in printf (done first in the
Bell Laboratories version).

HISTORICAL FEATURES
There is one feature of historical AWK implementations that gawk sup‐
ports: 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 too-large 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 mecha‐
nism).

· There is no facility for dynamically adding new functions written in
C (gawk’s @load mechanism).

· The \x escape sequence. (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 recog‐
nized.

· 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 and(), asort(), asorti(), bindtextdomain(), compl(), dcgettext(),
dcngettext(), gensub(), lshift(), mktime(), or(), patsplit(),
rshift(), strftime(), strtonum(), systime() and xor() functions.

· Localizable strings.

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,
–file, -i and –include options. If the initial search fails, the
path is searched again after appending .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 milliseconds 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 retries, and GAWK_MSEC_SLEEP and the interval between
retries. The interval is in milliseconds. On systems that do not sup‐
port usleep, the value is rounded up to an integral number of sec‐
onds.

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 4.1.

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 com‐
patible with the new version of UNIX awk. Arnold Robbins is the cur‐
rent maintainer.

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@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 set‐
ting 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. 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 main‐
tainer.

BUGS

The -F option is not necessary given the command line variable assign‐
ment feature; it remains only for backwards compatibility.

SEE ALSO

egrep, sed, getpid, getppid, getpgrp(2), getuid,
geteuid, getgid, getegid, getgroups, usleep

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 4.1, 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 provided valuable assistance during testing and debug‐
ging. 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, 2012, 2013, 2014
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 man‐
ual page into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a trans‐
lation approved by the Foundation.

Free Software Foundation Aug 03 2014 GAWK(1)

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