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GAWK(1) Utility Commands GAWK(1)
gawk - pattern scanning and processing language
gawk [ POSIX or GNU style options ] -f program-file [ -- ] file
...
gawk [ POSIX or GNU style options ] [ -- ] program-text file ...
Gawk is the GNU Project's implementation of the AWK programming
language. It conforms to the definition of the language in the
POSIX 1003.1 standard. This version in turn is based on the
description in The AWK Programming Language, by Aho, Kernighan,
and Weinberger. Gawk provides the additional features found in
the current version of 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.
This manual page is intentionally as terse as possible. Full
details are provided in GAWK: Effective AWK Programming, and you
should look there for the full story on any specific feature.
Where possible, links to the online version of the manual are
provided.
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.
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 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. 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.
-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. The default file is awkvars.out in
the current directory.
-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 commands for the debugger to execute non-
interactively.
In this mode of execution, gawk loads the AWK source code
and then prompts for debugging commands. Gawk can only
debug AWK program source provided with the -f and --include
options. The debugger is documented in GAWK: Effective AWK
Programming; see
https://www.gnu.org/software/gawk/manual/html_node/Debugger.html#Debugger .
-e program-text, --source program-text
Use program-text as AWK program source code. Each argument
supplied via -e is treated as if it begins with an implicit
@namespace "awk" statement.
-E file, --exec file
Similar to -f, however, this option is the last one
processed. This should be used with #! scripts,
particularly for CGI applications, to avoid passing in
options or source code (!) on the command line from a URL.
This option disables command-line variable assignments.
-g, --gen-pot
Scan and parse the AWK program, and generate a GNU .pot
(Portable Object Template) format file on standard output
with entries for all localizable strings in the program.
The program itself is not executed.
-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 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.
-I, --trace
Print the internal byte code names as they are executed
when running the program. The trace is printed to standard
error. Each ``op code'' is preceded by a + sign in the
output.
-k, --csv
Enable CSV special processing. See Comma Separated Values,
below, for more detail.
-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 implementations. See
https://www.gnu.org/software/gawk/manual/html_node/Options.html#Options
for the list of possible values for value.
-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 issues a
warning.)
NOTE: This feature is on parole. The primary gawk
maintainer is no longer supporting it, although there is a
member of the development team who is. If this situation
changes, the feature will be removed from gawk.
-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.
-o[file], --pretty-print[=file]
Output a pretty printed version of the program to file.
The default file is awkprof.out in the current directory.
This option implies --no-optimize.
-O, --optimize
Enable gawk's default optimizations upon the internal
representation of the program. 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 in the current
directory. The profile contains execution counts of each
statement in the program in the left margin and function
call counts for each user-defined function. Gawk runs more
slowly in this mode. This option implies --no-optimize.
-P, --posix
This turns on compatibility mode, and disables a number of
common extensions.
-r, --re-interval
Enable the use of interval expressions in regular
expression matching. Interval expressions are enabled by
default, but this option remains for backwards
compatibility.
-s, --no-optimize
Disable gawk's default optimizations upon the internal
representation of the program.
-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.
-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 when reporting
bugs. Per the GNU Coding Standards, these options cause an
immediate, successful exit.
-- Signal the end of options. This is useful to allow further
arguments to the AWK program itself to start with a “-”.
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.
For POSIX compatibility, the -W option may be used, followed by
the name of a long option.
An AWK program consists of a sequence of optional directives,
pattern-action statements, and optional function definitions.
@include "filename"
@nsinclude "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 together.
In addition, lines beginning with @include may be used to include
other source files into your program. This is equivalent to using
the --include option.
Lines beginning with @nsinclude also include files, but with two
differences. First, the namespace in the included file is not
reset to the default of awk, and such files may be included
multiple times.
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 specified 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 assignment. The variable var will be
assigned the value val. (This happens after any BEGIN rule(s)
have been run.)
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 rules
after processing the file.
For each record in the input, gawk tests to see if it matches any
pattern in the AWK program. For each pattern that the record
matches, 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.
For 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.
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. Additionally,
gawk allows variables to have regular-expression type. AWK also
has one dimensional arrays; arrays with multiple dimensions may be
simulated. However, gawk provides true arrays of arrays. 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. See
https://www.gnu.org/software/gawk/manual/html_node/Records.html
for the details.
Fields
As each input record is read, gawk splits the record into fields,
using the value of the FS variable as the field separator.
Additionally, FIELDWIDTHS and FPAT may be used to control input
field splitting. See the details, starting at
https://www.gnu.org/software/gawk/manual/html_node/Fields.html .
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.
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 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.
Comma Separated Values
When invoked with either the -k or the --csv option, gawk does not
use regular record determination and field splitting as described
above. Instead, records are terminated by unquoted newlines, and
fields are separated by commas. Double-quotes may be used to
enclose fields containing commas, newlines, or doubled double-
quotes. See
https://www.gnu.org/software/gawk/manual/html_node/Comma-Separated-Fields.html
for more details.
Built-in Variables
Gawk's built-in variables are listed below. This list is
purposely terse. For details, see
https://www.gnu.org/software/gawk/manual/html_node/Built_002din-Variables .
ARGC The number of command line arguments.
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.
BINMODE
On non-POSIX systems, specifies use of “binary” mode for
all file I/O. See
https://www.gnu.org/software/gawk/manual/html_node/PC-Using.html
for the details.
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.
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.
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.
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 expression, instead of
using the value of FS as the field separator.
FS The input field separator, a space by default. See
https://www.gnu.org/software/gawk/manual/html_node/Field-Separators.html
for the details.
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. See
https://www.gnu.org/software/gawk/manual/html_node/Case_002dsensitivity.html
for details.
LINT Provides dynamic control of the --lint option from within
an AWK program.
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. See
https://www.gnu.org/software/gawk/manual/html_node/Auto_002dset
for the details.
ROUNDMODE
The rounding mode to use for arbitrary precision arithmetic
on numbers, by default "N" (IEEE-754 roundTiesToEven mode).
See
https://www.gnu.org/software/gawk/manual/html_node/Setting-the-rounding-mode
for the details.
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.
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. You
may not use the delete statement with the SYMTAB array, nor
assign to elements 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 expression is an expression list (expr, expr
...) then the array subscript is a string consisting of the
concatenation of the (string) value of each expression, separated
by the value of the SUBSEP variable. This facility is used to
simulate multiply dimensioned arrays. For example:
i = "A"; j = "B"; k = "C"
x[i, j, k] = "hello, world\n"
assigns the string "hello, world\n" to the element of the array x
which is indexed by the string "A\034B\034C". All arrays in AWK
are associative, i.e., indexed by string values.
The special operator in may be used to test if an array has an
index consisting of a particular value:
if (val in array)
print array[val]
If the array has multiple subscripts, use (i, j) in array.
The in construct may also be used in a for loop to iterate over
all the elements of an array. 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++. See
https://www.gnu.org/software/gawk/manual/html_node/Arrays for
details.
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 variable 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
different.
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
https://www.gnu.org/software/gawk/manual/html_node/Namespaces.html#Namespaces .
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 context. If used in a
numeric expression, it will be treated as a number; if used as a
string it will be treated as a string.
To force a variable to be treated as a number, add 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 strtod(3). A number is converted to a string
by using the value of CONVFMT as a format string for sprintf(3),
with the numeric value of the variable as the argument. However,
even though all numbers in AWK are floating-point, integral values
are always converted as integers.
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. See
https://www.gnu.org/software/gawk/manual/html_node/Escape-Sequences
for the details.
Regexp Constants
A regular expression constant is a sequence of characters enclosed
between forward slashes (like /value/).
The escape sequences described in the manual may also be used
inside constant regular expressions (e.g., /[ \t\f\n\r\v]/ matches
whitespace characters).
Gawk provides strongly typed regular expression constants. These
are written with a leading @ 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.
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 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 combined with other patterns 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 essentially the same as those
in egrep(1). See
https://www.gnu.org/software/gawk/manual/html_node/Regexp.html for
the details on regular expressions.
A relational expression may use any of the operators defined below
in the section on actions. These generally test whether certain
fields match certain regular expressions.
The &&, ||, and ! operators are logical AND, logical OR, and
logical NOT, respectively, as in C. They do short-circuit
evaluation, also as in C, and are used for combining more
primitive pattern expressions. As in most languages, parentheses
may be used to change the order of evaluation.
The ?: operator is like the same operator in C. If the first
pattern is true then the pattern used for testing is the second
pattern, otherwise it is the third. Only one of the second and
third patterns is evaluated.
The pattern1, pattern2 form of an expression is called a range
pattern. It matches all input records starting with a record that
matches pattern1, and continuing until a record that matches
pattern2, inclusive. It does not combine with any other sort of
pattern expression.
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.
+ - ! 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.
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 an open 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. (The command can also be a
socket. See the subsection Special File Names, below.)
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.
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.
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
https://www.gnu.org/software/gawk/manual/html_node/I_002fO-Functions.html#I_002fO-Functions
for the full details on the exit status.
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 function 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 AWK versions of the printf statement and sprintf() function
are similar to those of C. For details, see
https://www.gnu.org/software/gawk/manual/html_node/Printf.html .
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.
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 number generator.
String Functions
Gawk has the following built-in string functions; details are
provided in
https://www.gnu.org/software/gawk/manual/html_node/String-Functions .
asort(s [, d [, how] ])
Return the number of elements in the source array s. Sort
the contents of s using gawk's normal rules for comparing
values, and replace the indices of the sorted values s with
sequential integers starting with 1. If the optional
destination array d is specified, first duplicate s into d,
and then sort d, leaving the indices of the source array s
unchanged. The optional string how controls the direction
and the comparison mode. Valid values for how are
described in
https://www.gnu.org/software/gawk/manual/html_node/String-Functions.html#String-Functions .
s and d are allowed to be the same array; this only makes
sense when supplying the third argument as well.
asorti(s [, d [, how] ])
Return the number of elements in the source array s. The
behavior is the same as that of asort(), except that the
array indices are used for sorting, not the array values.
When done, the array is indexed numerically, and the values
are those of the original indices. The original values are
lost; thus provide a second array if you wish to preserve
the original. The purpose of the optional string how is
the same as for asort(). Here too, s and d are allowed to
be the same array; this only makes sense when supplying the
third argument as well.
gensub(r, s, h [, t])
Search the target string t for matches of the regular
expression r. If h is a string beginning with g or G, then
replace all matches of r with s. Otherwise, h is a number
indicating which match of r to replace. If t is not
supplied, use $0 instead. Within the replacement text s,
the sequence \n, where n is a digit from 1 to 9, may be
used to indicate just the text that matched the n'th
parenthesized subexpression. The sequence \0 represents
the entire matched text, as does the character &. Unlike
sub() and gsub(), the modified string is returned as the
result of the function, and the original target string is
not changed.
gsub(r, s [, t])
For each substring matching the regular expression r in the
string t, substitute the string s, and return the number of
substitutions. If t is not supplied, use $0. An & in the
replacement text is replaced with the text that was
actually matched. Use \& to get a literal &. (This must
be typed as "\\&"; see
https://www.gnu.org/software/gawk/manual/html_node/Gory-Details.html#Gory-Details
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.)
length([s])
Return the length of the string s, or the length of $0 if s
is not supplied. 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. See
https://www.gnu.org/software/gawk/manual/html_node/String-Functions.html#String-Functions
for a description of how the array a is filled if it is
provided.
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 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.
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].
Splitting behaves identically to field splitting.
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
Gawk provides the following functions for obtaining time stamps
and formatting them. Details are provided in
https://www.gnu.org/software/gawk/manual/html_node/Time-Functions .
mktime(datespec [, utc-flag])
Turn datespec into a time stamp of the same form as
returned by systime(), and return the result. 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 datespec does not contain
enough elements or if the resulting time is out of range,
mktime() returns -1. See
https://www.gnu.org/software/gawk/manual/html_node/Time-Functions.html#Time-Functions
for the details of datespec.
strftime([format [, timestamp[, utc-flag]]])
Format timestamp according to the specification in format.
If utc-flag is present and is non-zero or non-null, the
result is in UTC, otherwise the result is in local time.
The timestamp should be of the same form as returned by
systime(). If timestamp is missing, the current time of
day is used. If format is missing, a default format
equivalent to the output of date(1) is used. The default
format is available in PROCINFO["strftime"]. See the
specification for the strftime() function in 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 converting the result back
to floating point. 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.
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
https://www.gnu.org/software/gawk/manual/html_node/I18N-Functions.html#I18N-Functions .
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. 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".
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".
Boolean Valued Functions
You can create special Boolean-typed values; see the manual for
how they work and why they exist.
mkbool(expression)
Based on the boolean value of expression return either a
true value or a false value. True values have numeric
value one. False values have numeric value zero.
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 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.
Local variables 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 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.
Functions may be called indirectly. To do this, assign the name of
the function to be called, as a string, to a variable. Then use
the variable as if it were the name of a function, prefixed with
an @ sign, like so:
function myfunc()
{
print "myfunc called"
...
}
{ ...
the_func = "myfunc"
@the_func() # call through the_func to myfunc
...
}
If --lint has been provided, gawk warns about calls to undefined
functions at parse time, instead of at run time. Calling an
undefined function at run time is a fatal error.
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
https://www.gnu.org/software/gawk/manual/html_node/Dynamic-Extensions.html#Dynamic-Extensions .
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.
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. See
https://www.gnu.org/software/gawk/manual/html_node/Internationalization.html#Internationalization
for the steps involved in producing and running a localizable AWK
program.
Gawk has a too-large number of extensions to POSIX awk. They are
described in
https://www.gnu.org/software/gawk/manual/html_node/POSIX_002fGNU.html .
All the extensions can be disabled by invoking gawk with the
--traditional or --posix options.
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 and
@nsinclude directives. 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_PERSIST_FILE environment variable, if present, specifies
a file to use as the backing store for persistent memory. This is
an experimental feature. See GAWK: Effective AWK Programming for
the details.
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 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.
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.
This man page documents gawk, version 5.3.
The original version of UNIX awk was designed and implemented by
Alfred Aho, Peter Weinberger, and Brian Kernighan of Bell
Laboratories. Ozan Yigit is the the current maintainer. Brian
Kernighan occasionally dabbles in its development.
Paul Rubin and Jay Fenlason, of the Free Software Foundation,
wrote gawk, to be compatible with the original version of awk
distributed in Seventh Edition UNIX. John Woods contributed a
number of bug fixes. David Trueman, with contributions from
Arnold Robbins, made gawk compatible with the new version of UNIX
awk. Arnold Robbins is the current maintainer.
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.
If you find a bug in gawk, please use the gawkbug(1) program to
report it.
Full instructions for reporting a bug are provided in
https://www.gnu.org/software/gawk/manual/html_node/Bugs.html .
Please carefully read and follow the instructions given there.
This will make bug reporting and resolution much easier for
everyone involved. Really.
On the other hand, if you have a question as to how to accomplish
a particular task using awk or gawk, send an email to help-
gawk@gnu.org with your request for help.
The -F option is not necessary given the command line variable
assignment feature; it remains only for backwards compatibility.
This manual page is too long; gawk has too many features.
egrep(1), sed(1), gawkbug(1), printf(3), and strftime(3).
The AWK Programming Language, second edition, Alfred V. Aho, Brian
W. Kernighan, Peter J. Weinberger, Addison-Wesley, 2023. ISBN
9-780138-269722.
GAWK: Effective AWK Programming, Edition 5.3, 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 .
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") }'
Copyright © 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, 2021, 2022, 2023,
2024 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.
This page is part of the gawk (GNU awk) project. Information
about the project can be found at
⟨http://www.gnu.org/software/gawk/⟩. If you have a bug report for
this manual page, see
⟨http://pkg-shadow.alioth.debian.org/getinvolved.php⟩. This page
was obtained from the project's upstream Git repository
⟨git://git.savannah.gnu.org/gawk.git⟩ on 2025-02-02. (At that
time, the date of the most recent commit that was found in the
repository was 2025-01-26.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
Free Software Foundation Aug 02 2024 GAWK(1)
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