gawk - Man Page

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.

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.

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.

When invoked with ether 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.

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 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:

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.

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.

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.

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 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.

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 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.

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.

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.

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 ]
}

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 command returns 1 on success, zero on end of file, and -1 on an error. If the errno(3) value indicates that the I/O operation may be retried, and PROCINFO["input", "RETRY"] is set, then -2 is returned instead of -1, and further calls to getline may be attempted. Upon an error, ERRNO is set to a string describing the problem.

NOTE: Failure in opening a two-way socket results in a non-fatal error being returned to the calling function. If using a pipe, coprocess, or socket to getline, or from print or printf within a loop, you must use close() to create new instances of the command or socket. AWK does not automatically close pipes, sockets, or coprocesses when they return EOF.

The 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.

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.

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.

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.

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).

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.

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".

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".

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.