gawk

According to POSIX, files named on the awk command line must be text files.  The behavior is “undefined” if they are not.  Most versions of awk treat a directory on the command line as a fatal error.

Starting with version 4.0 of gawk, a directory on the command line produces a warning, but is otherwise skipped.  If either of the --posix or --traditional options is given, then gawk reverts to treating directories on the command line as a fatal error.

Normally, records are separated by newline characters.  You can control how records are separated by assigning values to the built-in variable RS. If RS is any single character, that character separates records. Otherwise, RS is a regular expression.  Text in the input that matches this regular expression separates the record. However, in compatibility mode, only the first character of its string value is used for separating records. If RS is set to the null string, then records are separated by empty lines. When RS is set to the null string, the newline character always acts as a field separator, in addition to whatever value FS may have.

As each input record is read, gawk splits the record into fields, using the value of the FS variable as the field separator. If FS is a single character, fields are separated by that character. If FS is the null string, then each individual character becomes a separate field. Otherwise, FS is expected to be a full regular expression. In the special case that FS is a single space, fields are separated by runs of spaces and/or tabs and/or newlines. NOTE: The value of IGNORECASE (see below) also affects how fields are split when FS is a regular expression, and how records are separated when RS is a regular expression.

If the FIELDWIDTHS variable is set to a space-separated list of numbers, each field is expected to have fixed width, and gawk splits up the record using the specified widths.  Each field width may optionally be preceded by a colon-separated value specifying the number of characters to skip before the field starts. The value of FS is ignored. Assigning a new value to FS or FPAT overrides the use of FIELDWIDTHS.

Similarly, if the FPAT variable is set to a string representing a regular expression, each field is made up of text that matches that regular expression. In this case, the regular expression describes the fields themselves, instead of the text that separates the fields. Assigning a new value to FS or FIELDWIDTHS overrides the use of FPAT.

Each field in the input record may be referenced by its position: $1, $2, and so on. $0 is the whole record, including leading and trailing whitespace. Fields need not be referenced by constants:

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 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.

Gawk's built-in variables are:

ARGC

The number of command line arguments (does not include options to gawk, or the program source).

ARGIND

The index in ARGV of the current file being processed.

ARGV

Array of command line arguments.  The array is indexed from 0 to ARGC - 1. Dynamically changing the contents of ARGV can control the files used for data.

BINMODE

On non-POSIX systems, specifies use of “binary” mode for all file I/O. Numeric values of 1, 2, or 3, specify that input files, output files, or all files, respectively, should use binary I/O. String values of "r", or "w" specify that input files, or output files, respectively, should use binary I/O. String values of "rw" or "wr" specify that all files should use binary I/O. Any other string value is treated as "rw", but generates a warning message.

CONVFMT

The conversion format for numbers, "%.6g", by default.

ENVIRON

An array containing the values of the current environment. The array is indexed by the environment variables, each element being the value of that variable (e.g., ENVIRON["HOME"] might be "/home/arnold").

In POSIX mode, changing this array does not affect the environment seen by programs which gawk spawns via redirection or the system() function. Otherwise, gawk updates its real environment so that programs it spawns see the changes.

ERRNO

If a system error occurs either doing a redirection for getline, during a read for getline, or during a close(), then ERRNO is set to a string describing the error. The value is subject to translation in non-English locales. If the string in ERRNO corresponds to a system error in the errno(3) variable, then the numeric value can be found in PROCINFO["errno"]. For non-system errors, PROCINFO["errno"] will be zero.

FIELDWIDTHS

A whitespace-separated list of field widths.  When set, gawk parses the input into fields of fixed width, instead of using the value of the FS variable as the field separator. Each field width may optionally be preceded by a colon-separated value specifying the number of characters to skip before the field starts. See Fields, above.

FILENAME

The name of the current input file. If no files are specified on the command line, the value of FILENAME is “-”. However, FILENAME is undefined inside the BEGIN rule (unless set by getline).

FNR

The input record number in the current input file.

FPAT

A regular expression describing the contents of the fields in a record. When set, gawk parses the input into fields, where the fields match the regular expression, instead of using the value of FS as the field separator. See Fields, above.

FS

The input field separator, a space by default.  See Fields, above.

FUNCTAB

An array whose indices and corresponding values are the names of all the user-defined or extension functions in the program. NOTE: You may not use the delete statement with the FUNCTAB array.

IGNORECASE

Controls the case-sensitivity of all regular expression and string operations.  If IGNORECASE has a non-zero value, then string comparisons and pattern matching in rules, field splitting with FS and FPAT, record separating with RS, regular expression matching with ~ and !~, and the gensub(), gsub(), index(), match(), patsplit(), split(), and sub() built-in functions all ignore case when doing regular expression operations. NOTE: Array subscripting is not affected. However, the asort() and asorti() functions are affected.

Thus, if IGNORECASE is not equal to zero, /aB/ matches all of the strings "ab", "aB", "Ab", and "AB". As with all AWK variables, the initial value of IGNORECASE is zero, so all regular expression and string operations are normally case-sensitive.

LINT

Provides dynamic control of the --lint option from within an AWK program. When true, gawk prints lint warnings. When false, it does not. When assigned the string value "fatal", lint warnings become fatal errors, exactly like --lint=fatal. Any other true value just prints warnings.

NF

The number of fields in the current input record.

NR

The total number of input records seen so far.

OFMT

The output format for numbers, "%.6g", by default.

OFS

The output field separator, a space by default.

ORS

The output record separator, by default a newline.

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["argv"]

The command line arguments as received by gawk at the C-language level. The subscripts start from zero.

PROCINFO["egid"]

The value of the getegid(2) system call.

PROCINFO["errno"]

The value of errno(3) when ERRNO is set to the associated error message.

PROCINFO["euid"]

The value of the geteuid(2) system call.

PROCINFO["FS"]

"FS" if field splitting with FS is in effect, "FPAT" if field splitting with FPAT is in effect, "FIELDWIDTHS" if field splitting with FIELDWIDTHS is in effect, or "API" if API input parser field splitting is in effect.

PROCINFO["gid"]

The value of the getgid(2) system call.

PROCINFO["identifiers"]

A subarray, indexed by the names of all identifiers used in the text of the AWK program. The values indicate what gawk knows about the identifiers after it has finished parsing the program; they are not updated while the 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 --load.

"scalar"

The identifier is a scalar.

"untyped"

The identifier is untyped (could be used as a scalar or array, gawk doesn't know yet).

"user"

The identifier is a user-defined function.

PROCINFO["pgrpid"]

The value of the getpgrp(2) system call.

PROCINFO["pid"]

The value of the getpid(2) system call.

PROCINFO["platform"]

A string indicating the platform for which gawk was compiled.  It is one of:

"djgpp", "mingw"

Microsoft Windows, using either DJGPP, or MinGW, respectively.

"os2"

OS/2.

"posix"

GNU/Linux, Cygwin, Mac OS X, and legacy Unix systems.

"vms"

OpenVMS or Vax/VMS.

PROCINFO["ppid"]

The value of the getppid(2) system call.

PROCINFO["strftime"]

The default time format string for strftime(). Changing its value affects how strftime() formats time values when called with no arguments.

PROCINFO["uid"]

The value of the getuid(2) system call.

PROCINFO["version"]

The version of gawk.

The following elements are present if loading dynamic extensions is available:

PROCINFO["api_major"]

The major version of the extension API.

PROCINFO["api_minor"]

The minor version of the extension API.

The following elements are available if MPFR support is compiled into gawk:

PROCINFO["gmp_version"]

The version of the GNU GMP library used for arbitrary precision number support in gawk.

PROCINFO["mpfr_version"]

The version of the GNU MPFR library used for arbitrary precision number support in gawk.

PROCINFO["prec_max"]

The maximum precision supported by the GNU MPFR library for arbitrary precision floating-point numbers.

PROCINFO["prec_min"]

The minimum precision allowed by the GNU MPFR library for arbitrary precision floating-point numbers.

The following elements may set by a program to change gawk's behavior:

PROCINFO["NONFATAL"]

If this exists, then I/O errors for all redirections become nonfatal.

PROCINFO["name", "NONFATAL"]

Make I/O errors for name be nonfatal.

PROCINFO["command", "pty"]

Use a pseudo-tty for two-way communication with command instead of setting up two one-way pipes.

PROCINFO["input", "READ_TIMEOUT"]

The timeout in milliseconds for reading data from input, where input is a redirection string or a filename. A value of zero or less than zero means no timeout.

PROCINFO["input", "RETRY"]

If an I/O error that may be retried occurs when reading data from input, and this array entry exists, then getline returns -2 instead of following the default behavior of returning -1 and configuring input to return no further data. An I/O error that may be retried is one where errno(3) has the value EAGAIN, EWOULDBLOCK, EINTR, or ETIMEDOUT. This may be useful in conjunction with PROCINFO["input", "READ_TIMEOUT"] or in situations where a file descriptor has been configured to behave in a non-blocking fashion.

PROCINFO["sorted_in"]

If this element exists in PROCINFO, then its value controls the order in which array elements are traversed in for loops. Supported values are "@ind_str_asc", "@ind_num_asc", "@val_type_asc", "@val_str_asc", "@val_num_asc", "@ind_str_desc", "@ind_num_desc", "@val_type_desc", "@val_str_desc", "@val_num_desc", and "@unsorted". The value can also be the name (as a string) of any comparison function defined as 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.

ROUNDMODE

The rounding mode to use for arbitrary precision arithmetic on numbers, by default "N" (IEEE-754 roundTiesToEven mode). The accepted values are:

"A" or "a"

for rounding away from zero. These are only available if your version of the GNU MPFR library supports rounding away from zero.

"D" or "d"

for roundTowardNegative.

"N" or "n"

for roundTiesToEven.

"U" or "u"

for roundTowardPositive.

"Z" or "z"

for roundTowardZero.

RS

The input record separator, by default a newline.

RT

The record terminator. Gawk sets RT to the input text that matched the 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 string used to separate multiple subscripts in array elements, by default "\034".

SYMTAB

An array whose indices are the names of all currently defined global variables and arrays in the program.  The array may be used for indirect access to read or write the value of a variable:

foo = 5
SYMTAB["foo"] = 4
print foo    # prints 4

The typeof() function may be used to test if an element in SYMTAB is an array. You may not use the delete statement with the SYMTAB array, nor assign to 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++. For example:

a[1] = 5
a[2][1] = 6
a[2][2] = 7

NOTE: You may need to tell gawk that an array element is really a subarray in order to use it where gawk expects an array (such as in the second argument to split()). You can do this by creating an element in the subarray and then deleting it with the delete statement.

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 GAWK: Effective AWK Programming.

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.  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.

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.  These are:

\\

A literal backslash.

\a

The “alert” character; usually the ASCII BEL character.

\b

Backspace.

\f

Form-feed.

\n

Newline.

\r

Carriage return.

\t

Horizontal tab.

\v

Vertical tab.

\xhex digits

The character represented by the string of hexadecimal digits following the \x. Up to two following hexadecimal digits are considered part of the escape sequence. E.g., "\x1B" is the ASCII ESC (escape) character.

\ddd

The character represented by the 1-, 2-, or 3-digit sequence of octal digits. E.g., "\033" is the ASCII ESC (escape) character.

\c

The literal character c.

In compatibility mode, the characters represented by octal and hexadecimal escape sequences are treated literally when used in regular expression constants.  Thus, /a\52b/ is equivalent to /a\*b/.

A regular expression constant is a sequence of characters enclosed between forward slashes (like /value/). Regular expression matching is described more fully below; see Regular Expressions.

The escape sequences described earlier may also be used inside constant regular 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 the same as those in egrep(1), and are summarized below.

A relational expression may use any of the operators defined below in the section on actions. These generally test whether certain fields match certain regular expressions.

The &&, ||, and ! operators are logical AND, logical OR, and logical NOT, respectively, as in C. They do short-circuit evaluation, also as in C, and are used for combining more primitive pattern expressions.  As in most languages, parentheses may be used to change the order of evaluation.

The ?: operator is like the same operator in C.  If the first pattern is true then the pattern used for testing is the second pattern, otherwise it is the third.  Only one of the second and third patterns is evaluated.

The pattern1, pattern2 form of an expression is called a range pattern. It matches all input records starting with a record that matches pattern1, and continuing until a record that matches pattern2, inclusive.  It does not combine with any other sort of pattern expression.

Regular expressions are the extended kind found in egrep. They are composed of characters as follows:

c

Matches the non-metacharacter c.

\c

Matches the literal character c.

.

Matches any character including newline.

^

Matches the beginning of a string.

$

Matches the end of a string.

[abc...]

A character list: matches any of the characters abc.... You may include a range of characters by separating them with a dash. To include a literal dash in the list, put it first or last.

[^abc...]

A negated character list: matches any character except abc....

r1|r2

Alternation: matches either r1 or r2.

r1r2

Concatenation: matches r1, and then r2.

r+

Matches one or more r's.

r*

Matches zero or more r's.

r?

Matches zero or one r's.

(r)

Grouping: matches r.

r{n}

r{n,}

r{n,m}

One or two numbers inside braces denote an interval expression. If there is one number in the braces, the preceding regular expression r is repeated n times.  If there are two numbers separated by a comma, r is repeated n to m times. If there is one number followed by a comma, then r is repeated at least n times.

\y

Matches the empty string at either the beginning or the end of a word.

\B

Matches the empty string within a word.

\<

Matches the empty string at the beginning of a word.

\>

Matches the empty string at the end of a word.

\s

Matches any whitespace character.

\S

Matches any nonwhitespace character.

\w

Matches any word-constituent character (letter, digit, or underscore).

\W

Matches any character that is not word-constituent.

\`

Matches the empty string at the beginning of a buffer (string).

\'

Matches the empty string at the end of a buffer.

The escape sequences that are valid in string constants (see String Constants) are also valid in regular expressions.

Character classes are a feature introduced in the POSIX standard. A character class is a special notation for describing lists of characters that have a specific attribute, but where the actual characters themselves can vary from country to country and/or from character set to character set.  For example, the notion of what is an alphabetic character differs in the USA and in France.

A character class is only valid in a regular expression inside the brackets of a character list.  Character classes consist of [:, a keyword denoting the class, and :]. The character classes defined by the POSIX standard are:

[:alnum:]

Alphanumeric characters.

[:alpha:]

Alphabetic characters.

[:blank:]

Space or tab characters.

[:cntrl:]

Control characters.

[:digit:]

Numeric characters.

[:graph:]

Characters that are both printable and visible. (A space is printable, but not visible, while an a is both.)

[:lower:]

Lowercase alphabetic characters.

[:print:]

Printable characters (characters that are not control characters.)

[:punct:]

Punctuation characters (characters that are not letter, digits, control characters, or space characters).

[:space:]

Space characters (such as space, tab, and formfeed, to name a few).

[:upper:]

Uppercase alphabetic characters.

[:xdigit:]

Characters that are hexadecimal digits.

For example, before the POSIX standard, to match alphanumeric characters, you would have had to write /[A-Za-z0-9]/. If your character set had other alphabetic characters in it, this would not match them, and if your character set collated differently from ASCII, this might not even match the ASCII alphanumeric characters. With the POSIX character classes, you can write /[[:alnum:]]/, and this matches the alphabetic and numeric characters in your character set, no matter what it is.

Two additional special sequences can appear in character lists. These apply to non-ASCII character sets, which can have single symbols (called collating elements) that are represented with more than one character, as well as several characters that are equivalent for collating, or sorting, purposes.  (E.g., in French, a plain “e” and a grave-accented “e `” are equivalent.)

Collating Symbols

A collating symbol is a multi-character collating element enclosed in [. and .]. For example, if ch is a collating element, then [[.ch.]] is a regular expression that matches this collating element, while [ch] is a regular expression that matches either c or h.

Equivalence Classes

An equivalence class is a locale-specific name for a list of characters that are equivalent.  The name is enclosed in [= and =]. For example, the name e might be used to represent all of “e”, “e ´”, and “e `”. In this case, [[=e=]] is a regular expression that matches any of e, e ´, or e `.

These features are very valuable in non-English speaking locales. The library functions that gawk uses for regular expression matching currently only recognize POSIX character classes; they do not recognize collating symbols or equivalence classes.

The \y, \B, \<, \>, \s, \S, \w, \W, \`, and \' operators are specific to gawk; they are extensions based on facilities in the GNU regular expression libraries.

The various command line options control how gawk interprets characters in regular expressions.

No options

In the default case, gawk 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.

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 (** may also be used, and **= for the assignment operator).

+ - !

Unary plus, unary minus, and logical negation.

* / %

Multiplication, division, and modulus.

+ -

Addition and subtraction.

space

String concatenation.

|   |&

Piped I/O for getline, print, and printf.

< > <= >= == !=

The regular relational operators.

~ !~

Regular expression match, negated match. NOTE: Do not use a constant regular expression (/foo/) on the left-hand side of a ~ or !~. Only use one on the right-hand side.  The expression /foo/ ~ exp has the same meaning as (($0 ~ /foo/) ~ exp). This is usually not what you want.

in

Array membership.

&&

Logical AND.

||

Logical OR.

?:

The C conditional expression.  This has the form expr1 ? expr2 : expr3. If expr1 is true, the value of the expression is expr2, otherwise it is expr3. Only one of expr2 and expr3 is evaluated.

= += -= *= /= %= ^=

Assignment.  Both absolute assignment (var = value) and operator-assignment (the other forms) are supported.

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 file, pipe or coprocess. The optional how should only be used when closing one end of a two-way pipe to a coprocess. It must be a string value, either "to" or "from".

getline

Set $0 from the next input record; set NF, NR, FNR, RT.

getline <file

Set $0 from the next record of file; set NF, RT.

getline var

Set var from the next input record; set NR, FNR, RT.

getline var <file

Set var from the next record of file; set RT.

command | getline [var]

Run command, piping the output either into $0 or var, as above, and RT.

command |& getline [var]

Run command as a coprocess piping the output either into $0 or var, as above, and RT. Coprocesses are a gawk extension. (The command can also be a socket.  See the subsection Special File Names, below.)

next

Stop processing the current input record. Read the next input record and start processing over with the first pattern in the AWK program. Upon reaching the end of the input data, execute any END rule(s).

nextfile

Stop processing the current input file.  The next input record read comes from the next input file. Update FILENAME and ARGIND, reset FNR to 1, and start processing over with the first pattern in the AWK program. Upon reaching the end of the input data, execute any ENDFILE and END rule(s).

print

Print the current record. The output record is terminated with the value of ORS.

print expr-list

Print expressions. Each expression is separated by the value of OFS. The output record is terminated with the value of ORS.

print expr-list >file

Print expressions on file. Each expression is separated by the value of OFS. The output record is terminated with the value of ORS.

printf fmt, expr-list

Format and print. See The printf Statement, below.

printf fmt, expr-list >file

Format and print on file.

system(cmd-line)

Execute the command cmd-line, and return the exit status. (This may not be available on non-POSIX systems.) See GAWK: Effective AWK Programming for the full details on the exit status.

fflush([file])

Flush any buffers associated with the open output file or pipe file. If file is missing or if it is the null string, then flush all open output files and pipes.

Additional output redirections are allowed for print and printf.

print ... >> file

Append output to the file.

print ... | command

Write on a pipe.

print ... |& command

Send data to a coprocess or socket. (See also the subsection Special File Names, below.)

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

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

The AWK versions of the printf statement and sprintf() function (see below) accept the following conversion specification formats:

%a, %A

A floating point number of the form [-]0xh.hhhhp+-dd (C99 hexadecimal floating point format). For %A, uppercase letters are used instead of lowercase ones.

%c

A single character. If the argument used for %c is numeric, it is treated as a character and printed. Otherwise, the argument is assumed to be a string, and the only first character of that string is printed.

%d, %i

A decimal number (the integer part).

%e, %E

A floating point number of the form [-]d.dddddde[+-]dd. The %E format uses E instead of e.

%f, %F

A floating point number of the form [-]ddd.dddddd. If the system library supports it, %F is available as well. This is like %f, but uses capital letters for special “not a number” and “infinity” values. If %F is not available, gawk uses %f.

%g, %G

Use %e or %f conversion, whichever is shorter, with nonsignificant zeros suppressed. The %G format uses %E instead of %e.

%o

An unsigned octal number (also an integer).

%u

An unsigned decimal number (again, an integer).

%s

A character string.

%x, %X

An unsigned hexadecimal number (an integer). The %X format uses ABCDEF instead of abcdef.

%%

A single % character; no argument is converted.

Optional, additional parameters may lie between the % and the control letter:

count$

Use the count'th argument at this point in the formatting. This is called a positional specifier and is intended primarily for use in translated versions of format strings, not in the original text of an AWK program. It is a gawk extension.

-

The expression should be left-justified within its field.

space

For numeric conversions, prefix positive values with a space, and negative values with a minus sign.

+

The plus sign, used before the width modifier (see below), says to always supply a sign for numeric conversions, even if the data to be formatted is positive.  The + overrides the space modifier.

#

Use an “alternate form” for certain control letters. For %o, supply a leading zero. For %x, and %X, supply a leading 0x or 0X for a nonzero result. For %e, %E, %f and %F, the result always contains a decimal point. For %g, and %G, trailing zeros are not removed from the result.

0

A leading 0 (zero) acts as a flag, indicating that 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 significant digits.  For the %d, %i, %o, %u, %x, and %X formats, it specifies the minimum number of digits to print.  For the %s format, it specifies the maximum number of characters from the string that should be printed.

The dynamic width and prec capabilities of the ISO C printf() routines are supported. A * in place of either the width or prec specifications causes their values to be taken from the argument list to printf or sprintf(). To use a positional specifier with a dynamic width or precision, supply the count$ after the * in the format string. For example, "%3$*2$.*1$s".

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:

whereas you would otherwise have to use

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:

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 any of the strings valid for PROCINFO["sorted_in"]. It can also be the name of a user-defined comparison function as described in PROCINFO["sorted_in"].

asorti(s [, d [, how] ])

Return the number of elements in the source array s. The behavior is the same as that of asort(), except that the array indices are used for sorting, not the array values. When done, the array is indexed numerically, and the values are those of the original indices. The original values are lost; thus provide a second array if you wish to preserve the original. The purpose of the optional string how is the same as described previously for asort().

gensub(r, s, h [, t])

Search the target string t for matches of the regular expression r. If h is a string beginning with g or G, then replace all matches of r with s. Otherwise, h is a number indicating which match of r to replace. If t is not supplied, use $0 instead. Within the replacement text s, the sequence \n, where n is a digit from 1 to 9, may be used to indicate just the text that matched the n'th parenthesized subexpression.  The sequence \0 represents the entire matched text, as does the character &. Unlike sub() and gsub(), the modified string is returned as the result of the function, and the original target string is not changed.

gsub(r, s [, t])

For each substring matching the regular expression r in the string t, substitute the string s, and return the number of substitutions. If t is not supplied, use $0. An & in the replacement text is replaced with the text that was actually matched. Use \& to get a literal &. (This must be typed as "\\&"; see GAWK: Effective AWK Programming for a fuller discussion of the rules for ampersands and backslashes in the replacement text of sub(), gsub(), and gensub().)

index(s, t)

Return the index of the string t in the string s, or zero if t is not present. (This implies that character indices start at one.) It is a fatal error to use a regexp constant for t.

length([s])

Return the length of the string s, or the length of $0 if s is not supplied. As a non-standard extension, with an array argument, length() returns the number of elements in the array.

match(s, r [, a])

Return the position in s where the regular expression r occurs, or zero if r is not present, and set the values of RSTART and RLENGTH. Note that the argument order is the same as for the ~ operator: str ~ re. If array a is provided, a is cleared and then elements 1 through n are filled with the portions of s that match the corresponding parenthesized subexpression in r. The zero'th element of a contains the portion of s matched by the entire regular expression r. Subscripts a[n, "start"], and a[n, "length"] provide the starting index in the string and length respectively, of each matching substring.

patsplit(s, a [, r [, seps] ])

Split the string s into the array a and the separators array seps on the regular expression r, and return the number of fields. Element values are the portions of s that matched r. The value of seps[i] is the possibly null separator that appeared after a[i]. The value of seps[0] is the possibly null leading separator. If r is omitted, FPAT is used instead. The arrays a and seps are cleared first. Splitting behaves identically to field splitting with FPAT, described above.

split(s, a [, r [, seps] ])

Split the string s into the array a and the separators array seps on the regular expression r, and return the number of fields.  If r is omitted, FS is used instead. The arrays a and seps are cleared first. seps[i] is the field separator matched by r 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. In particular, if r is a single-character string, that string acts as the separator, even if it happens to be a regular expression metacharacter.

sprintf(fmt, expr-list)

Print expr-list according to fmt, and return the resulting string.

strtonum(str)

Examine str, and return its numeric value. If str begins with a leading 0, treat it as an octal number. If str begins with a leading 0x or 0X, treat it as a hexadecimal number. Otherwise, assume it is a decimal number.

sub(r, s [, t])

Just like gsub(), but replace only the first matching substring. Return either zero or one.

substr(s, i [, n])

Return the at most n-character substring of s starting at i. If n is omitted, use the rest of s.

tolower(str)

Return a copy of the string str, with all the uppercase characters in str translated to their corresponding lowercase counterparts. Non-alphabetic characters are left unchanged.

toupper(str)

Return a copy of the string str, with all the lowercase characters in str translated to their corresponding uppercase counterparts. Non-alphabetic characters are left unchanged.

Gawk is multibyte aware.  This means that index(), length(), substr() and match() all work in terms of characters, not bytes.

Since one of the primary uses of AWK programs is processing log files that contain time stamp information, gawk provides the following functions for obtaining time stamps and formatting them.

mktime(datespec [, utc-flag])

Turn datespec into a time stamp of the same form as returned by systime(), and return the result. The datespec is a string of the form YYYY MM DD HH MM SS[ DST]. The contents of the string are six or seven numbers representing respectively the full year including century, the month from 1 to 12, the day of the month from 1 to 31, the hour of the day from 0 to 23, the minute from 0 to 59, the second from 0 to 60, and an optional daylight saving flag. The values of these numbers need not be within the ranges specified; for example, an hour of -1 means 1 hour before midnight. The origin-zero Gregorian calendar is assumed, with year 0 preceding year 1 and year -1 preceding year 0. If utc-flag is present and is non-zero or non-null, the time is assumed to be in the UTC time zone; otherwise, the time is assumed to be in the local time zone. If the DST daylight saving flag is positive, the time is assumed to be daylight saving time; if zero, the time is assumed to be standard time; and if negative (the default), mktime() attempts to determine whether daylight saving time is in effect for the specified time. If datespec does not contain enough elements or if the resulting time is out of range, mktime() returns -1.

strftime([format [, timestamp[, utc-flag]]])

Format timestamp according to the specification in format. If utc-flag is present and is non-zero or non-null, the result is in UTC, otherwise the result is in local time. The timestamp should be of the same form as returned by systime(). If timestamp is missing, the current time of day is used. If format is missing, a default format equivalent to the output of date(1) is used. The default format is available in PROCINFO["strftime"]. See the specification for the strftime() function in 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.

NOTE: Passing negative operands to any of these functions causes a fatal error.

The functions are:

and(v1, v2 [, ...])

Return the bitwise AND of the values provided in the argument list. There must be at least two.

compl(val)

Return the bitwise complement of val.

lshift(val, count)

Return the value of val, shifted left by count bits.

or(v1, v2 [, ...])

Return the bitwise OR of the values provided in the argument list. There must be at least two.

rshift(val, count)

Return the value of val, shifted right by count bits.

xor(v1, v2 [, ...])

Return the bitwise XOR of the values provided in the argument list. There must be at least two.

The following functions provide type related information about their arguments.

isarray(x)

Return true if x is an array, false otherwise. This function is mainly for use with the elements of multidimensional arrays and with function parameters.

typeof(x)

Return a string indicating the type of x. The string will be one of "array", "number", "regexp", "string", "strnum", "unassigned", or "undefined".

The following functions may be used from within your AWK program for translating strings at run-time. For full details, see GAWK: Effective AWK Programming.

bindtextdomain(directory [, domain])

Specify the directory where gawk looks for the .gmo files, in case they will not or cannot be placed in the “standard” locations (e.g., during testing). It returns the directory where domain is “bound.”

The default domain is the value of TEXTDOMAIN. If directory is the null string (""), then bindtextdomain() returns the current binding for the given domain.

dcgettext(string [, domain [, category]])

Return the translation of string in text domain domain for locale category 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.