Gawk

gawk Summary

This appendix provides a brief summary of the gawk command line and the awk language. It is designed to serve as "quick reference." It is therefore terse, but complete.

Command Line Options Summary

The command line consists of options to gawk itself, the awk program text (if not supplied via the `-f' option), and values to be made available in the ARGC and ARGV predefined awk variables:

gawk [POSIX or GNU style options] -f source-file [--] file ...
gawk [POSIX or GNU style options] [--] 'program' file ...

The options that gawk accepts are:

-F fs

--field-separator fs

Use fs for the input field separator (the value of the FS predefined variable).

-f program-file

--file program-file

Read the awk program source from the file program-file, instead of from the first command line argument.

-mf=NNN

-mr=NNN

The `f' flag sets the maximum number of fields, and the `r' flag sets the maximum record size. These options are ignored by gawk, since gawk has no predefined limits; they are only for compatibility with the Bell Labs research version of Unix awk.

-v var=val

--assign var=val

Assign the variable var the value val before program execution begins.

-W traditional

-W compat

--traditional

--compat

Use compatibility mode, in which gawk extensions are turned off.

-W copyleft

-W copyright

--copyleft

--copyright

Print the short version of the General Public License on the error output. This option may disappear in a future version of gawk.

-W help

-W usage

--help

--usage

Print a relatively short summary of the available options on the error output.

-W lint

--lint

Give warnings about dubious or non-portable awk constructs.

-W lint-old

--lint-old

Warn about constructs that are not available in the original Version 7 Unix version of awk.

-W posix

--posix

Use POSIX compatibility mode, in which gawk extensions are turned off and additional restrictions apply.

-W re-interval

--re-interval

Allow interval expressions (see section Regular Expression Operators), in regexps.

-W source=program-text

--source program-text

Use program-text as awk program source code. This option allows mixing command line source code with source code from files, and is particularly useful for mixing command line programs with library functions.

-W version

--version

Print version information for this particular copy of gawk on the error output.

--

Signal the end of options. This is useful to allow further arguments to the awk program itself to start with a `-'. This is mainly for consistency with POSIX argument parsing conventions.

Any other options are flagged as invalid, but are otherwise ignored. See section Command Line Options, for more details.

Language Summary

An awk program consists of a sequence of zero or more pattern-action statements and optional function definitions. One or the other of the pattern and action may be omitted.

pattern    { action statements }
pattern
          { action statements }

function name(parameter list)     { action statements }

gawk first reads the program source from the program-file(s), if specified, or from the first non-option argument on the command line. The `-f' option may be used multiple times on the command line. gawk reads the program text from all the program-file files, effectively concatenating them in the order they are specified. This is useful for building libraries of awk functions, without having to include them in each new awk program that uses them. To use a library function in a file from a program typed in on the command line, specify `--source 'program'', and type your program in between the single quotes. See section Command Line Options.

The environment variable AWKPATH specifies a search path to use when finding source files named with the `-f' option. The default path, which is `.:/usr/local/share/awk'(23) is used if AWKPATH is not set. If a file name given to the `-f' option contains a `/' character, no path search is performed. See section The AWKPATH Environment Variable.

gawk compiles the program into an internal form, and then proceeds to read each file named in the ARGV array. The initial values of ARGV come from the command line arguments. If there are no files named on the command line, gawk reads the standard input.

If a "file" named on the command line has the form `var=val', it is treated as a variable assignment: the variable var is assigned the value val. If any of the files have a value that is the null string, that element in the list is skipped.

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, the associated action is executed.

Variables and Fields

awk variables are not declared; they come into existence when they are first used. Their values are either floating-point numbers or strings. awk also has one-dimensional arrays; multiple-dimensional arrays may be simulated. There are several predefined variables that awk sets as a program runs; these are summarized below.

Fields

As each input line is read, gawk splits the line 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. 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. If FS is the null string (""), then each individual character in the record becomes a separate field. Note that the value of IGNORECASE (see section Case-sensitivity in Matching) also affects how fields are split when FS is a regular expression.

Each field in the input line may be referenced by its position, $1, $2, and so on. $0 is the whole line. The value of a field may be assigned to as well. Field numbers need not be constants:

n = 5
print $n

prints the fifth field in the input line. The variable NF is set to the total number of fields in the input line.

References to non-existent fields (i.e. fields after $NF) return 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 value, and causes the value of $0 to be recomputed, with the fields being separated by the value of OFS. See section Reading Input Files.

Built-in Variables

gawk's built-in variables are:

ARGC

The number of elements in ARGV. See below for what is actually included in ARGV.

ARGIND

The index in ARGV of the current file being processed. When gawk is processing the input data files, it is always true that `FILENAME == ARGV[ARGIND]'.

ARGV

The array of command line arguments. The array is indexed from zero to ARGC - 1. Dynamically changing ARGC and the contents of ARGV can control the files used for data. A null-valued element in ARGV is ignored. ARGV does not include the options to awk or the text of the awk program itself.

CONVFMT

The conversion format to use when converting numbers to strings.

FIELDWIDTHS

A space separated list of numbers describing the fixed-width input data.

ENVIRON

An array of environment variable values. The array is indexed by variable name, each element being the value of that variable. Thus, the environment variable HOME is ENVIRON["HOME"]. One possible value might be `/home/arnold'. Changing this array does not affect the environment seen by programs which gawk spawns via redirection or the system function. (This may change in a future version of gawk.) Some operating systems do not have environment variables. The ENVIRON array is empty when running on these systems.

ERRNO

The system error message when an error occurs using getline or close.

FILENAME

The name of the current input file. If no files are specified on the command line, the value of FILENAME is the null string.

FNR

The input record number in the current input file.

FS

The input field separator, a space by default.

IGNORECASE

The case-sensitivity flag for string comparisons and regular expression operations. If IGNORECASE has a non-zero value, then pattern matching in rules, record separating with RS, field splitting with FS, regular expression matching with `~' and `!~', and the gensub, gsub, index, match, split and sub built-in functions all ignore case when doing regular expression operations, and all string comparisons are done ignoring case.

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 for the print statement, "%.6g" by default.

OFS

The output field separator, a space by default.

ORS

The output record separator, by default a newline.

RS

The input record separator, by default a newline. If RS is set to the null string, then records are separated by blank lines. When RS is set to the null string, then the newline character always acts as a field separator, in addition to whatever value FS may have. If RS is set to a multi-character string, it denotes a regexp; input text matching the regexp separates records.

RT

The input text that matched the text denoted by RS, the record separator.

RSTART

The index of the first character last matched by match; zero if no match.

RLENGTH

The length of the string last matched by match; -1 if no match.

SUBSEP

The string used to separate multiple subscripts in array elements, by default "\034".

See section Built-in Variables, for more information.

Arrays

Arrays are subscripted with an expression between square brackets (`[' and `]'). Array subscripts are always strings; numbers are converted to strings as necessary, following the standard conversion rules (see section Conversion of Strings and Numbers).

If you use multiple expressions separated by commas inside the square brackets, then the array subscript is a string consisting of the concatenation of the individual subscript values, converted to strings, separated by the subscript separator (the value of SUBSEP).

The special operator in may be used in a conditional context to see 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' to test for existence of an element.

The in construct may also be used in a for loop to iterate over all the elements of an array. See section Scanning All Elements of an Array.

You can remove an element from an array using the delete statement.

You can clear an entire array using `delete array'.

See section Arrays in awk.

Data Types

The value of an awk expression is always either a number or a string.

Some contexts (such as arithmetic operators) require numeric values. They convert strings to numbers by interpreting the text of the string as a number. If the string does not look like a number, it converts to zero.

Other contexts (such as concatenation) require string values. They convert numbers to strings by effectively printing them with sprintf. See section Conversion of Strings and Numbers, for the details.

To force conversion of a string value to a number, simply add zero to it. If the value you start with is already a number, this does not change it.

To force conversion of a numeric value to a string, concatenate it with the null string.

Comparisons are done numerically if both operands are numeric, or if one is numeric and the other is a numeric string. Otherwise one or both operands are converted to strings and a string comparison is performed. Fields, getline input, FILENAME, ARGV elements, ENVIRON elements and the elements of an array created by split are the only items that can be numeric strings. String constants, such as "3.1415927" are not numeric strings, they are string constants. The full rules for comparisons are described in section Variable Typing and Comparison Expressions.

Uninitialized variables have the string value "" (the null, or empty, string). In contexts where a number is required, this is equivalent to zero.

See section Variables, for more information on variable naming and initialization; see section Conversion of Strings and Numbers, for more information on how variable values are interpreted.

Patterns

An awk program is mostly composed of rules, each consisting of a pattern followed by an action. The action is enclosed in `{' and `}'. Either the pattern may be missing, or the action may be missing, but not both. If the pattern is missing, the action is executed for every input record. A missing action is equivalent to `{ print }', which prints the entire line.

Comments begin with the `#' character, and continue until the end of the line. Blank lines may be used to separate statements. Statements normally end with a newline; however, this is not the case for lines ending in a `,', `{', `?', `:', `&&', or `||'. Lines ending in do or else also have their statements automatically continued on the following line. In other cases, a line can be continued by ending it with a `\', in which case the newline is ignored.

Multiple statements may be put on one line by separating each one with a `;'. This applies to both the statements within the action part of a rule (the usual case), and to the rule statements.

See section Comments in awk Programs, for information on awk's commenting convention; see section awk Statements Versus Lines, for a description of the line continuation mechanism in awk.

Pattern Summary

awk patterns may be one of the following:

/regular expression/
relational expression
pattern && pattern
pattern || pattern
pattern ? pattern : pattern
(pattern)
! pattern
pattern1, pattern2
BEGIN
END

BEGIN and END are two special kinds of patterns that are not tested against the input. The action parts of all BEGIN rules are concatenated 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 concatenated, 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 rules cannot have missing action parts.

For /regular-expression/ patterns, the associated statement is executed for each input record that matches the regular expression. Regular expressions 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 matches, then the second pattern is matched against the input record; otherwise, the third is matched. Only one of the second and third patterns is matched.

The `pattern1, pattern2' form of a pattern is called a range pattern. It matches all input lines starting with a line that matches pattern1, and continuing until a line that matches pattern2, inclusive. A range pattern cannot be used as an operand of any of the pattern operators.

See section Pattern Elements.

Regular Expressions

Regular expressions are based on POSIX EREs (extended regular expressions). The escape sequences allowed in string constants are also valid in regular expressions (see section Escape Sequences). Regexps are composed of characters as follows:

c

matches the character c (assuming c is none of the characters listed below).

\c

matches the literal character c.

.

matches any character, including newline. In strict POSIX mode, `.' does not match the NUL character, which is a character with all bits equal to zero.

^

matches the beginning of a string.

$

matches the end of a string.

[abc...]

matches any of the characters abc... (character list).

[[:class:]]

matches any character in the character class class. Allowable classes are alnum, alpha, blank, cntrl, digit, graph, lower, print, punct, space, upper, and xdigit.

[[.symbol.]]

matches the multi-character collating symbol symbol. gawk does not currently support collating symbols.

[[=chars=]]

matches any of the equivalent characters in chars. gawk does not currently support equivalence classes.

[^abc...]

matches any character except abc... and newline (negated character list).

r1|r2

matches either r1 or r2 (alternation).

r1r2

matches r1, and then r2 (concatenation).

r+

matches one or more r's.

r*

matches zero or more r's.

r?

matches zero or one r's.

(r)

matches r (grouping).

r{n}

r{n,}

r{n,m}

matches at least n, n to any number, or n to m occurrences of r (interval expressions).

\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.

\w

matches any word-constituent character (alphanumeric characters and the underscore).

\W

matches any character that is not word-constituent.

\`

matches the empty string at the beginning of a buffer (same as a string in gawk).

\'

matches the empty string at the end of a buffer.

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

No options

In the default case, gawk provide all the facilities of POSIX regexps and the GNU regexp operators described above. However, interval expressions are not supported.

--posix

Only POSIX regexps are supported, the GNU operators are not special (e.g., `\w' matches a literal `w'). Interval expressions are allowed.

--traditional

Traditional Unix awk regexps are matched. The GNU operators are not special, interval expressions are not available, and neither are the POSIX character classes ([[:alnum:]] and so on). Characters described by octal and hexadecimal escape sequences are treated literally, even if they represent regexp metacharacters.

--re-interval

Allow interval expressions in regexps, even if `--traditional' has been provided.

See section Regular Expressions.

Actions

Action statements are enclosed in braces, `{' and `}'. A missing action statement is equivalent to `{ print }'.

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 similar to those in C.

Comments begin with the `#' character, and continue until the end of the line. Blank lines may be used to separate statements. Statements normally end with a newline; however, this is not the case for lines ending in a `,', `{', `?', `:', `&&', or `||'. Lines ending in do or else also have their statements automatically continued on the following line. In other cases, a line can be continued by ending it with a `\', in which case the newline is ignored.

Multiple statements may be put on one line by separating each one with a `;'. This applies to both the statements within the action part of a rule (the usual case), and to the rule statements.

See section Comments in awk Programs, for information on awk's commenting convention; see section awk Statements Versus Lines, for a description of the line continuation mechanism in awk.

Operators

The operators in awk, in order of decreasing precedence, are:

(...)

Grouping.

$

Field reference.

++ --

Increment and decrement, both prefix and postfix.

^

Exponentiation (`**' may also be used, and `**=' for the assignment operator, but they are not specified in the POSIX standard).

+ - !

Unary plus, unary minus, and logical negation.

* / %

Multiplication, division, and modulus.

+ -

Addition and subtraction.

space

String concatenation.

< <= > >= != ==

The usual relational operators.

~ !~

Regular expression match, negated match.

in

Array membership.

&&

Logical "and".

||

Logical "or".

?:

A 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.

See section Expressions.

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 }

See section Control Statements in Actions.

I/O Statements

The Input/Output statements are as follows:

getline

Set $0 from next input record; set NF, NR, FNR. See section Explicit Input with getline.

getline <file

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

getline var

Set var from next input record; set NF, FNR.

getline var <file

Set var from next record of file.

command | getline

Run command, piping its output into getline; sets $0, NF, NR.

command | getline var

Run command, piping its output into getline; sets var.

next

Stop processing the current input record. The next input record is read and processing starts over with the first pattern in the awk program. If the end of the input data is reached, the END rule(s), if any, are executed. See section The next Statement.

nextfile

Stop processing the current input file. The next input record read comes from the next input file. FILENAME is updated, FNR is set to one, ARGIND is incremented, and processing starts over with the first pattern in the awk program. If the end of the input data is reached, the END rule(s), if any, are executed. Earlier versions of gawk used `next file'; this usage is still supported, but is considered to be deprecated. See section The nextfile Statement.

print

Prints the current record. See section Printing Output.

print expr-list

Prints expressions.

print expr-list > file

Prints expressions to file. If file does not exist, it is created. If it does exist, its contents are deleted the first time the print is executed.

print expr-list >> file

Prints expressions to file. The previous contents of file are retained, and the output of print is appended to the file.

print expr-list | command

Prints expressions, sending the output down a pipe to command. The pipeline to the command stays open until the close function is called.

printf fmt, expr-list

Format and print.

printf fmt, expr-list > file

Format and print to file. If file does not exist, it is created. If it does exist, its contents are deleted the first time the printf is executed.

printf fmt, expr-list >> file

Format and print to file. The previous contents of file are retained, and the output of printf is appended to the file.

printf fmt, expr-list | command

Format and print, sending the output down a pipe to command. The pipeline to the command stays open until the close function is called.

getline returns zero on end of file, and -1 on an error. In the event of an error, getline will set ERRNO to the value of a system-dependent string that describes the error.

printf Summary

Conversion specification have the form %[flag][width][.prec]format. Items in brackets are optional.

The awk printf statement and sprintf function accept the following conversion specification formats:

%c

An ASCII 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

A floating point number of the form [-]ddd.dddddd.

%g

%G

Use either the `%e' or `%f' formats, whichever produces a shorter string, with non-significant zeros suppressed. `%G' will use `%E' instead of `%e'.

%o

An unsigned octal number (again, an integer).

%s

A character string.

%x

%X

An unsigned hexadecimal number (an integer). The `%X' format uses `A' through `F' instead of `a' through `f' for decimal 10 through 15.

%%

A single `%' character; no argument is converted.

There are optional, additional parameters that may lie between the `%' and the control letter:

-

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 non-zero result. For `e', `E', and `f', the result will always contain a decimal point. For `g', and `G', trailing zeros are not removed from the result.

0

A leading `0' (zero) acts as a flag, that indicates output should be padded with zeros instead of spaces. This applies even to non-numeric output formats. This flag only has an effect when the field width is wider than the value to be printed.

width

The field should be padded to this width. The field is normally padded with spaces. If the `0' flag has been used, it is padded with zeros.

.prec

A number that specifies the precision to use when printing. For the `e', `E', 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', `o', `i', `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.

Either or both of the width and prec values may be specified as `*'. In that case, the particular value is taken from the argument list.

See section Using printf Statements for Fancier Printing.

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 file names internally. These file names allow access to open file descriptors inherited from gawk's parent process (usually the shell). The file names are:

`/dev/stdin'

The standard input.

`/dev/stdout'

The standard output.

`/dev/stderr'

The standard error output.

`/dev/fd/n'

The file denoted by the open file descriptor n.

In addition, reading the following files provides process related information about the running gawk program. All returned records are terminated with a newline.

`/dev/pid'

Returns the process ID of the current process.

`/dev/ppid'

Returns the parent process ID of the current process.

`/dev/pgrpid'

Returns the process group ID of the current process.

`/dev/user'

At least four space-separated fields, containing the return values of the getuid, geteuid, getgid, and getegid system calls. If there are any additional fields, they are the group IDs returned by getgroups system call. (Multiple groups may not be supported on all systems.)

These file names may also be used on the command line to name data files. These file names are only recognized internally if you do not actually have files with these names on your system.

See section Special File Names in gawk, for a longer description that provides the motivation for this feature.

Built-in Functions

awk provides a number of built-in functions for performing numeric operations, string related operations, and I/O related operations.

The built-in arithmetic functions are:

atan2(y, x)

the arctangent of y/x in radians.

cos(expr)

the cosine in radians.

exp(expr)

the exponential function (e ^ expr).

int(expr)

truncates to integer.

log(expr)

the natural logarithm of expr.

rand()

a random number between zero and one.

sin(expr)

the sine in radians.

sqrt(expr)

the square root function.

srand([expr])

use expr as a new seed for the random number generator. If no expr is provided, the time of day is used. The return value is the previous seed for the random number generator.

awk has the following built-in string functions:

gensub(regex, subst, how [, target])

If how is a string beginning with `g' or `G', then replace each match of regex in target with subst. Otherwise, replace the how'th occurrence. If target is not supplied, use $0. The return value is the changed string; the original target is not modified. Within subst, `\n', where n is a digit from one to nine, can be used to indicate the text that matched the n'th parenthesized subexpression.

gsub(regex, subst [, target])

for each substring matching the regular expression regex in the string target, substitute the string subst, and return the number of substitutions. If target is not supplied, use $0.

index(str, search)

returns the index of the string search in the string str, or zero if search is not present.

length([str])

returns the length of the string str. The length of $0 is returned if no argument is supplied.

match(str, regex)

returns the position in str where the regular expression regex occurs, or zero if regex is not present, and sets the values of RSTART and RLENGTH.

split(str, arr [, regex])

splits the string str into the array arr on the regular expression regex, and returns the number of elements. If regex is omitted, FS is used instead. regex can be the null string, causing each character to be placed into its own array element. The array arr is cleared first.

sprintf(fmt, expr-list)

prints expr-list according to fmt, and returns the resulting string.

sub(regex, subst [, target])

just like gsub, but only the first matching substring is replaced.

substr(str, index [, len])

returns the len-character substring of str starting at index. If len is omitted, the rest of str is used.

tolower(str)

returns a copy of the string str, with all the upper-case characters in str translated to their corresponding lower-case counterparts. Non-alphabetic characters are left unchanged.

toupper(str)

returns a copy of the string str, with all the lower-case characters in str translated to their corresponding upper-case counterparts. Non-alphabetic characters are left unchanged.

The I/O related functions are:

close(expr)

Close the open file or pipe denoted by expr.

fflush([expr])

Flush any buffered output for the output file or pipe denoted by expr. If expr is omitted, standard output is flushed. If expr is the null string (""), all output buffers are flushed.

system(cmd-line)

Execute the command cmd-line, and return the exit status. If your operating system does not support system, calling it will generate a fatal error. `system("")' can be used to force awk to flush any pending output. This is more portable, but less obvious, than calling fflush.

Time Functions

The following two functions are available for getting the current time of day, and for formatting time stamps.

systime()

returns the current time of day as the number of seconds since a particular epoch (Midnight, January 1, 1970 UTC, on POSIX systems).

strftime([format[, timestamp]])

formats timestamp according to the specification in format. The current time of day is used if no timestamp is supplied. A default format equivalent to the output of the date utility is used if no format is supplied. See section Functions for Dealing with Time Stamps, for the details on the conversion specifiers that strftime accepts.

See section Built-in Functions, for a description of all of awk's built-in functions.

String Constants

String constants in awk are sequences of characters enclosed in double quotes ("). 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

Formfeed.

\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'. As in ANSI C, all following hexadecimal digits are considered part of the escape sequence. E.g., "\x1B" is a string containing the ASCII ESC (escape) character. (The `\x' escape sequence is not in POSIX awk.)

\ddd

The character represented by the one, two, or three digit sequence of octal digits. Thus, "\033" is also a string containing the ASCII ESC (escape) character.

\c

The literal character c, if c is not one of the above.

The escape sequences may also be used inside constant regular expressions (e.g., the regexp /[ \t\f\n\r\v]/ matches whitespace characters).

See section Escape Sequences.

User-defined Functions

Functions in awk are defined as follows:

function name(parameter list) { statements }

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.

If there are fewer arguments passed than there are names in parameter-list, the extra names are given the null string as their value. Extra names have the effect of local variables.

The open-parenthesis in a function call of a user-defined function must immediately follow the function name, without any intervening white space. This is to avoid a syntactic ambiguity with the concatenation operator.

The word func may be used in place of function (but not in POSIX awk).

Use the return statement to return a value from a function.

See section User-defined Functions.

Historical Features

There are two features of historical awk implementations that gawk supports.

First, it is possible to call the length built-in function not only with no arguments, but even without parentheses!

a = length

is the same as either of

a = length()
a = length($0)

For example:

$ echo abcdef | awk '{ print length }'
-| 6

This feature is marked as "deprecated" in the POSIX standard, and gawk will issue a warning about its use if `--lint' is specified on the command line. (The ability to use length this way was actually an accident of the original Unix awk implementation. If any built-in function used $0 as its default argument, it was possible to call that function without the parentheses. In particular, it was common practice to use the length function in this fashion, and this usage was documented in the awk manual page.)

The other historical feature is the use of either the break statement, or the continue statement outside the body of a while, for, or do loop. Traditional awk implementations have treated such usage as equivalent to the next statement. More recent versions of Unix awk do not allow it. gawk supports this usage if `--traditional' has been specified.

See section Command Line Options, for more information about the `--posix' and `--lint' options.