awk(1)

NAME

awk − pattern-directed scanning and processing language

SYNOPSIS

awk [-F fs] [-v var=value ] [prog | -f file ...] [file ...]

DESCRIPTION

awk scans each input file for lines that match any of a set of patterns specified literally in prog or in one or more files specified as -f file. With each pattern there can be an associated action that is to be performed when a line in a file matches the pattern. Each line is matched against the pattern portion of every pattern-action statement, and the associated action is performed for each matched pattern. The file name - means the standard input. Any file of the form var=value is treated as an assignment, not a filename. An assignment is evaluated at the time it would have been opened if it were a filename, unless the -v option is used.

An input line is made up of fields separated by white space, or by regular expression FS. The fields are denoted $1, $2, ...; $0 refers to the entire line.

Options

awk recognizes the following options and arguments:

-F fs Specify regular expression used to separate fields. The default is to recognize space and tab characters, and to discard leading spaces and tabs. If the -F option is used, leading input field separators are no longer discarded.

-f file Specify an awk program file. Up to 100 program files can be specified. The pattern-action statements in these files are executed in the same order as the files were specified.

-v var=value Cause var=value assignment to occur before the BEGIN action (if it exists) is executed.

Statements

A pattern-action statement has the form:

pattern { action }

A missing { action } means print the line; a missing pattern always matches. Pattern-action statements are separated by new-lines or semicolons.

An action is a sequence of statements. A statement can be one of the following:

if(expression) statement [else statement]
while(expression) statement
for(expression;expression; expression) statement
for(var in array) statement
do statement while(expression)
break
continue
{ [statement ...] }
expression# commonly var=expression
print [expression-list] [> expression]
printf format [, expression-list] [> expression]
return [expression]
next # skip remaining patterns on this input line.
delete array [expression] # delete an array element.
exit [expression] # exit immediately; status is expression.

Statements are terminated by semicolons, newlines or right braces. An empty expression-list stands for $0. String constants are quoted (""), with the usual C escapes recognized within. Expressions take on string or numeric values as appropriate, and are built using the operators +, -, *, /, %, ^ (exponentiation), and concatenation (indicated by a blank). The operators ++, --, +=, -=, *=, /=, %=, ^=, **=, >, >=, <, <=, ==, !=, and ?: are also available in expressions. Variables can be scalars, array elements (denoted x[i] ) or fields. Variables are initialized to the null string. Array subscripts can be any string, not necessarily numeric (this allows for a form of associative memory). Multiple subscripts such as [i, j,k] are permitted. The constituents are concatenated, separated by the value of SUBSEP.

The print statement prints its arguments on the standard output (or on a file if >file or >>file is present or on a pipe if |cmd is present), separated by the current output field separator, and terminated by the output record separator. file and cmd can be literal names or parenthesized expressions. Identical string values in different statements denote the same open file. The printf statement formats its expression list according to the format (see printf(3)).

Built-In Functions

The built-in function close(expr) closes the file or pipe expr. This function returns zero if successful, otherwise, it returns non-zero.

The customary functions exp, log, sqrt, sin, cos, atan2 are built in. Other built-in functions are:

length([s]) Length of its associated argument (in characters) taken as a string, or of $0 if no argument.

rand() Random number on (0,1)

srand([expr]) Sets and returns seed for rand, and returns the seed set. If no argument is given, the time of day is used as the seed value; otherwise, [expr] is used.

int(x) Truncates to an integer value

substr(s, m[, n])
n-character substring of s that begins at position m counted from 1. If n is omitted, the substring is limited by the length of string s.

index(s, t) Position in s where the string t occurs, or 0 if it does not.

match(s, ere) Position in s where the extended regular expression ere occurs, or 0 if it does not. The variables RSTART and RLENGTH are set to the position and length of the matched string.

split(s, a[, fs])
Splits the string s into array elements a[1] , a[2] , ..., a[n] , and returns n. The separation is done with the regular expression fs, or with the field separator FS if fs is not given.

sub(ere, repl [, in])
Substitutes repl for the first occurrence of the extended regular expression ere in the string in. If in is not given, $0 is used.

gsub ame as sub except that all occurrences of the regular expression are replaced; sub and gsub return the number of replacements.

sprintf(fmt, expr, ...)
String resulting from formatting expr ... according to the printf(3S) format fmt

system(cmd) Executes cmd and returns its exit status

toupper(s) Converts the argument string s to uppercase and returns the result.

tolower(s) Converts the argument string s to lowercase and returns the result.

The built-in function getline sets $0 to the next input record from the current input file; getline < file sets $0 to the next record from file. getline x sets variable x instead. Finally, cmd | getline pipes the output of cmd into getline; each call of getline returns the next line of output from cmd. In all cases, getline returns 1 for a successful input, 0 for end of file, and −1 for an error.

Patterns

Patterns are arbitrary Boolean combinations (with ! || &&) of regular expressions and relational expressions. awk supports Extended Regular Expressions as described in regexp(5). Isolated regular expressions in a pattern apply to the entire line. Regular expressions can also occur in relational expressions, using the operators ~ and !~. /re/ is a constant regular expression; any string (constant or variable) can be used as a regular expression, except in the position of an isolated regular expression in a pattern.

A pattern can consist of two patterns separated by a comma; in this case, the action is performed for all lines from an occurrence of the first pattern though an occurrence of the second.

A relational expression is one of the following:

expression matchop regular-expression
expression relop expression
expression in array-name
(expr,expr,...) in array-name

where a relop is any of the six relational operators in C, and a matchop is either ~ (matches) or !~ (does not match). A conditional is an arithmetic expression, a relational expression, or a Boolean combination of the two.

The special patterns BEGIN and END can be used to capture control before the first input line is read and after the last. BEGIN and END do not combine with other patterns.

Special Characters

The following special escape sequences are recognized by awk in both regular expressions and strings:

Escape Meaning

\a alert character

\b backspace character

\f form-feed character

\n new-line character

\r carriage-return character

\t tab character

\v vertical-tab character

\nnn 1- to 3-digit octal value nnn

\xhhh 1- to n-digit hexadecimal number

Variable Names

Variable names with special meanings are:

FS Regular expression used to separate fields; also settable by option -Ffs.

NF Number of fields in the current record

NR Ordinal number of the current record

FNR Ordinal number of the current record in the current file

FILENAME Name of the current input file

RS Input record separator (default newline)

OFS Output field separator (default blank)

ORS Output record separator (default newline)

OFMT Output format for numbers (default %.6g). If the value of OFMT is not a floating-point format specification, the results are unspecified.

CONVFMT Internal conversion format for numbers (default %.6g). If the value of CONVFMT is not a floating-point format specification, the results are unspecified.

SUBSEP Separates multiple subscripts (default 034)

ARGC Argument count, assignable

ARGV Argument array, assignable; non-null members are taken as filenames

ENVIRON Array of environment variables; subscripts are names. For example, if environment variable V=thing, ENVIRON["V"] produces thing.

Functions can be defined (at the position of a pattern-action statement) as follows:

function foo(a, b, c) { ...; return x }

Parameters are passed by value if scalar, and by reference if array name. Functions can be called recursively. Parameters are local to the function; all other variables are global.

Note that if pattern-action statements are used in an HP-UX command line as an argument to the awk command, the pattern-action statement must be enclosed in single quotes to protect it from the shell. For example, to print lines longer than 72 characters, the pattern-action statement as used in a script (-f file command form) is:

length > 72

The same pattern action statement used as an argument to the awk command is quoted in this manner:

awk ’length > 72’

EXTERNAL INFLUENCES

Environment Variables

LC_COLLATE determines the collating sequence used when evaluating regular expressions and by the relational operators when performing comparisons on string values.

LC_CTYPE determines the interpretation of text as single and/or multi-byte characters, and the characters matched by character-class expressions in regular expressions.

If LC_COLLATE or LC_CTYPE is not specified in the environment or is set to the empty string, the value of LANG is used as a default for each unspecified or empty variable. If LANG is not specified or is set to the empty string, a default of "C" (see lang(5)) is used instead of LANG. If any internationalization variable contains an invalid setting, awk behaves as if all internationalization variables are set to "C". See environ(5).

The LC_NUMERIC category determines the radix character used to print floating-point numbers.

LANG defines the search path when looking for commands executed by system(expr).

International Code Set Support

Single- and multi-byte character code sets are supported except that variable names must contain only ASCII characters and regular expressions must contain only single-byte characters.

DIAGNOSTICS

awk supports up to 199 fields ($1, $2, ..., $199) per record.

EXAMPLES

Print lines longer than 72 characters:

length > 72

Print first two fields in opposite order:

{ print $2, $1 }

Same, with input fields separated by comma and/or blanks and tabs:

BEGIN { FS = ",[ \t]*|[ \t]+" }
{ print $2, $1 }

Add up first column, print sum and average:

{ s += $1 }"
END { print "sum is", s, " average is", s/NR }

Print all lines between start/stop pairs:

/start/, /stop/

Simulate echo command (see echo(1)):

BEGIN   {                             # Simulate echo(1)
        for (i = 1; i < ARGC; i++) printf "%s ", ARGV[i]
        printf "\n"
        exit }

BUGS

There are no explicit conversions between numbers and strings. To force an expression to be treated as a number add 0 to it; to force it to be treated as a string, concatenate to it.

Scope rules for variables in functions are not well defined.

AUTHOR

awk was developed by AT&T.

STANDARDS CONFORMANCE

awk: SVID2, XPG2, XPG3, POSIX.2

Hewlett-Packard Company — HP-UX Release 9.0: August 1992

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