awk – pattern–directed scanning and processing language|
awk [ –F fs ] [ –d ] [ –mf n ] [ –mr n ] [ –safe ] [ –v var=value ]
[ –f progfile | prog ] [ file ... ]|
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 progfile. With each pattern there can be an associated action
that will be performed when a line of a file matches the pattern.
Each line is matched against the pattern portion of every
pattern–action statement; 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 file name, and is executed at the time it would have been opened
if it were a file name. The option –v followed by
var=value is an assignment to be done before the program is executed;
any number of –v options may be present. –F fs option defines the
input field separator to be the regular expression fs. |
An input line is normally made up of fields separated by white space, or by regular expression FS. The fields are denoted $1, $2, ..., while $0 refers to the entire line. If FS is null, the input line is split into one field per character.
To compensate for inadequate implementation of storage management, the –mr option can be used to set the maximum size of the input record, and the –mf option to set the maximum number of fields.
The –safe option causes awk to run in ``safe mode,'' in which it is not allowed to run shell commands or open files and the environment is not made available in the ENVIRON variable.
A pattern–action statement has the form
An action is a sequence of statements. A statement can be one of the following:
if( expression ) statement [ else statement ]
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 white space). The operators ! ++ –– += –= *= /= %= ^= > >= < <= == != ?: are also available in expressions. Variables may be scalars, array elements (denoted x[i]) or fields. Variables are initialized to the null string. Array subscripts may 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 may 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 fprintf(2)). The built–in function close(expr) closes the file or pipe expr. The built–in function fflush(expr) flushes any buffered output for the file or pipe expr. If expr is omitted or is a null string, all open files are flushed.
The mathematical functions exp, log, sqrt, sin, cos, and atan2
are built in. Other built–in functions:
sprintf(fmt, expr, ...)
Patterns are arbitrary Boolean combinations (with ! || &&) of regular expressions and relational expressions. Regular expressions are as in regexp(6). Isolated regular expressions in a pattern apply to the entire line. Regular expressions may also occur in relational expressions, using the operators ~ and !~. /re/ is a constant regular expression; any string (constant or variable) may be used as a regular expression, except in the position of an isolated regular expression in a pattern.
A pattern may 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:
The special patterns BEGIN and END may 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.
Variable names with special meanings:
Functions may be defined (at the position of a pattern–action statement)
length($0) > 72|
A. V. Aho, B. W. Kernighan, P. J. Weinberger, The AWK Programming Language, Addison–Wesley, 1988. ISBN 0–201–07981–X
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.|
The scope rules for variables in functions are a botch; the syntax is worse.
UTF is not always dealt with correctly, though awk does make an attempt to do so. The split function with an empty string as final argument now copes with UTF in the string being split.