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awk scans each of its input filenames for lines that match any of a set of patterns specified in program. The input filenames are read in order; the standard input is read if there are no filenames. The filename `-' means the standard input.
The set of patterns may either appear literally on the command line as program, or, by using the `-f program-file' option, the set of patterns may be in a program-file; a program-file of `-' means the standard input. If the program is specified on the command line, it should be enclosed in single quotes (') to protect it from the shell.
awk variables may be set on the command line using arguments of the form variable=value. This sets the awk variable variable to value before the first record of the next filename argument is read.
With each pattern in program there can be an associated action that will be performed when a line of a filename matches the pattern. See the discussion below for the format of input lines and the awk language. Each line in each input filename is matched against the pattern portion of every pattern-action statement; the associated action is performed for each matched pattern.
An input line is made up of fields separated by white space. The field separator can be changed by using FS -- see Special Variable Names below. Fields are denoted $1, $2, and so forth. $0 refers to the entire line.
A pattern-action statement has the form
An action is a sequence of statements. A statement can be one of the following:
statements are terminated by semicolons, NEWLINE characters or right braces. An empty expression-list stands for the whole line.
expressions take on string or numeric values as appropriate, and are built using the operators +, -, *, /, %, and concatenation (indicated by a blank). The C operators ++ , -- , += , -= , *= , /= , and %= are also available in expressions.
variable 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, providing a form of associative memory. String constants are quoted "... ".
The print statement prints its arguments on the standard output (or on a file if >filename is present), separated by the current output field separator, and terminated by the output record separator. The printf statement formats its expression list according to the format template format (see printf.3v for a description of the formatting control characters).
The built-in function length returns the length of its argument taken as a string, or of the whole line if no argument. There are also built-in functions exp, log, sqrt, and int, where int truncates its argument to an integer. `substr( s, m, n )' returns the n-character substring of s that begins at position m. `sprintf (format, expression, expression, ...)' formats the expressions according to the printf format given by format, and returns the resulting string.
Patterns are arbitrary Boolean combinations (!, ||, &&, and parentheses) of regular expressions and relational expressions. Regular expressions must be surrounded by slashes and are as in egrep (see grep.1v Isolated regular expressions in a pattern apply to the entire line. Regular expressions may also occur in relational expressions.
A pattern may consist of two patterns separated by a comma; in this case, the action is performed for all lines between an occurrence of the first pattern and the next occurrence of the second.
A relational expression is one of the following:
expression matchop regular-expression expression relop expression
where a relop is any of the six relational operators in C, and a matchop is either ~ (contains) or !~ (does not contain). A conditional is an arithmetic expression, a relational expression, or a Boolean combination of these.
The special pattern BEGIN may be used to capture control before the first input line is read, in which case BEGIN must be the first pattern. The special pattern END may be used to capture control after the last input line is read, in which case END must be the last pattern.
A single character c may be used to separate the fields by starting the program with
BEGIN {FS = "c" }
or by using the -Fc option.
Other variable names with special meanings include NF, the number of fields in the current record; NR, the ordinal number of the current record; FILENAME, the name of the current input file; OFS, the output field separator (default blank); ORS, the output record separator (default NEWLINE); and OFMT, the output format for numbers (default %.6g).
Print lines longer than 72 characters:
length > 72
Print first two fields in opposite order:
{ print $2, $1 }
Add up first column, print sum and average:
{ s += $1 }
END { print "sum is", s, " average is", s/NR }
Print fields in reverse order:
{ for (i = NF; i > 0; --i) print $i }
Print all lines between start/stop pairs:
/start/, /stop/
Print all lines whose first field is different from previous one:
$1 != prev { print; prev = $1 }
[a manual with the abbreviation TEXT]
A. V. Aho, B. W. Kerninghan, P. J. Weinberger, The AWK Programming Language Addison-Wesley, 1988.
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 the null string ("") to it.
There is no escape sequence that prints a double-quote. A workaround is to use the sprintf (see printf.3v function to store the character into a variable by its ASCII sequence.
dq = sprintf("%c", 34)
Syntax errors result in the cryptic message `awk: bailing out near line 1'.
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Created by unroff & hp-tools. © somebody (See intro for details). All Rights Reserved. Last modified 11/5/97