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awk [-Ffieldsep] [-v var=value] [-f progfile | 'prog'] [file . . .]
awk processes supplementary code set characters in pattern-action statements and comments, and recognizes supplementary code set characters as field separators (see below) according to the locale specified in the LC_CTYPE environment variable (see LANG on environ(5)). In regular expressions, pattern searches are performed on characters, not bytes, as described on grep(1).
Each input line is matched against the pattern portion of every pattern-action statement; the associated action is performed for each matched pattern. Any file of the form var=value is treated as an assignment, not a filename, and is executed at the time it would have been opened if it were a filename, and is executed at the time it would have been opened if it were a filename. The option -v followed by var=value is an assignment to be done before prog is executed; any number of -v options may be present.
An input line is normally made up of fields separated by white space. (This default can be changed by using the FS built-in variable or the -Ffieldsep option.) The fields are denoted $1, $2, ... ; $0 refers to the entire line.
A pattern-action statement has the form:
pattern { action }
Either pattern or action may be omitted. If there is no action with a pattern, the matching line is printed. If there is no pattern with an action, the action is performed on every input line. Pattern-action statements are separated by newlines or semicolons.
As noted, patterns are arbitrary Boolean combinations ( ``!'', ``||'', ``&&'', and parentheses) of relational expressions and extended regular expressions. A relational expression is one of the following:
expression relop expression expression matchop regular_expression expression in array-name (expression,expression, ... ) in array-name
where a relop is any of the six relational operators in C, and a matchop is either ``~'' (contains) or ``!~'' (does not contain). An expression is an arithmetic expression, a relational expression, the special expression
var in array
or a Boolean combination of these.
In patterns extended regular expressions must be surrounded by slashes. Isolated regular expressions in a pattern apply to the entire line. Extended 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 pattern.
The special patterns BEGIN and END may be used to capture control before the first input line has been read and after the last input line has been read respectively. These keywords do not combine with any other patterns.
An extended regular expression may be used to separate fields by using the -F fieldsep option or by assigning the expression to the built-in variable FS. The default is to ignore leading blanks and to separate fields by blanks and/or tab characters. However, if FS is assigned a value, leading blanks are no longer ignored.
Other built-in variables include:
A numeric value that is exactly equal to the value of an integer will be converted to a string by the equivalent of a call to the sprintf built-in function with the string %d as the fmt parameter and the numeric value being converted as the first and only expr parameter. Any other numeric value will be converted to a string by the equivalent of a call to the sprintf built-in function with the value of the variable CONVFMT as the fmt parameter and the numeric value being converted as the first and only expr parameter.
The field separators specified with the -F option or with the variables OFS, ORS, and FS may be supplementary code set characters.
An action is a sequence of statements. A statement may be one of the following:
if ( expression ) statement [else statement] while ( expression ) statement do statement while ( expression ) for ( expression ; expression ; expression ) statement for ( var in array ) statement delete array[subscript] #delete an array element break continue { [statement] ... } expression # commonly variable = expression print [expression-list] [>expression] printf format [, expression-list] [>expression] next # skip remaining patterns on this input line exit [expr] # skip the rest of the input; exit status is expr return [expr]
Statements are terminated by semicolons, new-lines, or right braces. An empty expression-list stands for the whole input line. Expressions take on string or numeric values as appropriate, and are built using the operators ``+'', ``-'', ``'', ``/'', ``%'', ``^'' and concatenation (indicated by a blank). 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 or zero. 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. String constants are quoted (``""''), with the usual C escapes recognized within.
The print statement prints its arguments on the standard output, or on a file if >expression is present, or on a pipe if | cmd is present. The arguments are 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 (see fprintf(3S)). The built-in function close(expr) closes the file or pipe expr.
The mathematical functions: atan2, cos, exp, log, sin, sqrt, are built-in.
Other built-in functions include:
The input/output built-in functions are:
All forms of getline return 1 for successful input, 0 for end of file, and -1 for an error.
awk also provides user-defined functions. Such functions may be defined (in the pattern position of a pattern-action statement) as
function name(args,...) { stmts }or
func name(args,...) { stmts }Function arguments are passed by value if scalar and by reference if array name. Argument names are local to the function; all other variable names are global. Function calls may be nested and functions may be recursive. The return statement may be used to return a value.
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 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 }
Simulate echo(1):
BEGIN { for (i = 1; i < ARGC; i++) printf "%s", ARGV[i] printf "\n" exit }
Print a file, filling in page numbers starting at 5:
/Page/ { $2 = n++; } { print }
Assuming this program is in a file named
prog,
the following command line prints the file
input
numbering its pages starting at 5:
awk -f prog n=5 input.
A. V. Aho, B. W. Kernighan, P. J. Weinberger, The awk Programming Language Addison-Wesley, 1988
Input white space is not preserved on output if fields are involved.
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.