awk(1)

awk -- pattern scanning and processing language

Synopsis

   awk  [-Ffieldsep] [-v var=value] [-f progfile | 'prog'] [file . . .]

Description

awk scans each input file for lines that match any of a set of patterns specified in prog. The prog string must be enclosed in single quotes (') to protect it from the shell. Patterns are arbitrary Boolean combinations of extended regular expressions (see grep(1)) and relational expressions. For each pattern in prog there may be an associated action performed when a line of a file matches the pattern. The set of pattern-action statements may appear literally as prog or in a file specified with the -f progfile option. Input files are read in order; if there are no files, the standard input is read. The file name - means the standard input.

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:

ARGC: command line argument count
ARGV: command line argument array
CONVFMT: a conversion format for non-integer numeric values; see the description of the sprintf built-in function
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.
ENVIRON: array of environment variables; subscripts are names
FILENAME: name of the current input file
FNR: ordinal number of the current record in the current file
FS: input field separator regular expression (default blank and tab)
NF: number of fields in the current record
NR: ordinal number of the current record
OFMT: output format for numbers (default %.6g)
OFS: output field separator (default blank)
ORS: output record separator (default new-line)
RS: input record separator (default new-line)
SUBSEP: separates multiple subscripts (default is 034)

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:

gsub(for, repl, in): behaves like sub (see below), except that it replaces successive occurrences of the regular expression (like the ed global substitute command).
index(s, t): returns the position in string s where string t first occurs, or 0 if it does not occur at all.
int: truncates to an integer value.
length(s): returns the length in bytes of its argument taken as a string, or of the whole line if there is no argument.
match(s, re): returns the position in string s where the regular expression re occurs, or 0 if it does not occur at all. RSTART is set to the starting position (which is the same as the returned value), and RLENGTH is set to the length of the matched string.
rand: random number on (0, 1).
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.
srand: sets the seed for rand and returns the previous seed.
sprintf(fmt, expr, expr,...): formats the expressions according to the fprintf(3S) format given by fmt and returns the resulting string.
sub(for, repl, in): substitutes the string repl in place of the first instance of the regular expression for in string in and returns the number of substitutions. If in is omitted, awk substitutes in the current record ($0).
substr(s, m, n): returns the n-character substring of s that begins at position m.
tolower(s): returns a string in which each upper case character in string s is replaced by a lower case character.
toupper(s): returns a string in which each lower case character in string s is replaced by an upper case character.

The input/output built-in functions are:

close(filename): closes the file or pipe named filename and returns the status.
cmd | getline: pipes the output of cmd into getline; each successive call to getline returns the next line of output from cmd.
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.
getline x <file: sets x from the next record of file.
system(cmd): executes cmd and returns its exit status.

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 }

   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.

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 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.

Files

/usr/lib/locale/locale/LC_MESSAGES/uxawk: language-specific message file (See LANG on environ(5).)

References

fprintf(3S), grep(1), lex(1), sed(1)

A. V. Aho, B. W. Kernighan, P. J. Weinberger, The awk Programming Language Addison-Wesley, 1988

Notices

nawk is equivalent to awk.

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.