NAME

awk − pattern scanning and processing language

SYNOPSIS

awk [−F re] [parameter...] [’prog’] [−f progfile] [file...]

DESCRIPTION

awk is a new version of oawk that provides capabilities unavailable in previous versions.  This version will become the default version of awk in the next major UNIX system release. 

The −F re option defines the input field separator to be the regular expression re. 

Parameters, in the form x=... y=... may be passed to awk, where x and y are awk built-in variables (see list below). 

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.  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.  Each input line is matched against the pattern portion of every pattern-action statement; the associated action is performed for each matched pattern. 

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 −F re 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. 

Patterns are arbitrary Boolean combinations ( !, ││, &&, and parentheses) of relational expressions and regular expressions.  A relational expression is one of the following:

expression relop expression
expression matchop regular 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, the special expression

var in array,

or a Boolean combination of these. 

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. 

Regular expressions are as in egrep [see grep(1)]. In patterns they must be surrounded by slashes. 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 pattern.

A regular expression may be used to separate fields by using the −F re 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

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)

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)

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

if ( conditional ) statement [ else statement ]
while ( conditional ) statement
do statement while ( conditional )
for ( expression ; conditional ; expression ) statement
for ( var in array ) statement
delete array[subscript]
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 both.  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 C operators ++, −−, +=, −=, ∗=, /=, and %= 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.  String constants are quoted ("). 

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 printf(3S) in the Programmer’s Reference Manual]. 

awk has a variety of built-in functions: arithmetic, string, input/output, and general. 

The arithmetic functions are: atan2, cos, exp, int, log, rand, sin, sqrt, and srand.  int truncates its argument to an integer.  rand returns a random number between 0 and 1.  srand ( expr ) sets the seed value for rand to expr or uses the time of day if expr is omitted. 

The string functions are:

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. 

length(s) returns the length 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. 

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. 

sprintf(fmt, expr, expr, ...)
formats the expressions according to the printf(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.

The input/output and general functions are:

close(filename)
closes the file or pipe named filename.

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 var sets variable var instead. 

getline var <file
sets var 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 file, filling in page numbers starting at 5:

/Page/ { $2 = n++; }
       { print }

     command line:  awk −f program n=5 input

SEE ALSO

grep(1), lex(1), oawk(1), printf(3S), sed(1)

BUGS

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. 

September 29, 2021