NAME

awk — pattern-directed scanning and processing language

SYNOPSIS

awk [-safe] [-V] [-d[n]] [-F fs | --csv] [-v var=value] [prog | -f progfile] file ...

DESCRIPTION

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 filename, and is executed at the time it would have been opened if it were a filename.

The options are as follows:

--csv: Process records using the (more or less) standard comma-separated values (CSV) format instead of the input field separator. When the --csv option is specified, attempts to change the input field separator or record separator are ignored.
-d[n]: Debug mode. Set debug level to n, or 1 if n is not specified. A value greater than 1 causes awk to dump core on fatal errors.
-F fs: Define the input field separator to be the regular expression fs.
-f progfile: Read program code from the specified file progfile instead of from the command line.
-safe: Disable file output (print >, print >>), process creation (cmd | getline, print |, system) and access to the environment (ENVIRON; see the section on variables below). This is a first (and not very reliable) approximation to a “safe” version of awk.
-V: Print the version number of awk to standard output and exit.
-v var=value: Assign value to variable var before prog is executed; any number of -v options may be present.

The input is normally made up of input lines (records) separated by newlines, or by the value of RS. If RS is null, then any number of blank lines are used as the record separator, and newlines are used as field separators (in addition to the value of FS). This is convenient when working with multi-line records.

An input line is normally made up of fields separated by whitespace, or by the value of the field separator FS at the time the line is read. The fields are denoted $1, $2, ..., while $0 refers to the entire line. FS may be set to either a single character or a regular expression. As a special case, if FS is a single space (the default), fields will be split by one or more whitespace characters. If FS is null, the input line is split into one field per character.

Normally, any number of blanks separate fields. In order to set the field separator to a single blank, use the -F option with a value of ‘[ ]’. If a field separator of ‘t’ is specified, awk treats it as if ‘\t’ had been specified and uses ⟨TAB⟩ as the field separator. In order to use a literal ‘t’ as the field separator, use the -F option with a value of ‘[t]’. The field separator is usually set via the -F option or from inside a BEGIN block so that it takes effect before the input is read.

A pattern-action statement has the form:

pattern { action }

A missing { action } means print the line; a missing pattern always matches. Pattern-action statements are separated by newlines or semicolons.

Newlines are permitted after a terminating statement or following a comma (‘,’), an open brace (‘{’), a logical AND (‘&&’), a logical OR (‘||’), after the ‘do’ or ‘else’ keywords, or after the closing parenthesis of an ‘if’, ‘for’, or ‘while’ statement. Additionally, a backslash (‘\’) can be used to escape a newline between tokens.

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

if (expression) statement [else statement]
while (expression) statement
for (expression; expression; expression) statement
for (var in array) statement
do statement while (expression)
break
continue
{ [statement ...] }
expression # commonly var = expression
print [expression-list] [>expression]
printf format [..., expression-list] [>expression]
return [expression]
next # skip remaining patterns on this input line
nextfile # skip rest of this file, open next, start at top
delete array[expression] # delete an array element
delete array # delete all elements of array
exit [expression] # exit processing, and perform END processing; status is expression

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 (see printf(1) for a complete list of these). Expressions take on string or numeric values as appropriate, and are built using the operators + - * / % ^ (exponentiation), and concatenation (indicated by whitespace). 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 (see the section on variables below).

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 printf(1)).

Patterns are arbitrary Boolean combinations (with ! || &&) of regular expressions and relational expressions. awk supports extended regular expressions (EREs). See re_format(7) for more information on regular expressions. 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 through an occurrence of the second.

A relational expression is one of the following:

expression matchop regular-expression
expression relop expression
expression in array-name
(expr, expr, ...) in array-name

where a relop is any of the six relational operators in C, and a matchop is either ~ (matches) or !~ (does not match). 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. The special pattern END may be used to capture control after processing is finished. BEGIN and END do not combine with other patterns. They may appear multiple times in a program and execute in the order they are read by awk.

Variable names with special meanings:

ARGC: Argument count, assignable.
ARGV: Argument array, assignable; non-null members are taken as filenames.
CONVFMT: Conversion format when converting numbers (default "%.6g").
ENVIRON: Array of environment variables; subscripts are names.
FILENAME: The name of the current input file.
FNR: Ordinal number of the current record in the current file.
FS: Regular expression used to separate fields (default whitespace); also settable by option -F fs.
NF: Number of fields in the current record. $NF can be used to obtain the value of the last field 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 newline).
RLENGTH: The length of the string matched by the match() function.
RS: Input record separator (default newline). If empty, blank lines separate records. If more than one character long, RS is treated as a regular expression, and records are separated by text matching the expression.
RSTART: The starting position of the string matched by the match() function.
SUBSEP: Separates multiple subscripts (default 034).

FUNCTIONS

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

Functions may be defined (at the position of a pattern-action statement) thusly:

function foo(a, b, c) { ...; return x
  }

Parameters are passed by value if scalar, and by reference if array name; functions may be called recursively. Parameters are local to the function; all other variables are global. Thus local variables may be created by providing excess parameters in the function definition.

Arithmetic Functions

atan2(y, x): Return the arctangent of y/x in radians.
cos(x): Return the cosine of x, where x is in radians.
exp(x): Return the exponential of x.
int(x): Return x truncated to an integer value.
log(x): Return the natural logarithm of x.
rand(): Return a random number, n, such that 0≤n<1. Random numbers are non-deterministic unless a seed is explicitly set with srand().
sin(x): Return the sine of x, where x is in radians.
sqrt(x): Return the square root of x.
srand(expr): Sets seed for rand() to expr and returns the previous seed. If expr is omitted, rand() will return non-deterministic random numbers.

String Functions

gensub(r, s, h, [t]): Search the target string t for matches of the regular expression r. If h is a string beginning with g or G, then replace all matches of r with s. Otherwise, h is a number indicating which match of r to replace. If no t is supplied, $0 is used instead. Unlike sub() and gsub(), the modified string is returned as the result of the function, and the original target is not changed. Note that \n sequences within the replacement string s, as supported by GNU awk, are not supported at this time.
gsub(r, t, s): The same as sub() except that all occurrences of the regular expression are replaced. gsub() returns the number of replacements.
index(s, t): The position in s where the string t occurs, or 0 if it does not.
length(s): The length of s taken as a string, number of elements in an array for an array argument, or length of $0 if no argument is given.
match(s, r): The position in s where the regular expression r occurs, or 0 if it does not. The variable RSTART is set to the starting position of the matched string (which is the same as the returned value) or zero if no match is found. The variable RLENGTH is set to the length of the matched string, or -1 if no match is found.
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. An empty string as field separator splits the string into one array element per character.
sprintf(fmt, expr, ...): The string resulting from formatting expr, ... according to the printf(1) format fmt.
sub(r, t, s): Substitutes t for the first occurrence of the regular expression r in the string s. If s is not given, $0 is used. An ampersand (‘&’) in t is replaced in string s with regular expression r. A literal ampersand can be specified by preceding it with two backslashes (‘\\’). A literal backslash can be specified by preceding it with another backslash (‘\\’). sub() returns the number of replacements.
substr(s, m, n): Return at most the n-character substring of s that begins at position m counted from 1. If n is omitted, or if n specifies more characters than are left in the string, the length of the substring is limited by the length of s.
tolower(str): Returns a copy of str with all upper-case characters translated to their corresponding lower-case equivalents.
toupper(str): Returns a copy of str with all lower-case characters translated to their corresponding upper-case equivalents.

Time Functions

This version of awk provides the following functions for obtaining and formatting time stamps.

mktime(datespec)

Converts datespec into a timestamp in the same form as a value returned by systime(). The datespec is a string composed of six or seven numbers separated by whitespace:

YYYY MM DD HH MM SS [DST]

The fields in datespec are as follows:

YYYY: Year: a four-digit year, including the century.
MM: Month: a number from 1 to 12.
DD: Day: a number from 1 to 31.
HH: Hour: a number from 0 to 23.
MM: Minute: a number from 0 to 59.
SS: Second: a number from 0 to 60 (permitting a leap second).
DST: Daylight Saving Time: a positive or zero value indicates that DST is or is not in effect. If DST is not specified, or is negative, mktime() will attempt to determine the correct value.

strftime([format [, timestamp]])

Formats timestamp according to the string format. The format string may contain any of the conversion specifications described in the strftime(3) manual page, as well as any arbitrary text. The timestamp must be in the same form as a value returned by mktime() and systime(). If timestamp is not specified, the current time is used. If format is not specified, a default format equivalent to the output of date(1) is used.

systime()

Returns the value of time in seconds since 0 hours, 0 minutes, 0 seconds, January 1, 1970, Coordinated Universal Time (UTC).

Input/Output and General Functions

close(expr): Closes the file or pipe expr. expr should match the string that was used to open the file or pipe.
cmd | getline [var]: Read a record of input from a stream piped from the output of cmd. If var is omitted, the variables $0 and NF are set. Otherwise var is set. If the stream is not open, it is opened. As long as the stream remains open, subsequent calls will read subsequent records from the stream. The stream remains open until explicitly closed with a call to close(). getline returns 1 for a successful input, 0 for end of file, and -1 for an error.
fflush([expr]): Flushes any buffered output for the file or pipe expr, or all open files or pipes if expr is omitted. expr should match the string that was used to open the file or pipe.
getline: Reads the next record from the current input file. This form of getline sets the variables $0, NF, NR, and FNR. getline returns 1 for a successful input, 0 for end of file, and -1 for an error.
getline var: Reads the next record from the current input file. This form of getline sets the variables var, NR and FNR. getline returns 1 for a successful input, 0 for end of file, and -1 for an error.
getline [var] < file: Reads the next record from file. If var is omitted, the variables $0 and NF are set. Otherwise var is set. If file is not open, it is opened. As long as the stream remains open, subsequent calls will read subsequent records from file. file remains open until explicitly closed with a call to close().
system(cmd): Executes cmd and returns its exit status. This will be -1 upon error, cmd's exit status upon a normal exit, 256 + sig if cmd was terminated by a signal, where sig is the number of the signal, or 512 + sig if there was a core dump.

Bit-Operation Functions

compl(x): Returns the bitwise complement of integer argument x.
and(x, y): Performs a bitwise AND on integer arguments x and y.
or(x, y): Performs a bitwise OR on integer arguments x and y.
xor(x, y): Performs a bitwise Exclusive-OR on integer arguments x and y.
lshift(x, n): Returns integer argument x shifted by n bits to the left.
rshift(x, n): Returns integer argument x shifted by n bits to the right.

ENVIRONMENT

The following environment variables affect the execution of awk:

LC_CTYPE: The character encoding locale(1). It decides which byte sequences form characters, which characters are letters, and how letters are mapped from lower to upper case and vice versa. If unset or set to "C", "POSIX", or an unsupported value, each byte is treated as a character, and non-ASCII bytes are not regarded as letters.
POSIXLY_CORRECT: When set, behave in accordance with the standard, even when it conflicts with historical behavior.

EXIT STATUS

The awk utility exits 0 on success, and >0 if an error occurs.

But note that the exit expression can modify the exit status.

EXAMPLES

Print lines longer than 72 characters:

length($0) > 72

Print first two fields in opposite order:

{ print $2, $1 }

Same, with input fields separated by comma and/or spaces 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 all lines between start/stop pairs:

/start/, /stop/

Simulate echo(1):

BEGIN { # Simulate echo(1)
        for (i = 1; i < ARGC; i++) printf "%s ", ARGV[i]
        printf "\n"
        exit }

Print an error message to standard error:

{ print "error!" > "/dev/stderr" }

UNUSUAL FLOATING-POINT VALUES

awk was designed before IEEE 754 arithmetic defined Not-A-Number (NaN) and Infinity values, which are supported by all modern floating-point hardware.

Because awk uses strtod(3) and atof(3) to convert string values to double-precision floating-point values, modern C libraries also convert strings starting with inf and nan into infinity and NaN values respectively. This led to strange results, with something like this:

echo nancy | awk '{ print $1 + 0 }'

printing nan instead of zero.

awk now follows GNU awk, and prefilters string values before attempting to convert them to numbers, as follows:

Hexadecimal values: Hexadecimal values (allowed since C99) convert to zero, as they did prior to C99.
NaN values: The two strings “+NAN” and “-NAN” (case independent) convert to NaN. No others do. (NaNs can have signs.)
Infinity values: The two strings “+INF” and “-INF” (case independent) convert to positive and negative infinity, respectively. No others do.

STANDARDS

The awk utility is compliant with the specification except that consecutive backslashes in the replacement string argument for sub() and gsub() are not collapsed and a slash (‘/’) does not need to be escaped in a bracket expression. Also, the behaviour of rand() and srand() has been changed to support non-deterministic random numbers.

The flags [-dV], [--csv], and [-safe], support for regular expressions in RS, as well as the functions fflush(), gensub(), compl(), and(), or(), xor(), lshift(), rshift(), mktime(), strftime() and systime() are extensions to that specification.

HISTORY

An awk utility appeared in Version 7 AT&T UNIX.

BUGS

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.