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AWK(1) General Commands Manual AWK(1)
NAME
awk - pattern-directed scanning and processing language
SYNOPSIS
awk [-F fs] [-v var=value] [-safe] [-d[N]] [prog | -f progfile] file ...
awk -version
DESCRIPTION
awk is the Bell Labs' implementation of the AWK programming language as
described in the The AWK Programming Language by A.V. Aho,
B.W. Kernighan, P.J. Weinberger.
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 option -v followed by
var=value is an assignment to be done before prog is executed; any number
of -v options may be present. The -F fs option defines the input field
separator to be the regular expression fs.
The options are as follows:
-d[N] Set debug level to specified number N. If the number is
omitted, debug level is set to 1.
-f filename
Read the AWK program source from specified file filename,
instead of the first command line argument. Multiple -f
options may be specified.
-F fs Set the input field separator FS to the regular expression
fs.
-mr NNN, -mf NNN
Obsolete, no longer needed options. Set limit on maximum
record or fields number.
-safe Potentially unsafe functions such as system() make the
program abort (with a warning message).
-v var=value
Assign the value value to the variable var before prog is
executed. Any number of -v options may be present.
-version Print awk version on standard output and exit.
An input line is normally made up of fields separated by white space, or
by the regular expression the built-in variable FS is set to. If FS is
null, the input line is split into one field per character. The fields
are denoted $1, $2, ..., while $0 refers to the entire line. Setting any
other field causes the re-evaluation of $0 Assigning to $0 resets the
values of all other fields and the NF built-in variable.
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.
An action is a sequence of statements. 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. Expressions take on string or numeric values as
appropriate, and are built using the Operators (see next subsection).
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.
Operators
awk operators, in order of decreasing precedence, are:
(...) Grouping
$ Field reference
++ -- Increment and decrement, can be used either as postfix or
prefix.
^ Exponentiation (the ** form is also supported, and **= for
the assignment operator).
+ - ! Unary plus, unary minus and logical negation.
* / % Multiplication, division and modulus.
+ - Addition and subtraction.
space String concatenation.
< >
<= >=
!= == Regular relational operators
~ !~ Regular expression match and not match
in Array membership
&& Logical AND
|| Logical OR
?: C conditional expression. This is used as expr1 ? expr2 :
expr3. If expr1 is true, the result value is expr2,
otherwise it is expr3. Only one of expr2 and expr3 is
evaluated.
= += -=
*= /= %= ^=
Assignment and Operator-Assignment
Control Statements
The control statements are as follows:
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
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 an array
exit [expression]
Exit immediately; status is expression
I/O Statements
The input/output statements are as follows:
close(expr) Closes the file or pipe expr. Returns zero on success;
otherwise nonzero.
fflush(expr) Flushes any buffered output for the file or pipe expr.
Returns zero on success; otherwise nonzero.
getline [var] Set var (or $0 if var is not specified) to the next
input record from the current input file. getline
returns 1 for a successful input, 0 for end of file,
and -1 for an error.
getline [var] < file
Set var (or $0 if var is not specified) to the next
input record from the specified file file.
expr | getline Pipes the output of expr into getline; each call of
getline returns the next line of output from expr.
print [expr-list] [redirection]
Print arguments separated by the current output field
separator OFS, and terminated by the output record
separator ORS.
printf format[, expr-list] [redirection]
Format and print its expression list according to
format. See printf(3) for list of supported formats
and their meaning.
Both print and printf statements write to standard output by default.
The output is written to the file or pipe specified by redirection if one
is supplied, as follows: > file, >> file, or | expr. Both file and expr
may be literal names or parenthesized expressions; identical string
values in different statements denote the same open file. For that
purpose the file names /dev/stdin, /dev/stdout, and /dev/stderr refer to
the program's stdin, stdout, and stderr respectively (and are unrelated
to the fd(4) devices of the same names).
Mathematical and Numeric Functions
AWK has the following mathematical and numerical functions built-in:
atan2(x, y) Returns the arctangent of x/y in radians. See also
atan2(3).
cos(expr) Computes the cosine of expr, measured in radians. See
also cos(3).
exp(expr) Computes the exponential value of the given argument
expr. See also exp(3).
int(expr) Truncates expr to integer.
log(expr) Computes the value of the natural logarithm of argument
expr. See also log(3).
rand() Returns random number between 0 and 1.
sin(expr) Computes the sine of expr, measured in radians. See
also sin(3).
sqrt(expr) Computes the non-negative square root of expr. See
also sqrt(3).
srand([expr]) Sets seed for random number generator (rand()) and
returns the previous seed.
String Functions
AWK has the following string functions built-in:
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 the `\n' sequences within
replacement string s supported by GNU awk are not
supported at this moment.
gsub(r, s [t]) Same as sub() except that all occurrences of the
regular expression are replaced; sub() and gsub()
return the number of replacements.
index(s, t) The position in s where the string t occurs, or 0 if it
does not.
length[([string])]
The length of its argument taken as a string, or of $0
if no argument.
match(s, r) The position in s where the regular expression r
occurs, or 0 if it does not. The variables RSTART and
RLENGTH are set to the position and 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. An empty string as field
separator splits the string into one array element per
character.
sprintf(fmt, expr, ...)
Returns the string resulting from formatting expr
according to the printf(3) format fmt.
sub(r, s [t]) Substitutes s for the first occurrence of the regular
expression r in the target string t. If t is not
given, $0 is used.
substr(s, m [n]) Returns the at most n-character substring of s starting
at position m, counted from 1. If n is omitted, the
rest of s is returned.
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 awk provides the following two functions for obtaining time stamps
and formatting them:
systime() Returns the value of time in seconds since the start of
Unix Epoch (midnight, January 1, 1970, Coordinated
Universal Time). See also time(3).
strftime([format [timestamp]])
Formats the time timestamp according to the string
format. timestamp should be in same form as value
returned by systime(). If timestamp is missing,
current time is used. If format is missing, a default
format equivalent to the output of date(1) would be
used. See the specification of ANSI C strftime(3) for
the format conversions which are supported.
Other built-in functions
system(cmd) Executes cmd and returns its exit status.
Patterns
Patterns are arbitrary Boolean combinations (with ! || &&) of regular
expressions and relational expressions. Regular expressions are as in
egrep(1). 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 though 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 patterns BEGIN and END may be used to capture control before
the first input line is read and after the last. BEGIN and END do not
combine with other patterns.
If an awk program consists of only actions with the pattern BEGIN, and
the BEGIN action contains no getline statement, awk exits without reading
its input when the last statement in the last BEGIN action is executed.
If an awk program consists of only actions with the pattern END or only
actions with the patterns BEGIN and END, the input is read before the
statements in the END actions are executed.
Built-in Variables
Variable names with special meanings:
ARGC argument count, assignable
ARGV argument array, assignable; non-null members are taken as
filenames
CONVFMT conversion format used 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; also settable
by option -F fs.
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 newline)
RS input record separator (default newline)
RSTART position of the first character matched by match(); 0 if no
match.
RLENGTH length of the string matched by match(); -1 if no match.
SUBSEP separates multiple subscripts (default 034)
Functions
Functions may be defined (at the position of a pattern-action statement)
thus:
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.
EXAMPLES
Print lines longer than 72 characters. length() defaults to $0 and the
empty parens can also be omitted in this case:
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 all lines between start/stop pairs:
/start/, /stop/
Simulate echo(1):
BEGIN {
for (i = 1; i < ARGC; ++i)
printf("%s%s", ARGV[i], i==ARGC-1?"\n":" ")
}
Another way to do the same that demonstrates field assignment and $0 re-
evaluation:
BEGIN { for (i = 1; i < ARGC; ++i) $i = ARGV[i]; print }
Print an error message to standard error:
{ print "error!" > "/dev/stderr" }
SEE ALSO
egrep(1), lex(1), sed(1), atan2(3), cos(3), exp(3), log(3), sin(3),
sqrt(3), strftime(3), time(3)
A.V. Aho, B.W. Kernighan, P.J. Weinberger, The AWK Programming Language,
Addison-Wesley, 1988. ISBN 0-201-07981-X
AWK Language Programming, Edition 1.0, published by the Free Software
Foundation, 1995
HISTORY
nawk has been the default system awk since NetBSD 2.0, replacing the
previously used GNU awk.
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.
Only eight-bit characters sets are handled correctly.
NetBSD 10.99 July 5, 2022 NetBSD 10.99