NAME
awk
- pattern-directed scanning and processing language
SYNOPSIS
awk
[-F fs]
[-v var=value]
[-safe]
[-d[N]]
[prog | -f filename]
awk
-V
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, and 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 filename.
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:
- -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.
- -V
-
Print
awk
version on standard output and exit.
An input line is normally made up of fields separated by white space,
or by regular expression
FS.
The fields are denoted
$1,
$2,
..., while
$0
refers to the entire line.
If
FS
is null, the input line is split into one field per character.
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:
- Xo
-
if ( expression ) statement
[else statement]
- while( expression ) statement
-
- Xo
-
for( expression ; expression ;
expression )
statement
- Xo
-
for( var in array
)
statement
- Xo
-
do statement
while( expression )
- break
-
- continue
-
- delete array [expression]
-
- delete array
-
- exit [expression]
-
expression
- return [expression]
-
- {[statement ...
- ]}
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.
- Xo
-
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.
- next
-
Skip remaining patterns on this input line.
- nextfile
-
Skip rest of this file, open next, start at top.
- Xo
-
print
[expr-list]
[> file]
The
print
statement prints its arguments on the standard output (or to a file
if
> file
or to a pipe if
| expr
is present),
separated by the current output field separator
OFS,
and terminated by the
output record separator
ORS.
Both
file
and
expr
may be literal names or parenthesized expressions; identical string values in
different statements denote the same open file.
- Xo
-
printf
format
[,expr-list]
[> file]
Format and print its expression list according to
format.
See
printf(3)
for list of supported formats and their meaning.
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
, t
, [s]
) -
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
, 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.
- 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.
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 not 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
- length($0) > 72
-
Print lines longer than 72 characters.
- {print $2, $1
- }
Print first two fields in opposite order.
- BEGIN{ FS = ",[ \t]*|[ \t]+"
- }
- { print $2, $1
- }
Same, with input fields separated by comma and/or blanks and tabs.
- { s += $1
- }
- Xo
-
END{ print "sum is", s, " average is ", s/NR }
Add up first column, print sum and average.
- /start/, /stop/
-
Print all lines between start/stop pairs.
- BEGIN{ # Simulate echo(1)
-
- for (i = 1; i < ARGC; i++) printf "%s ", ARGV[i]
-
- printf "\n"
-
- exit
- }
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
NetBSD2.0,
replacing the previously used GNU
.
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.