awk - pattern-directed scanning and processing language
awk [
-Ffs ] [
-v var=value ] [
-mrn ] [
-mfn ] [
-f prog [
prog ] [
file ... ]
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 file. 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 file name, and is executed at the time it
would have been opened if it were a file name. 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.
-F fs
option defines the input field separator to be the regular expression
fs.
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.
To compensate for inadequate implementation of storage management, the
-mr option can be used to set the maximum size of the input record, and
the
-mf option to set the maximum number of fields.
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. 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 immediately; 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. Expressions
take on string or numeric values as appropriate, and are built using the
operators
+ - * / % ^ (exponentiation), and concatenation (indicated by
white space). 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.
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
fprintf(2))
. The built-in function
close(expr) closes the file or pipe
expr. The
built-in function
fflush(expr) flushes any buffered
output for the file or pipe
expr.
The mathematical functions
exp,
log,
sqrt,
sin,
cos, and
atan2 are built in. Other built-in functions:
- length
- the length of its argument taken as a string, or of
$0 if no argument.
- rand
- random number on (0,1)
- srand
- sets seed for rand and returns the previous
seed.
- int
- truncates to an integer value
- utf
- converts its numerical argument, a character number, to a
UTF string
-
substr(s, m,
n)
- the n-character substring of s that begins at
position m counted from 1.
-
index(s, t)
- the position in s where the string t occurs,
or 0 if it does not.
-
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.
-
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.
- gsub
- same as sub except that all occurrences of the
regular expression are replaced; sub and gsub return the
number of replacements.
-
sprintf(fmt, expr,
...)
- the string resulting from formatting expr ...
according to the printf format fmt
-
system(cmd)
- executes cmd and returns its exit status
-
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.
The ``function''
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. Finally,
cmd | getline pipes the output of
cmd into
getline; each call of
getline returns the next
line of output from
cmd. In all cases,
getline returns 1 for a
successful input, 0 for end of file, and -1 for an error.
Patterns are arbitrary Boolean combinations (with
! || &&) of
regular expressions and relational expressions. Regular expressions are as in
regexp(6). 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.
Variable names with special meanings:
- CONVFMT
- conversion format used when converting numbers (default
%.6g)
- FS
- regular expression used to separate fields; also settable
by option -Ffs.
- NF
- number of fields in the current record
- NR
- ordinal number of the current record
- FNR
- ordinal number of the current record in the current
file
- FILENAME
- the name of the current input file
- RS
- input record separator (default newline)
- OFS
- output field separator (default blank)
- ORS
- output record separator (default newline)
- OFMT
- output format for numbers (default %.6g)
- SUBSEP
- separates multiple subscripts (default 034)
- ARGC
- argument count, assignable
- ARGV
- argument array, assignable; non-null members are taken as
file names
- ENVIRON
- array of environment variables; subscripts are names.
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.
- 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 }
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 }
/sys/src/cmd/awk
sed(1),
regexp(6),
A. V. Aho, B. W. Kernighan, P. J. Weinberger,
The AWK Programming
Language, Addison-Wesley, 1988. ISBN 0-201-07981-X
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.