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NAME
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awk – pattern-directed scanning and processing language
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SYNOPSIS
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awk [ −F fs ] [ −d ] [ −mf n ] [ −mr n ] [ −safe ] [ −v var=value
] [ −f progfile | prog ] [ file ... ]
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DESCRIPTION
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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 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 the program 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.
The −safe option causes awk to run in “safe mode,” in which it
is not allowed to run shell commands or open files and the environment
is not made available in the ENVIRON variable.
A pattern-action statement has the form
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(3)). 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. If expr is omitted or is a null string, all open files are
flushed.
The mathematical functions exp, log, sqrt, sin, cos, and atan2
are built in. Other built-in functions:
length If its argument is a string, the string’s length is returned.
If its argument is an array, the number of subscripts in the array
is returned. If no argument, the length of $0 is returned.
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)
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the n-character substring of s that begins at position m counted
from 1.
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index(s, t)
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the position in s where the string t occurs, or 0 if it does not.
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match(s, r)
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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.
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split(s, a, fs)
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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.
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sub(r, t, s)
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substitutes t for the first occurrence of the regular expression
r in the string s. If s is not given, $0 is used.
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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, ...)
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the string resulting from formatting expr ... according to the
printf format fmt
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system(cmd)
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executes cmd and returns its exit status
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tolower(str)
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returns a copy of str with all upper-case characters translated
to their corresponding lower-case equivalents.
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toupper(str)
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returns a copy of str with all lower-case characters translated
to their corresponding upper-case equivalents.
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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(7). 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:
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expression matchop regular-expression
expression relop expression
expression in array-name
(expr,expr,...) in array-name
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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:
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function foo(a, b, c) { ...; return x }
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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.
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EXAMPLES
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length($0) > 72
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Print lines longer than 72 characters.
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{ print $2, $1 }
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Print first two fields in opposite order.
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BEGIN { FS = ",[ \t]*|[ \t]+" }
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Same, with input fields separated by comma and/or blanks and tabs.
{ s += $1 }
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END { print "sum is", s, " average is", s/NR }
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Add up first column, print sum and average.
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/start/, /stop/
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Print all lines between start/stop pairs.
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BEGIN { # Simulate echo(1)
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for (i = 1; i < ARGC; i++) printf "%s ", ARGV[i]
printf "\n"
exit }
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SOURCE
SEE ALSO
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sed(1), regexp(7),
A. V. Aho, B. W. Kernighan, P. J. Weinberger, The AWK Programming
Language, Addison-Wesley, 1988. ISBN 0-201-07981-X
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BUGS
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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.
UTF is not always dealt with correctly, though awk does make an
attempt to do so. The split function with an empty string as final
argument now copes with UTF in the string being split.
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