awk.1

unix v7是最后一个广泛发布的研究型UNIX版本

.TH AWK 1 
.SH NAME
awk \- pattern scanning and processing language
.SH SYNOPSIS
.B awk
[
.BI \-F c
]
[ prog ] [ file ] ...
.SH DESCRIPTION
.I Awk
scans each input
.I file
for lines that match any of a set of patterns specified in
.IR prog .
With each pattern in
.I prog
there can be an associated action that will be performed
when a line of a
.I file
matches the pattern.
The set of patterns may appear literally as
.I prog,
or in a file
specified as
.B \-f
.IR file .
.PP
Files are read in order;
if there are no files, the standard input is read.
The file name `\-'
means the standard input.
Each line is matched against the
pattern portion of every pattern-action statement;
the associated action is performed for each matched pattern.
.PP
An input line is made up of fields separated by white space.
(This default can be changed by using FS,
.IR "vide infra" ".)"
The fields are denoted $1, $2, ... ;
$0 refers to the entire line.
.PP
.PP
A pattern-action statement has the form
.PP
	pattern { action }
.PP
A missing { action } means print the line;
a missing pattern always matches.
.PP
An action is a sequence of statements.
A statement can be one of the following:
.PP
.nf
	if ( conditional ) statement [ else statement ]
	while ( conditional ) statement
	for ( expression ; conditional ; expression ) statement
	break
	continue
	{ [ statement ] ... }
	variable = expression
	print [ expression-list ] [ >expression ]
	printf format [ , expression-list ] [ >expression ]
	next	# skip remaining patterns on this input line 
	exit	# skip the rest of the input
.fi
.PP
Statements are terminated by
semicolons, newlines or right braces.
An empty expression-list stands for the whole line.
Expressions take on string or numeric values as appropriate,
and are built using the operators
+, \-, *, /, %,  and concatenation (indicated by a blank).
The C operators ++, \-\-, +=, \-=, *=, /=, and %=
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.
String constants are quoted "...".
.PP
The 
.I print
statement prints its arguments on the standard output
(or on a file if 
.I >file
is present), separated by the current output field separator,
and terminated by the output record separator.
The
.I printf
statement formats its expression list according to the format
(see
.IR printf (3)).
.PP
The built-in function
.I length
returns the length of its argument
taken as a string,
or of the whole line if no argument.
There are also built-in functions
.I exp,
.I log,
.I sqrt,
and
.IR int .
The last truncates its argument to an integer.
.IR substr(s,\ m,\ n)
returns the 
.IR n -character
substring of
.I s
that begins at position
.IR m .
The function
.IR sprintf(fmt,\ expr,\ expr,\ ...)
formats the expressions
according to the
.IR printf (3)
format given by
.I fmt
and returns the resulting string.
.PP
Patterns are arbitrary Boolean combinations
(!, \(or\(or, &&, and parentheses) of 
regular expressions and
relational expressions.
Regular expressions must be surrounded
by slashes and are as in
.IR egrep .
Isolated regular expressions
in a pattern apply to the entire line.
Regular expressions may also occur in
relational expressions.
.PP
A pattern may consist of two patterns separated by a comma;
in this case, the action is performed for all lines
between an occurrence of the first pattern
and the next occurrence of the second.
.PP
.nf
A relational expression is one of the following:
.PP
.nf
	expression matchop regular-expression
	expression relop expression
.PP
.fi
where a relop is any of the six relational operators in C,
and a matchop is either ~ (for contains)
or !~ (for does not contain).
A conditional is an arithmetic expression,
a relational expression,
or a Boolean combination
of these.
.PP
The special patterns
BEGIN
and
END
may be used to capture control before the first input line is read
and after the last.
BEGIN must be the first pattern, END the last.
.PP
A single character
.I c
may be used to separate the fields by starting
the program with
.PP
	BEGIN { FS = "c" }
.PP
or by using the
.BI \-F c
option.
.PP
Other variable names with special meanings
include NF, the number of fields in the current record;
NR, the ordinal number of the current record;
FILENAME, the name of the current input file;
OFS, the output field separator (default blank);
ORS, the output record separator (default newline);
and
OFMT, the output format for numbers (default "%.6g").
.PP
.SH EXAMPLES
.PP
Print lines longer than 72 characters:
.PP
.nf
	length > 72
.fi
.PP
Print first two fields in opposite order:
.PP
.nf
	{ print $2, $1 }
.fi
.PP
Add up first column, print sum and average:
.PP
.nf
		{ s += $1 }
	END	{ print "sum is", s, " average is", s/NR }
.fi
.PP
Print fields in reverse order:
.PP
.nf
	{ for (i = NF; i > 0; \-\-i) print $i }
.fi
.PP
Print all lines between start/stop pairs:
.PP
.nf
	/start/, /stop/
.fi
.PP
Print all lines whose first field is different from previous one:
.PP
.nf
	$1 != prev { print; prev = $1 }
.fi
.SH SEE ALSO
.PP
lex(1), sed(1)
.br
A. V. Aho, B. W. Kernighan, P. J. Weinberger,
.I
Awk \- a pattern scanning and processing language
.SH 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.