awk(1) CLIX awk(1)
NAME
awk - Executes a pattern scanning and processing language
SYNOPSIS
awk [-Fc] [prog] [parameters] [files]
DESCRIPTION
The awk command executes a pattern scanning and processing language. The
awk command scans each input file for lines that match any of a set of
patterns specified in prog. With each pattern in prog there can be an
associated action that will be performed when a line of a file matches the
pattern. The set of patterns may appear literally as prog, or in a file
specified as -ffile. The prog string should be enclosed in single quotes
(') to protect it from the shell.
Parameters, in the form x=..., y=..., and so on, may be passed to awk.
Files are read in order; if there are no files, stdin is read. The
filename - means stdin. Each line is matched against the pattern portion
of every pattern-action statement; the associated action is performed for
each matched pattern.
An input line is made up of fields separated by white space. (This
default can be changed by using FS; see below). The fields are denoted
$1, $2, ...; $0 refers to the entire line.
A pattern-action statement has the form:
pattern { action }
A missing action means display the line; a missing pattern always matches.
An action is a sequence of statements. A statement can be one of the
following:
⊕ if (conditional) statement [else statement]
⊕ while (conditional) statement
⊕ for (expression ; conditional ; expression) statement
⊕ break
⊕ continue
⊕ { [statement] ... }
⊕ variable = expression
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awk(1) CLIX awk(1)
⊕ print [expression-list] [>expression]
⊕ printf format [, expression-list] [>expression]
⊕ next # skip remaining patterns on this input line
⊕ exit # skip the rest of the input
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 (").
The print statement displays its arguments on stdout (or on a file if
>expr is present), separated by the current output field separator, and
terminated by the output record separator. The printf statement formats
its expression list according to the format (see printf in the
Programmer's Reference Manual).
The built-in function 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 exp, log, sqrt, and int. The last truncates its argument to an
integer; substr(s,m,n) returns the n-character substring of s that begins
at position m. The function sprintf(fmt,expr,expr, ... ) formats the
expressions according to the printf format given by fmt and returns the
resulting string.
Patterns are arbitrary Boolean combinations (!, ||, &&, and parentheses)
of regular expressions and relational expressions. Regular expressions
must be surrounded by slashes and are as in egrep (see grep). Isolated
regular expressions in a pattern apply to the entire line. Regular
expressions may also occur in relational expressions. 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.
A relational expression is one of the following:
⊕ expression matchop regular-expression
⊕ expression relop expression
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.
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awk(1) CLIX awk(1)
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.
A single character c may be used to separate the fields by starting the
program with:
BEGIN { FS = c }
or by using the -Fc flag.
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).
EXAMPLES
1. To display lines longer than 72 characters:
length > 72
2. To display the first two fields in opposite order:
{ print $2, $1 }
3. To add up first column, display sum and average:
{ s += $1 }
END { print "sum is", s, " average is", s/NR }
4. To display fields in reverse order:
{ for (i = NF; i > 0; --i) print $i }
5. To display all lines between start/stop pairs:
/start/, /stop/
6. To display all lines whose first field is different from previous one:
$1 != prev { print; prev = $1 }
7. To display a file, filling in page numbers starting at 5:
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awk(1) CLIX awk(1)
awk n=5 file '/Page/ { $2 = n++; }
{ print }'
NOTES
Input white space is not preserved on output if fields are involved.
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 the null string (" ") to it.
DIAGNOSTICS
The following messages may be displayed if an error occurs:
awk: can't open file file
One of the files listed does not have read permission or does not
exist.
awk: syntax error near line number
A line which contained an unrecognizable awk command was found.
EXIT VALUES
The command returns a value of 0 if successful. If unsuccessful, the
command returns a value of 1.
RELATED INFORMATION
Commands: grep(1), sed(1), lex(1)
Functions: printf(3)
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