awk.html

unix 下的C开发手册,还用详细的例程。

input file.
The name "-" indicates the standard input.
If an argument matches the format of an
<i>assignment</i>
operand,
this argument
will be treated as an assignment rather than a
<i>file</i>
argument.

<dt><b>CONVFMT</b><dd>The
<b>printf</b>
format for converting numbers to strings (except for output statements,
where
<b>OFMT</b>
is used);
%.6g
by default.

<dt><b>ENVIRON</b><dd>The variable
<b>ENVIRON</b>
is an array representing the value of the environment,
as described in the <b>XSH</b> specification
under the
<i>exec</i>
functions.
The indices of the array are strings
consisting of the names of the environment variables,
and the value of each array element is
a string consisting of the value of that variable.
If the value of an environment variable is considered a
<i>numeric string</i>
(see
<xref href=awkexpr><a href="#tag_000_000_108_002">
Expressions in awk
</a></xref>),
the array element will also have its numeric value.

In all cases where the behaviour of
<i>awk</i>
is affected by environment variables
(including the environment of any commands that
<i>awk</i>
executes via the
<b>system</b>
function or via pipeline redirections with the
<b>print</b>
statement, the
<b>printf</b>
statement, or the
<b>getline</b>
function),
the environment used will be the environment at the time
<i>awk</i>
began executing;
it is implementation-dependent whether any modification of
<b>ENVIRON</b>
affects this environment.

<dt><b>FILENAME</b><dd>A pathname of the current input file.
Inside a
<b>BEGIN</b>
action the value is undefined.
Inside an
<b>END</b>
action the value is the name of the last
input file processed.

<dt><b>FNR</b><dd>The ordinal number of the current record
in the current file.
Inside a
<b>BEGIN</b>
action the value is zero.
Inside an
<b>END</b>
action the value is the number of the last record
processed in the last file processed.

<dt><b>FS</b><dd><index term="regular expressions"></index>
Input field separator regular expression;
a space character
by default.

<dt><b>NF</b><dd>The number of fields in the current record.
Inside a
<b>BEGIN</b>
action, the use of
<b>NF</b>
is undefined unless a
<b>getline</b>
function without a
<i>var</i>
argument is executed previously.
Inside an
<b>END</b>
action,
<b>NF</b>
will retain the value it had for the last record read, unless
a subsequent, redirected,
<b>getline</b>
function without a
<i>var</i>
argument is performed prior to entering the
<b>END</b>
action.

<dt><b>NR</b><dd>The ordinal number of the current record
from the start of input.
Inside a
<b>BEGIN</b>
action the value is zero.
Inside an
<b>END</b>
action
the value is the number of the last record processed.

<dt><b>OFMT</b><dd>The
<b>printf</b>
format for converting numbers to strings in
output statements (see
<xref href=awkout><a href="#tag_000_000_108_010">
Output Statements
</a></xref>);
%.6g
by default.
The result of the conversion is unspecified if the value of
<b>OFMT</b>
is not a floating-point format specification.

<dt><b>OFS</b><dd>The
<b>print</b>
statement output field separation;
a
space character
by default.

<dt><b>ORS</b><dd>The
<b>print</b>
statement output record separator;
a
newline character
by default.

<dt><b>RLENGTH</b><dd>The length of the string matched by the
<b>match</b>
function.

<dt><b>RS</b><dd>The first character of the string value of
<b>RS</b>
is the input record separator;
a
newline character
by default.
If
<b>RS</b>
contains more than one character,
the results are unspecified.
If
<b>RS</b>
is null, then records are separated by
sequences of one or more blank lines,
leading or trailing blank lines do not result in empty records at
the beginning or end of the input,
and a
newline character
is always a field separator, no matter what the value of
<b>FS</b>
is.

<dt><b>RSTART</b><dd>The starting position of the string matched by the <b>match</b>
function, numbering from 1.
This is always equivalent
to the return value of the <b>match</b> function.

<dt><b>SUBSEP</b><dd>The subscript separator string for multi-dimensional arrays;
the default value is implementation-dependent.

</dl>
<h5><a name = "tag_000_000_108_004">&nbsp;</a>Regular Expressions</h5>
<xref type="5" name="awkre"></xref>
The
<i>awk</i>
utility makes use of the extended regular expression notation (see
the <b>XBD</b> specification, <a href="../xbd/re.html#tag_007_004"><b>Extended Regular Expressions</b>&nbsp;</a> ) except that it will allow the use of
C-language conventions for escaping special characters within the EREs,
as specified in the table in the <b>XBD</b> specification, <a href="../xbd/notation.html"><b>File Format Notation</b>&nbsp;</a> 
(\\,
\a,
\b,
\f,
\n,
\r,
\t,
\v)
and the following table;
these escape sequences will be recognised both inside and outside
bracket expressions.
Note that records need not be separated by newline characters
and string constants can contain newline characters, so even the
\n
sequence is valid in
<i>awk</i>
EREs.
Using a slash character within the regular expression
requires the escaping shown in the following table:
<pre>
<table  bordercolor=#000000 border=1 align=center>
<tr valign=top><th align=center><b>Escape Sequence</b>
<th align=center><b>Description</b>
<th align=center><b>Meaning</b>
<tr valign=top><td align=center><b>\"</b>
<td align=left> Backslash quotation-mark 
<td align=left> Quotation-mark character 
<tr valign=top><td align=center><b>\/</b>
<td align=left> Backslash slash 
<td align=left> Slash character 
<tr valign=top><td align=center><b>\<i>ddd</i></b>
<td align=left> A backslash character followed by the longest sequence of one, two or three octal-digit characters (01234567). If all of the digits are 0, (that is, representation of the NUL character), the behaviour is undefined. 
<td align=left> The character whose encoding is represented by the one-, two- or three-digit octal integer. If the size of a byte on the system is greater than nine bits, the valid escape sequence used to represent a byte is implementation-dependent. Multi-byte characters require multiple, concatenated escape sequences of this type, including the leading \ for each byte. 
<tr valign=top><td align=center><b>\<i>c</i></b>
<td align=left> A backslash character followed by any character not described in this table or in the table in the <b>XBD</b> specification, <a href="../xbd/notation.html"><b>File Format Notation</b>&nbsp;</a>  (<code>\\,\a,\b,\f,\n,\r,\t,\v</code>)
<td align=left>Undefined
</table>
</pre>
<h6 align=center><xref table="Escape Sequences in <I>awk</i>"></xref>Table: Escape Sequences in <i>awk</i></h6>
<xref type="7" name="awkesc"></xref>
<p>
A regular expression can be matched against
a specific field or string by using one of the
two regular expression matching operators,
~
and
!~.
These operators interpret
their right-hand operand as a regular expression
and their left-hand operand as a string.
If the regular expression matches the string, the
~
expression
will evaluate to a value of
1,
and the
!~
expression will evaluate to a value of
0.
(The regular expression matching operation
is as defined by the term
matched in the <b>XBD</b> specification, <a href="../xbd/re.html#tag_007_001"><b>Regular Expression Definitions</b>&nbsp;</a> ,
where a match occurs on any part of the string
unless the regular expression is limited with the
circumflex or dollar sign special characters.)
If the regular expression does not match the string, the
~
expression will evaluate to a value of
0,
and the
!~
expression will evaluate to a value of
1.
If the right-hand operand is any expression other than the lexical token
<b>ERE</b>,
the string value of the expression will be interpreted as an extended
regular expression, including the escape conventions described above.
Note that these same escape conventions also will be
applied in the determining the
value of a string literal (the lexical token
<b>STRING</b>),
and thus will be applied a second time when a string literal is used in this
context.
<p>
When an
<b>ERE</b>
token appears as an expression in any context other than as the right-hand
of the
~
or
!~
operator or as one of the built-in
function arguments described below, the value of the resulting
expression will be the equivalent of:
<pre>
<code>
$0  ~  /<i>ere</i>/
</code>
</pre>
<p>
The
<i>ere</i>
argument to the
<b>gsub</b>,
<b>match</b>,
<b>sub</b>
functions, and the
<i>fs</i>
argument to the
<b>split</b>
function (see
<xref href=awkstr><a href="#tag_000_000_108_013">
String Functions
</a></xref>)
will be interpreted as extended regular expressions.
These can be either
<b>ERE</b>
tokens or arbitrary expressions,
and will be interpreted in the same manner as the right-hand side of the
~
or
!~
operator.
<p>
An extended regular expression
can be used to separate fields by
using the
<b>-F</b>&nbsp;<i>ERE</i>
option or by assigning
a string containing the expression to
the built-in variable
<b>FS</b>.
The default
value of the
<b>FS</b>
variable will be a single
space
character.
The following describes
<b>FS</b>
behaviour:
<ol>
<p>
<li>
If
<b>FS</b>
is a single character:
<ol type = a>
<p>
<li>
If
<b>FS</b>
is the
space character,
skip leading and trailing
blank characters;
fields will be delimited by sets of one or more
blank characters.
<p>
<li>
Otherwise, if
<b>FS</b>
is any other character
<i>c</i>,
fields will be delimited by
each single occurrence of
<i>c .</i>
<p>
</ol>
<p>
<li>
Otherwise,
the string value of
<b>FS</b>
will be considered to be an extended regular expression.
Each occurrence of
a sequence matching
the extended regular expression will delimit fields.
<p>
</ol>
<p>
Except in the
<b>gsub</b>,
<b>match</b>,
<b>split</b>
and
<b>sub</b>
built-in functions, regular expression matching will be based on input records;
that is, record separator characters (the first character of the value of
the variable
<b>RS</b>,
a
newline character
by default) cannot be embedded in the expression,
and no expression will match the record separator character.
If the record separator is not a
newline character,
newline characters embedded in the expression can be matched.
In those four built-in functions, regular expression matching will be
based on text strings; that is, any character
(including the
newline character
and the record separator) can be embedded in the pattern
and an appropriate pattern will match any character.
However, in all
<i>awk</i>
regular expression matching, the use of one or more NUL characters
in the pattern, input record or text string produces
undefined results.
<h5><a name = "tag_000_000_108_005">&nbsp;</a>Patterns</h5>
A
<i>pattern</i>
is any valid
<i>expression</i>,
a range specified by two expressions separated by comma,
or one of the two special patterns
<b>BEGIN</b>
or
<b>END</b>.
<h5><a name = "tag_000_000_108_006">&nbsp;</a>Special Patterns</h5>
The
<i>awk</i>
utility recognises two special patterns,
<b>BEGIN</b>
and
<b>END</b>.
Each
<b>BEGIN</b>
pattern will be matched once and its associated action
executed before the first
record of input is read
(except possibly by use of the
<b>getline</b>
function (see
<xref href=awkio><a href="#tag_000_000_108_014">
Input/Output and General Functions
</a></xref>)
in a prior
<b>BEGIN</b>
action) and before command line assignment is done.
Each
<b>END</b>
pattern will be matched once
and its associated action executed
after the last record of input has been read.
These two patterns will have associated actions.
<p>
<b>BEGIN</b>
and
<b>END</b>
will not combine with other patterns.
Multiple
<b>BEGIN</b>
and
<b>END</b>
patterns are allowed.
The actions associated with the
<b>BEGIN</b>
patterns will be
executed in the order specified in the program, as are the
<b>END</b>
actions.
An
<b>END</b>
pattern can precede a
<b>BEGIN</b>
pattern in a program.
<p>
If an
<i>awk</i>
program consists of only
actions with the pattern
<b>BEGIN</b>,
and the
<b>BEGIN</b>
action contains no
<b>getline</b>
function,
<i>awk</i>
will exit without reading its input when the last statement in the last
<b>BEGIN</b>
action is executed.
If an
<i>awk</i>
program consists of only
actions with the pattern
<b>END</b>
or only
actions with the patterns
<b>BEGIN</b>
and
<b>END</b>,
the input will be read before the statements in the
<b>END</b>
actions are executed.
<h5><a name = "tag_000_000_108_007">&nbsp;</a>Expression Patterns</h5>
An expression pattern will be evaluated
as if it were an expression in a Boolean context.