247 lines
8.4 KiB
Groff
247 lines
8.4 KiB
Groff
.CD "awk \(en pattern matching language"
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.SX "awk \fIrules\fR [\fIfile\fR] ...
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.FL "\fR(none)"
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.EX "awk rules input" "Process \fIinput\fR according to \fIrules\fR"
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.EX "awk rules \(en >out" "Input from terminal, output to \fIout\fR"
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.PP
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AWK is a programming language devised by Aho, Weinberger, and Kernighan
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at Bell Labs (hence the name).
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\fIAwk\fR programs search files for
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specific patterns and performs \*(OQactions\*(CQ for every occurrence
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of these patterns. The patterns can be \*(OQregular expressions\*(CQ
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as used in the \fIed\fR editor. The actions are expressed
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using a subset of the C language.
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.PP
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The patterns and actions are usually placed in a \*(OQrules\*(CQ file
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whose name must be the first argument in the command line,
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preceded by the flag \fB\(enf\fR. Otherwise, the first argument on the
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command line is taken to be a string containing the rules
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themselves. All other arguments are taken to be the names of text
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files on which the rules are to be applied, with \fB\(en\fR being the
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standard input. To take rules from the standard input, use \fB\(enf \(en\fR.
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.PP
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The command:
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.HS
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.Cx "awk rules prog.\d\s+2*\s0\u"
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.HS
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would read the patterns and actions rules from the file \fIrules\fR
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and apply them to all the arguments.
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.PP
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The general format of a rules file is:
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.HS
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~~~<pattern> { <action> }
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~~~<pattern> { <action> }
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~~~...
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.HS
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There may be any number of these <pattern> { <action> }
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sequences in the rules file. \fIAwk\fR reads a line of input from
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the current input file and applies every <pattern> { <action> }
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in sequence to the line.
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.PP
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If the <pattern> corresponding to any { <action> } is missing,
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the action is applied to every line of input. The default
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{ <action> } is to print the matched input line.
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.SS "Patterns"
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.PP
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The <pattern>s may consist of any valid C expression. If the
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<pattern> consists of two expressions separated by a comma, it
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is taken to be a range and the <action> is performed on all
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lines of input that match the range. <pattern>s may contain
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\*(OQregular expressions\*(CQ delimited by an @ symbol. Regular
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expressions can be thought of as a generalized \*(OQwildcard\*(CQ
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string matching mechanism, similar to that used by many
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operating systems to specify file names. Regular expressions
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may contain any of the following characters:
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.HS
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.in +0.75i
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.ta +0.5i
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.ti -0.5i
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x An ordinary character
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.ti -0.5i
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\\ The backslash quotes any character
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.ti -0.5i
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^ A circumflex at the beginning of an expr matches the beginning of a line.
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.ti -0.5i
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$ A dollar-sign at the end of an expression matches the end of a line.
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.ti -0.5i
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\&. A period matches any single character except newline.
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.ti -0.5i
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* An expression followed by an asterisk matches zero or more occurrences
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of that expression: \*(OQfo*\*(CQ matches \*(OQf\*(CQ, \*(OQfo\*(CQ, \*(OQfoo\*(CQ, \*(OQfooo\*(CQ, etc.
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.ti -0.5i
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+ An expression followed by a plus sign matches one or more occurrences
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of that expression: \*(OQfo+\*(CQ matches \*(OQfo\*(CQ, \*(OQfoo\*(CQ, \*(OQfooo\*(CQ, etc.
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.ti -0.5i
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[] A string enclosed in square brackets matches any single character in that
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string, but no others. If the first character in the string is a circumflex, the
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expression matches any character except newline and the characters in the
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string. For example, \*(OQ[xyz]\*(CQ matches \*(OQxx\*(CQ and \*(OQzyx\*(CQ, while
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\*(OQ[^xyz]\*(CQ matches \*(OQabc\*(CQ but not \*(OQaxb\*(CQ. A range of characters may be
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specified by two characters separated by \*(OQ-\*(CQ.
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.in -0.75i
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.SS "Actions"
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.PP
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Actions are expressed as a subset of the C language. All
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variables are global and default to int's if not formally
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declared.
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Only char's and int's and pointers and arrays of
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char and int are allowed. \fIAwk\fR allows only decimal integer
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constants to be used\(emno hex (0xnn) or octal (0nn). String
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and character constants may contain all of the special C
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escapes (\\n, \\r, etc.).
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.PP
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\fIAwk\fR supports the \*(OQif\*(CQ, \*(OQelse\*(CQ,
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\*(OQwhile\*(CQ and \*(OQbreak\*(CQ flow of
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control constructs, which behave exactly as in C.
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.PP
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Also supported are the following unary and binary operators,
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listed in order from highest to lowest precedence:
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.HS
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.ta 0.25i 1.75i 3.0i
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.nf
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\fB Operator Type Associativity\fR
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() [] unary left to right
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.tr ~~
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! ~ ++ \(en\(en \(en * & unary right to left
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.tr ~
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* / % binary left to right
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+ \(en binary left to right
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<< >> binary left to right
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< <= > >= binary left to right
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== != binary left to right
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& binary left to right
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^ binary left to right
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| binary left to right
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&& binary left to right
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|| binary left to right
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= binary right to left
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.fi
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.HS
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Comments are introduced by a '#' symbol and are terminated by
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the first newline character. The standard \*(OQ/*\*(CQ and \*(OQ*/\*(CQ
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comment delimiters are not supported and will result in a
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syntax error.
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.SP 0.5
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.SS "Fields"
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.SP 0.5
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.PP
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When \fIawk\fR reads a line from the current input file, the
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record is automatically separated into \*(OQfields.\*(CQ A field is
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simply a string of consecutive characters delimited by either
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the beginning or end of line, or a \*(OQfield separator\*(CQ character.
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Initially, the field separators are the space and tab character.
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The special unary operator '$' is used to reference one of the
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fields in the current input record (line). The fields are
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numbered sequentially starting at 1. The expression \*(OQ$0\*(CQ
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references the entire input line.
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.PP
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Similarly, the \*(OQrecord separator\*(CQ is used to determine the end
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of an input \*(OQline,\*(CQ initially the newline character. The field
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and record separators may be changed programatically by one of
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the actions and will remain in effect until changed again.
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.PP
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Multiple (up to 10) field separators are allowed at a time, but
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only one record separator.
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.PP
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Fields behave exactly like strings; and can be used in the same
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context as a character array. These \*(OQarrays\*(CQ can be considered
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to have been declared as:
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.SP 0.15
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.HS
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~~~~~char ($n)[ 128 ];
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.HS
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.SP 0.15
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In other words, they are 128 bytes long. Notice that the
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parentheses are necessary because the operators [] and $
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associate from right to left; without them, the statement
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would have parsed as:
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.HS
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.SP 0.15
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~~~~~char $(1[ 128 ]);
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.HS
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.SP 0.15
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which is obviously ridiculous.
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.PP
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If the contents of one of these field arrays is altered, the
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\*(OQ$0\*(CQ field will reflect this change. For example, this
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expression:
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.HS
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.SP 0.15
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~~~~~*$4 = 'A';
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.HS
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.SP 0.15
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will change the first character of the fourth field to an upper-
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case letter 'A'. Then, when the following input line:
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.HS
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.SP 0.15
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~~~~~120 PRINT "Name address Zip"
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.SP 0.15
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.HS
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is processed, it would be printed as:
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.HS
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.SP 0.15
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~~~~~120 PRINT "Name Address Zip"
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.HS
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.SP 0.15
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Fields may also be modified with the strcpy() function (see
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below). For example, the expression:
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.HS
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~~~~~strcpy( $4, "Addr." );
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.HS
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applied to the same line above would yield:
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.HS
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~~~~~120 PRINT "Name Addr. Zip"
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.HS
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.SS "Predefined Variables"
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.PP
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The following variables are pre-defined:
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.HS
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.in +1.5i
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.ta +1.25i
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.ti -1.25i
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FS Field separator (see below).
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.ti -1.25i
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RS Record separator (see below also).
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.ti -1.25i
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NF Number of fields in current input record (line).
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.ti -1.25i
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NR Number of records processed thus far.
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.ti -1.25i
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FILENAME Name of current input file.
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.ti -1.25i
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BEGIN A special <pattern> that matches the beginning of input text.
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.ti -1.25i
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END A special <pattern> that matches the end of input text.
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.in -1.5i
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.HS
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\fIAwk\fR also provides some useful built-in functions for string
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manipulation and printing:
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.HS
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.in +1.5i
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.ta +1.25i
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.ti -1.25i
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print(arg) Simple printing of strings only, terminated by '\\n'.
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.ti -1.25i
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printf(arg...) Exactly the printf() function from C.
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.ti -1.25i
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getline() Reads the next record and returns 0 on end of file.
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.ti -1.25i
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nextfile() Closes the current input file and begins processing the next file
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.ti -1.25i
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strlen(s) Returns the length of its string argument.
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.ti -1.25i
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strcpy(s,t) Copies the string \*(OQt\*(CQ to the string \*(OQs\*(CQ.
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.ti -1.25i
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strcmp(s,t) Compares the \*(OQs\*(CQ to \*(OQt\*(CQ and returns 0 if they match.
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.ti -1.25i
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toupper(c) Returns its character argument converted to upper-case.
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.ti -1.25i
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tolower(c) Returns its character argument converted to lower-case.
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.ti -1.25i
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match(s,@re@) Compares the string \*(OQs\*(CQ to the regular expression \*(OQre\*(CQ and
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returns the number of matches found (zero if none).
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.in -1.5i
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.SS "Authors"
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.PP
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\fIAwk\fR was written by Saeko Hirabauashi and Kouichi Hirabayashi.
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