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Minix/2.0.0/wwwman/man9/awk.9.html
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Minix/2.0.0/wwwman/man9/awk.9.html
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<HTML>
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<HEAD>
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<TITLE>awk(9)</TITLE>
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</HEAD>
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<BODY>
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<H1>awk(9)</H1>
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<HR>
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<PRE>
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<STRONG>Command:</STRONG> <STRONG>awk</STRONG> <STRONG>-</STRONG> <STRONG>pattern</STRONG> <STRONG>matching</STRONG> <STRONG>language</STRONG>
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<STRONG>Syntax:</STRONG> <STRONG>awk</STRONG> <EM>rules</EM> [<EM>file</EM>] ...
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<STRONG>Flags:</STRONG> (none)
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<STRONG>Examples:</STRONG> awk rules input # Process <EM>input</EM> according to <EM>rules</EM>
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awk rules - >out # Input from terminal, output to <EM>out</EM>
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AWK is a programming language devised by Aho, Weinberger, and
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Kernighan at Bell Labs (hence the name). <EM>Awk</EM> programs search files for
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specific patterns and performs 'actions' for every occurrence of these
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patterns. The patterns can be 'regular expressions' as used in the <EM>ed</EM>
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editor. The actions are expressed using a subset of the C language.
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The patterns and actions are usually placed in a 'rules' file whose
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name must be the first argument in the command line, preceded by the
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flag <STRONG>-f</STRONG>. Otherwise, the first argument on the command line is taken to
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be a string containing the rules themselves. All other arguments are
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taken to be the names of text files on which the rules are to be
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applied, with <STRONG>-</STRONG> being the standard input. To take rules from the
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standard input, use <STRONG>-f</STRONG> <STRONG>-</STRONG>.
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The command:
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<STRONG>awk</STRONG> <STRONG>rules</STRONG> <STRONG>prog.d*u</STRONG>
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would read the patterns and actions rules from the file <EM>rules</EM> and apply
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them to all the arguments.
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The general format of a rules file is:
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<pattern> { <action> } <pattern> { <action> } ...
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There may be any number of these <pattern> { <action> } sequences in the
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rules file. <EM>Awk</EM> reads a line of input from the current input file and
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applies every <pattern> { <action> } in sequence to the line.
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If the <pattern> corresponding to any { <action> } is missing, the
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action is applied to every line of input. The default { <action> } is
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to print the matched input line.
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<STRONG>Patterns</STRONG>
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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 is taken
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to be a range and the <action> is performed on all lines of input that
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match the range. <pattern>s may contain 'regular expressions' delimited
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by an @ symbol. Regular expressions can be thought of as a generalized
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'wildcard' string matching mechanism, similar to that used by many
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operating systems to specify file names. Regular expressions may
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contain any of the following characters:
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x An ordinary character
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\ The backslash quotes any character
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^ A circumflex at the beginning of an expr matches the beginning
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of a line.
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$ A dollar-sign at the end of an expression matches the end of a
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line.
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. A period matches any single character except newline.
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* An expression followed by an asterisk matches zero or more
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occurrences of that expression: 'fo*' matches 'f', 'fo', 'foo',
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'fooo', etc.
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+ An expression followed by a plus sign matches one or more
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occurrences of that expression: 'fo+' matches 'fo', 'foo',
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'fooo', etc.
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[] A string enclosed in square brackets matches any single
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character in that string, but no others. If the first
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character in the string is a circumflex, the expression matches
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any character except newline and the characters in the string.
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For example, '[xyz]' matches 'xx' and 'zyx', while '[^xyz]'
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matches 'abc' but not 'axb'. A range of characters may be
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specified by two characters separated by '-'.
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<STRONG>Actions</STRONG>
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Actions are expressed as a subset of the C language. All variables
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are global and default to int's if not formally declared. Only char's
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and int's and pointers and arrays of char and int are allowed. <EM>Awk</EM>
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allows only decimal integer constants to be used----no hex (0xnn) or
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octal (0nn). String and character constants may contain all of the
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special C escapes (\n, \r, etc.).
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<EM>Awk</EM> supports the 'if', 'else', 'while' and 'break' flow of control
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constructs, which behave exactly as in C.
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Also supported are the following unary and binary operators, listed
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in order from highest to lowest precedence:
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<STRONG>Operator</STRONG> <STRONG>Type</STRONG> <STRONG>Associativity</STRONG>
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() [] unary left to right
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! ~ ++ -- - * & unary right to left
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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 left to right
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|| binary left to right
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= binary right to left
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Comments are introduced by a '#' symbol and are terminated by the first
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newline character. The standard '/*' and '*/' comment delimiters are
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not supported and will result in a syntax error.
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<STRONG>Fields</STRONG>
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When <EM>awk</EM> reads a line from the current input file, the record is
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automatically separated into 'fields.' A field is simply a string of
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consecutive characters delimited by either the beginning or end of line,
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or a 'field separator' character. Initially, the field separators are
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the space and tab character. The special unary operator '$' is used to
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reference one of the fields in the current input record (line). The
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fields are numbered sequentially starting at 1. The expression '$0'
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references the entire input line.
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Similarly, the 'record separator' is used to determine the end of
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an input 'line,' initially the newline character. The field and record
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separators may be changed programatically by one of the actions and will
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remain in effect until changed again.
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Multiple (up to 10) field separators are allowed at a time, but
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only one record separator.
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Fields behave exactly like strings; and can be used in the same
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context as a character array. These 'arrays' can be considered to have
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been declared as:
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char ($n)[ 128 ];
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In other words, they are 128 bytes long. Notice that the parentheses
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are necessary because the operators [] and $ associate from right to
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left; without them, the statement would have parsed as:
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char $(1[ 128 ]);
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which is obviously ridiculous.
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If the contents of one of these field arrays is altered, the '$0'
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field will reflect this change. For example, this expression:
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*$4 = 'A';
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will change the first character of the fourth field to an upper- case
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letter 'A'. Then, when the following input line:
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120 PRINT "Name address Zip"
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is processed, it would be printed as:
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120 PRINT "Name Address Zip"
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Fields may also be modified with the strcpy() function (see below). For
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example, the expression:
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strcpy( $4, "Addr." );
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applied to the same line above would yield:
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120 PRINT "Name Addr. Zip"
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<STRONG>Predefined</STRONG> <STRONG>Variables</STRONG>
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The following variables are pre-defined:
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FS Field separator (see below).
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RS Record separator (see below also).
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NF Number of fields in current input record (line).
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NR Number of records processed thus far.
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FILENAME Name of current input file.
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BEGIN A special <pattern> that matches the beginning of
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input text.
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END A special <pattern> that matches the end of input
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text.
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<EM>Awk</EM> also provides some useful built-in functions for string manipulation
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and printing:
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print(arg) Simple printing of strings only, terminated by '\n'.
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printf(arg...) Exactly the printf() function from C.
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getline() Reads the next record and returns 0 on end of file.
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nextfile() Closes the current input file and begins processing
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the next file
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strlen(s) Returns the length of its string argument.
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strcpy(s,t) Copies the string 't' to the string 's'.
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strcmp(s,t) Compares the 's' to 't' and returns 0 if they match.
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toupper(c) Returns its character argument converted to upper-
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case.
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tolower(c) Returns its character argument converted to lower-
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case.
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match(s,@re@) Compares the string 's' to the regular expression 're'
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and returns the number of matches found (zero if
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none).
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<STRONG>Authors</STRONG>
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<EM>Awk</EM> was written by Saeko Hirabauashi and Kouichi Hirabayashi.
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</PRE>
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</BODY>
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</HTML>
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