Editing AWK (section)

=== Match pattern from command line ===

This program can be represented in several ways. The first one uses the [[Bourne shell]] to make a shell script that does everything. It is the shortest of these methods:
<syntaxhighlight lang="bash">
#!/bin/sh

pattern="$1"
shift
awk '/'"$pattern"'/ { print FILENAME ":" $0 }' "$@"
</syntaxhighlight>

The <code>$pattern</code> in the awk command is not protected by single quotes so that the shell does expand the variable but it needs to be put in double quotes to properly handle patterns containing spaces.  A pattern by itself in the usual way checks to see if the whole line (<code>$0</code>) matches. <code>FILENAME</code> contains the current filename. awk has no explicit concatenation operator; two adjacent strings concatenate them. <code>$0</code> expands to the original unchanged input line.

There are alternate ways of writing this. This shell script accesses the environment directly from within awk:
<syntaxhighlight lang="bash">
#!/bin/sh

export pattern="$1"
shift
awk '$0 ~ ENVIRON["pattern"] { print FILENAME ":" $0 }' "$@"
</syntaxhighlight>

This is a shell script that uses <code>ENVIRON</code>, an array introduced in a newer version of the One True awk after the book was published. The subscript of <code>ENVIRON</code> is the name of an environment variable; its result is the variable's value. This is like the [[getenv]] function in various standard libraries and [[POSIX]]. The shell script makes an environment variable <code>pattern</code> containing the first argument, then drops that argument and has awk look for the pattern in each file.

<code>~</code> checks to see if its left operand matches its right operand; <code>!~</code> is its inverse. A regular expression is just a string and can be stored in variables.

The next way uses command-line variable assignment, in which an argument to awk can be seen as an assignment to a variable:
<syntaxhighlight lang="bash">
#!/bin/sh

pattern="$1"
shift
awk '$0 ~ pattern { print FILENAME ":" $0 }' pattern="$pattern" "$@"
</syntaxhighlight>

Or You can use the ''-v var=value'' command line option (e.g. ''awk -v pattern="$pattern" ...'').

Finally, this is written in pure awk, without help from a shell or without the need to know too much about the implementation of the awk script (as the variable assignment on command line one does), but is a bit lengthy:
<syntaxhighlight lang="awk">
BEGIN {
    pattern = ARGV[1]
    for (i = 1; i < ARGC; i++) # remove first argument
        ARGV[i] = ARGV[i + 1]
    ARGC--
    if (ARGC == 1) { # the pattern was the only thing, so force read from standard input (used by book)
        ARGC = 2
        ARGV[1] = "-"
    }
}
$0 ~ pattern { print FILENAME ":" $0 }
</syntaxhighlight>

The <code>BEGIN</code> is necessary not only to extract the first argument, but also to prevent it from being interpreted as a filename after the <code>BEGIN</code> block ends. <code>ARGC</code>, the number of arguments, is always guaranteed to be ≥1, as <code>ARGV[0]</code> is the name of the command that executed the script, most often the string <code>"awk"</code>. <code>ARGV[ARGC]</code> is the empty string, <code>""</code>. <code>#</code> initiates a comment that expands to the end of the line.

Note the <code>if</code> block. awk only checks to see if it should read from standard input before it runs the command. This means that
 awk 'prog'
only works because the fact that there are no filenames is only checked before <code>prog</code> is run! If you explicitly set <code>ARGC</code> to 1 so that there are no arguments, awk will simply quit because it feels there are no more input files. Therefore, you need to explicitly say to read from standard input with the special filename <code>-</code>.