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13.9. A Multiline Context grep Using sed

[One weakness of the grep family of programs is that they are line oriented. They read only one line at a time, so they can't find patterns (such as phrases) that are split across two lines. agrep (Section 13.6) can do multiline searches. One advantage of the cgrep script is that it shows how to handle multiple-line patterns in sed and can be adapted for work other than searches. -- JP]

Figure Go to http://examples.oreilly.com/upt3 for more information on: cgrep

It may surprise you to learn that a fairly decent context grep (Section 13.8) program can be built using sed. As an example, the following command line:

$ cgrep -10 system main.c

will find all lines containing the word system in the file main.c and show ten additional lines of context above and below each match. (The -context option must be at least one, and it defaults to two lines.) If several matches occur within the same context, the lines are printed as one large "hunk" rather than repeated smaller hunks. Each new block of context is preceded by the line number of the first occurrence in that hunk. This script, which can also search for patterns that span lines:

$ cgrep -3 "awk.*perl"

will find all occurrences of the word "awk" where it is followed by the word "perl" somewhere within the next three lines. The pattern can be any simple regular expression, with one notable exception: because you can match across lines, you should use \n in place of the ^ and $ metacharacters.

[While this is a wonderful example of some neat sed techniques, if this is all you're trying to do, use perl. It has features designed to do exactly this sort of thing very efficiently, and it will be much faster. -- DH]

-- GU

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