grepping a range of lines using perl.

I was asked how to use grep to select everything in a file starting with a pattern, and ending with a different one. The file is our diagnostic log and if this has originated with one of our system testers could be massive (a few hundred thousand lines long). gnu-grep can be used for this. You could do something like:

grep -A 9999999 'some first expression' < wayToBigFile | grep -B 9999999 'some other expression'

Here 9999999 is some number of lines that is guessed big enough to contain all the lines of interest (not known ahead of time), so the command says “give me everything after the expression, and then give me everything before the other expression in that output”

Perl gives you a nicer way of doing this:

#!/usr/bin/perl -n

$foundIt = 1 if ( /some first expression/ ) ;

next if ( !$foundIt ) ;

print ;

exit if ( /some other expression/ ) ;

# done.

The -n flag says to run the whole script as if it is in a ‘while (<>){ … }’ loop.  Until the initial pattern is seen $foundIt is false, and nothing will be printed, and we bail if the second pattern is seen. Note that this relies on perl’s lazy variable initialization, since $foundIt = 0 until modified.

Observe also that this script is actually also it’s own test case.

myPrompt$ chmod 755 ./theScript
myPrompt$ ./theScript < ./theScript
$foundIt = 1 if ( /some first expression/ ) ;

next if ( !$foundIt ) ;

print ;

exit if ( /some other expression/ ) ;