02-24-2011
penalty for case insensitive grep
I just found out there were a big performance penalty for case insensitive "grep" on big files.
It would be understandable, except that the penalty seems to be exaggerated out of proportion.
A real example, if I only grep a single letter "V" (or "v") , without "-i", on a big file, (file is doctored so only a few "V" or "v" exist). It takes 0.157 user time to finish.
Then I grep the same letter "V", but with "-i" option, it takes 32.0 user time to finish. That is 200 times longer than without "-i" for a single character.
Can someone provide some insight why this is the case?
Thanks.
NB Phil
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ZGREP(1) BSD General Commands Manual ZGREP(1)
NAME
zgrep, zegrep, zfgrep -- print lines matching a pattern in gzip-compressed files
SYNOPSIS
zgrep [grep-flags] [--] pattern [files ...]
zegrep [grep-flags] [--] pattern [file ...]
zfgrep [grep-flags] [--] pattern [file ...]
DESCRIPTION
zgrep runs grep(1) on files or stdin, if no files argument is given, after decompressing them with zcat(1).
The grep-flags and pattern arguments are passed on to grep(1). If an -e flag is found in the grep-flags, zgrep will not look for a pattern
argument.
zegrep calls egrep(1), while zfgrep calls fgrep(1).
EXIT STATUS
In case of missing arguments or missing pattern, 1 will be returned, otherwise 0.
SEE ALSO
egrep(1), fgrep(1), grep(1), gzip(1), zcat(1)
AUTHORS
Thomas Klausner <wiz@NetBSD.org>
BSD
December 28, 2003 BSD