is one of
Unix's most useful tools. As a result, all users
seem to want their own, slightly different version that solves a
different piece of the problem. (Maybe this is a problem in itself;
there really should be only one grep, as the
manpage says.) Three versions of grep come with
every Unix system; in addition, there are six or seven freely
available versions that we'll mention here, as well
as probably dozens of others that you can find kicking around the
Here are the different versions of grep and what
they offer. We'll start with the standard versions:
- Plain old grep
Great for searching with regular expressions (Section 13.2).
- Extended grep (or egrep)
Handles extended regular expressions. It is also,
arguably, the fastest of the standard greps
- Fixed grep (or fgrep)
So named because it matches fixed strings. It
is sometimes inaccurately called "fast
grep"; often it is really the
slowest of them all. It is useful to search for patterns with literal
backslashes, asterisks, and so on that you'd
otherwise have to escape somehow. fgrep has the
interesting ability to search for multiple strings (Section 13.5).
Of course, on many modern Unixes all three are the same executable,
just with slightly different behaviors, and so you may not see
dramatic speed differences between them. Now for the freeware
- agrep, or "approximate grep"
A tool that finds lines that "more or
less" match your search string. A very interesting
and useful tool, it's part of the
glimpse package, which is an indexing and query
system for fast searching of huge amounts of text.
agrep is introduced in Section 13.6.
- Very fast versions of grep, such as GNU grep/egrep/fgrep
Most free Unixes use GNU grep as their main
Searches through RCS files (Section 39.5) (Section 13.7).
In addition, you can simulate the action of
grep with sed,
awk, and perl. These utilities
allow you to write such variations as a grep that
searches for a pattern that can be split across
several lines (Section 13.9) and other
programs (Section 41.12), which show you a few lines before and after
the text you find. (Normal greps just show the
lines that match.)