Regular expressions (regexps) are a very powerful tool, allowing you to look for text strings matching a particular pattern. In this first part of a two-part series, I'm going to look at using them on the command line. The next part will cover regexps in editors and other programs.

The shell built-in wildcard provision

There's some basic regexp-type provision built into the shell: the most basic example of this is the * wildcard. This example will list every file in the current directory which has a .jpg extension:

ls *.jpg

ls file1.jpg file2.jpg ...

ls | grep '.*\.jpg'

The single quotes are very important! Without the single quotes, the shell will try to do expansion before running the command, and strange things will result. Always single-quote your regular expressions on the command line.

The shell wildcard provision can be very useful. The important thing is to remember that the syntax isn't quite the same as for 'proper' regular expressions. Here's another shell built-in example, which will move all of your old logs (which on my system are named like mail.log.0.gz, system.log.1.bz2, etc) to a subdirectory:

mv *.log.[0-9].* logarchive/

Next: Full Regular Expressions »