bash Cookbook

Testing with Regular Expressions

Problem

Sometimes even the extended pattern matching of the extglob option isn’t enough. What you really need are regular expressions. Let’s say that you rip a CD of classical music into a directory, ls that directory, and see these names:

$ ls
Ludwig Van Beethoven - 01 - Allegro.ogg
Ludwig Van Beethoven - 02 - Adagio un poco mosso.ogg
Ludwig Van Beethoven - 03 - Rondo - Allegro.ogg
Ludwig Van Beethoven - 04 - "Coriolan" Overture, Op. 62.ogg
Ludwig Van Beethoven - 05 - "Leonore" Overture, No. 2 Op. 72.ogg
$

You’d like to write a script to rename these files to something simple, such as just the track number. How can you do that?

Solution

Use the regular expression matching of the =~ operator. Once it has matched the string, the various parts of the pattern are available in the shell variable $BASH_REMATCH. Here is the part of the script that deals with the pattern match:

#!/usr/bin/env bash
# cookbook filename: trackmatch
#
for CDTRACK in *
do
    if [[ "$CDTRACK" =~ "([[:alpha:][:blank:]]*)- ([[:digit:]]*) - (.*)$" ]]
    then
        echo Track ${BASH_REMATCH[2]} is ${BASH_REMATCH[3]}
        mv "$CDTRACK" "Track${BASH_REMATCH[2]}"
    fi
done

Warning

Caution: this requires bash version 3.0 or newer because older versions don’t have the =~ operator. In addition, bash version 3.2 unified the handling of the pattern in the == and =~ conditional command operators but introduced a subtle quoting bug that was corrected in 3.2 patch #3. If the solution above fails, you may be using bash ...