The files I had, had the following pattern as filename ARTIST – SONG – TRACK – ALBUM

James Brown - I Got You (I Feel Good).ogg  - 01 - Classic James Brown

I wanted to rename it to ARTIST – ALBUM – TRACK – NAME

James Brown - Classic James Brown - 01 - I Got You (I Feel Good).ogg

Describing the change as a sed program is easy:

s/\(.*\) - \(.*\) - \(.*\) - \(.*\).ogg/\1 - \4 - \3 - \2.ogg/

Now all that has to be done is to pass each filename to mv and pass it again after it went through the sed script. This could be done like this:

for i in *; do
  mv "$i" "`echo $i | sed "s/\(.*\) - \(.*\) - \(.*\) - \(.*\).ogg/\1 - \4 - \3 - \2.ogg/"`";
done

The important part is the

`echo $i | sed "s/\(.*\) - \(.*\) - \(.*\) - \(.*\).ogg/\1 - \4 - \3 - \2.ogg/"`

which pipes the filename to sed and returns it as an argument for mv.

To see what renaming will be done one can alter a bit the above command, and get

for i in *; do
  echo "$i" "->" "`echo $i | sed "s/\(.*\) - \(.*\) - \(.*\) - \(.*\).ogg/\1 - \4 - \3 - \2.ogg/"`";
done

While will effectively print a list of lines of the form oldname -> newname.

Of course this technique isn’t limited to the renaming I’ve done. By changing the pattern given to sed, one can do any kind of renaming that can be described as a regular expression replacement. Also one can change the globbing (the *) in the for loop to operate only on specific files, that match a given pattern, in the directory instead of all of them.