Change ID3 tags in the command line with id3v2.

retag.sh

#!/bin/bash
# Copy the Artist tag to Artist Album.
# Requires id3v2 [brew install id3vw]
#

DIR="500 Greatest Songs"

find "$DIR" -name "*mp3" | while read song
do
    artist=`id3v2 --list "$song" | grep TPE1 | cut -d: -f 2 | sed -e 's/^ //'`
    echo $artist
    id3v2 --TPE2 "${artist}" "$song"
done