A very simple Bash script to download a YouTube video and extract the music file from it.

youtube2mp3.sh

#!/bin/bash 

address=$1 
regex='v=(.*)' 

if [[ $address =~ $regex ]]; then 

    video_id=${BASH_REMATCH[1]}
    video_id=$(echo $video_id | cut -d'&' -f1) 
    video_title="$(youtube-dl --get-title $address)" 
    youtube-dl $address 

    echo "Starting FFMPEG for:  " $video_id "  *********\n\n\n\n\n"
    ffmpeg -i $video_id.flv "$video_title".wav 

    echo "Starting LAME &&&&&&&&&\n\n\n\n\n" 
    lame "$video_title".wav "$video_title".mp3 
    rm $video_id.$ext "$video_title".wav 
else 
    echo "Sorry but the system encountered a problem." 
fi