FOR LINUX PCS' 
Downloading from youTube
Install
youtube-dl from the official repository in Ubuntu:
sudo apt-get install youtube-dl To download a video execute the following:
youtube-dl <url> E.g
youtube-dl http://www.youtube.com/watch?v=_z-hEyVQDRA You can also specify other options
-b Best quality -m Mobile version -d High Definition -g Don’t download, just show the url -c Resume download of a video which was interrupted before -w Don’t overwrite existing file For complete list of switches execute man youtube-dl in terminal.
Information from
http://www.absolutelytech.com/2010/06/26/howto-download-youtube-video-from-command-line-ubuntu/
Converting to mp3
Now install
ffmpeg from the official repo.
sudo apt-get install ffmpeg Next have a look to make sure the flv file has downloaded from youtube by typing
ls *flv This will list all the flv files you have. What you should see is a flv file with the same name as the 'v' component of the youtube url you downloaded.
E.g if you downloaded
http://www.youtube.com/watch?v=_z-hEyVQDRA then you should have a file called _z-hEyVQDRA.flv
Next you need to copy that file name and then put it in an ffmpg command like
ffmpeg -i <file name>.flv -acodec libmp3lame <song name>.mp3 So for this example you would type
ffmpeg -i _z-hEyVQDRA.flv -acodec libmp3lame MasterOfPuppets.mp3 Once this has successfully completed you will now have the mp3 file you were after.
Note For cleanup you may want to type rm _z-hEyVQDRA.flv to get rid of the flv file that you no longer need.
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