Well, any linux would do but thats what most people use. ffmpeg is a command line tool for converting between different media formats. You would have to do some extra steps to make 3gp conversion work with it.
Step by step instructions for ubuntu are given for it here
Command line tools are very useful especially when giving commands on a remote computer using ssh. When on commanding remote computer means you most probably have some files to transfer. Command line ftp can be very useful but dont you think it is a little too. Well there is something call ncftp which is easy to use and powerful. I wanted to copy my whole website structure from another host but I could not figure out how to do that using the normal ftp command line tool so i found out about this and it worked great. I have used it on my desktop machine aswell because i think gui ftp clients are very unreliable.
