Ubuntu: convert video files

Install more codecs:
sudo apt-get install ubuntu-restricted-extras

Example:
ffmpeg -i file.flv file.mp4

Convert all FLV videos to AVi/x264/mp3:
for i in *.flv; do ffmpeg -i "$i" -qmin 3 -qmax 10 -vcodec libx264 -acodec libmp3lame "${i%flv}avi"; done