Linux – Changing subtitle format with ffmpeg

The answer as given will (lossily) re-encode the audio and video of the input file, and it will only select a single audio and video track (so if you have multiple languages, you're S-O-L). The correct way to do this in modern ffmpeg (or avconv for Ubuntu/Debian users, same syntax) is:

ffmpeg -i input.avi -i input.srt -map 0 -map 1 -c copy output.mkv

MKVmerge, a part of mkvtoolsnix, can do this perfectly well too

mkvmerge -o output.mkv input.avi subtitle.srt

I recently stumbled upon this post and I do know the answer to it. I hope it can help other people as well.

What you basically do is also put the mkv file in the subtitles variable. You can add :si=0 (subtitle index) that takes the first subtitle stream it can find. You'll have to use the double and single quote if the file has spaces in it.

ffmpeg -i video.mkv -vf "subtitles='video.mkv':si=0" out.mp4