Extract Subtitles

ffmpeg is smart enough to figure this out for you:

ffmpeg -i SomeVideo.mkv SomeVideo.en.srt

Note the added 'en' that helps some servers (such as plex) display the language in the interface. A good script is:

for X in `ls *.mkv`; do Y=`echo $X | sed 's/.mkv/.en.srt/'`;ffmpeg -i "$X"  "$Y";  done

If you get the error 

    Output file #0 does not contain any stream

Check your stream mapping

Stream mapping:
  Stream #0:2 -> #0:0 (subrip (srt) -> subrip (srt))

If it says dvdsub on the source side, you'll have to use the vobsub conversion method