Here’s a bunch of useful commands I use for extracting, converting subtitles from mostly mkv and mp4 files with mostly SubRip/SRT and VTT formatting.
I use these on both macOS and Linux (Ubuntu).
Extracting subtitles from mp4 files
Check subtitle track ID with:
MP4Box -info video.mp4
Then extract SRT subtitles with (where number is the track ID):
MP4Box -srt 3 video.mp4
Extracting subtitles from mkv files
Check the subtitle track ID with:
mkvmerge -i video.mkv
Or get a verbose JSON output to for track language:
mkvmerge -i -J video.mkv
Extract with (where number is the track ID):
mkvextract tracks video.mkv 3:subtitle.srt
Convert SRT to VTT
ffmpeg -i subtitle.srt subtitle.vtt
To be continued..