mkvmerge

Merge and extract multimedia streams. More information: https://mkvtoolnix.download/doc/mkvmerge.html.

mkvmerge --identify path/to/file.mkv

mkvextract tracks path/to/file.mkv 1:path/to/output.webm

mkvextract tracks path/to/file.mkv 3:path/to/subs.srt

mkvmerge --output path/to/output.mkv path/to/file.mkv path/to/subs.srt