mkvtoolnix

MKVToolNix GUI
is a Qt GUI for mkvmerge and a successor of mmg.
mkvmerge
merges multimedia streams into a Matroska file.
mkvinfo
lists all elements contained in a Matroska file.
mkvextract
extracts specific parts from a Matroska file to other formats.
mkvpropedit
allows to analyze and modify some Matroska file properties.

General Format for mkvmerge:

mkvmerge -o video_merged.mkv original.mkv original.srt

where -o or –output file-name is write to the file file-name.

add subs to a video:

mkvmerge -o Max\ Headroom\ -\ S01E01\ -\ Blipverts-merged.mkv Max\ Headroom\ -\ S01E01\ -\ Blipverts.mkv Max\ Headroom\ -\ S01E01\ -\ Blipverts.srt

extract subs from simple file:

mkvmerge -i Star\ Trek\ Lower\ Decks\ -\ S01E01\ -\ Second\ Contact.mkv

mkvextract tracks Star\ Trek\ Lower\ Decks\ -\ S01E01\ -\ Second\ Contact.mkv 2:Star\ Trek\ Lower\ Decks\ -\ S01E01\ -\ Second\ Contact.srt

extract subs from complex file:

mkvmerge -i Cowboy\ Bebop\ (2021)\ -\ S01E01\ -\ Cowboy\ Gospel.mkv

mkvextract tracks Cowboy\ Bebop\ (2021)\ -\ S01E01\ -\ Cowboy\ Gospel.mkv 7:Cowboy\ Bebop\ (2021)\ -\ S01E01\ -\ Cowboy\ Gospel.srt

convert mp4 to mkv with srt:

mkvmerge -o "result.mkv" "orig.mp4" "subs.srt" << reboot