How to embed external subtitles in a video files using mkvtoolnix.

First, make sure that mkvmerge is installed on your system by running the following command:

  apt-get install mkvtoolnix

Next, navigate to the directory where the movie file and the subtitle file are located using the cd command.

Then, use the following mkvmerge command to add the subtitle file to the movie file: 

mkvmerge -o output.mkv movie.mp4 subtitle.srt

In this command, "output.mkv" is the output movie file with the subtitles embedded, "movie.mp4" is the input movie file, and "subtitle.srt" is the input subtitle file.

Further customization

https://askubuntu.com/questions/119325/how-can-i-embed-subtitles-into-a-movie-file