mp4box

MPEG-4 Systems Toolbox - Muxes streams into MP4 container.

Display information about an existing MP4 file:

mp4box -info {{filename}}

Add an SRT subtitle file into an MP4 file:

mp4box -add {{input_subs.srt}}:lang=eng -add {{input.mp4}} {{output.mp4}}

Combine audio from one file and video from another:

mp4box -add {{input1.mp4}}#audio -add {{input2.mp4}}#video {{output.mp4}