get-folder-duration
get time duration of folder with mp4 files
ffmpeg -i "1. Introduction.mp4" 2>&1 | grep Duration
https://askubuntu.com/questions/224237/how-to-check-how-long-a-video-mp4-is-using-the-shell
https://stackoverflow.com/questions/818255/in-the-shell-what-does-21-mean