This command will chop out from start ss for t time a bit of video. Make sure you have input and out extensions same or you will see this: Could not find input stream matching output stream.
ffmpeg -ss 00:19:55 -i blackmass.mkv -vcodec copy -acodec copy -t 106 snip.mkv
For some reason ffmpeg isnt in the repo for ubunt 14 so run this tuff to add it.
sudo apt-add-repository ppa:mc3man/trusty-media
sudo apt-get update
sudo apt-get install ffmpeg gstreamer0.10-ffmpeg