Extract part of a video with a one-line command

This can be done using mencoder or ffmpeg.

mencoder

Say that you want to cut out a part starting at 00:00:30 into the original file with a 5 seconds length and that it shouldn't be reencoded (if you want to re-encode it you'll have to replace copy with audio and video codecs, for a list of available audio codecs issue mencoder -oac help, for a list of available video codecs issue mencoder -ovc help), then you issue:

mencoder -ss 00:30:00 -endpos 00:00:05 -oac pcm -ovc copy originalfile -o newfile

You'll have to replace orginalfile and newfile with actual file names, the latter is the name of the file that is created to hold the cut-out part.

ffmpeg

Say that you want to cut out a part starting at 00:00:30 into the original file with a 5 seconds length and that it shouldn't be re-encoded (if you want to re-encode it you'll have to replace copy with audio and video codecs, for a list of available codecs issue ffmpeg -formats -E), then you issue:

ffmpeg -ss 00:00:30 -i orginalfile -t 00:00:05 -vcodec copy -acodec copy newfile

You'll have to replace orginalfile and newfile with actual file names, the later is the name of the file that is created to hold the cut out part.

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_{For reference see http://lazyxiong-tech.blogspot.com/2007/05/using-mencoder-to-cut-out-pieces-of.html and "7. Copy Only A Part Of Video" in http://segfault.in/2010/10/ffmpeg-tricks-you-should-know-about/}

Do you need to cut video with re-encoding or without re-encoding mode? You can try to following below command.

Synopsis: ffmpeg -i [input_file] -ss [start_seconds] -t [duration_seconds] [output_file]

use FFmpeg cut mp4 video without re-encoding

Example:
ffmpeg -i source.mp4 -ss 00:00:05 -t 00:00:10 -c copy cut_video.mp4

use FFmpeg cut mp4 video with re-encoding

Example:
ffmpeg -i source.mp4 -ss 00:00:05 -t 00:00:10 -async 1 -strict -2 cut_video.mp4

If you want to cut off section from the beginning, simply drop -t 00:00:10 from the command