[ffmpeg] clip video & audio

get a clip from the whole movie, such as, i just need 20 seconds: from 00:00:30 to 00:00:50

ffmpeg -i input.mkv -acodec copy -vcodec copy -ss 00:00:30 -t 00:00:20 output.mkv

this command will not change the audio and video format (just copy).