#       Start time     Offset     input file                           output file
ffmpeg -ss 00:00:00 -t 00:30:00 -i input.avi -vcodec copy -acodec copy output1.avi

#or

#     Start time [s]    Offset [s]  input file                           output file
ffmpeg -ss    0     -t     10     -i input.avi -vcodec copy -acodec copy output1.avi