Is there a free program to join and cut AVI movie files?
You can also use the concat demuxer with ffmpeg:

ffmpeg -f concat -i &lt;(for f in $PWD/*.avi;do echo "file '$f'";done) -c copy output.avi


-c copy disables re-encoding the streams.

You can install ffmpeg with brew install ffmpeg after installing Homebrew.

See https://trac.ffmpeg.org/wiki/How%20to%20concatenate%20%28join,%20merge%29%20media%20files for more information.