How do I merge two *.avi files into one

There's a dedicated tool to do this, avimerge:

avimerge -o cd.avi -i cd1.avi cd2.avi

If not installed, install transcode: Avimerge is part of transcode package: https://manpages.debian.org/jessie/transcode/avimerge.1.en.html http://manpages.ubuntu.com/manpages/bionic/man1/avimerge.1.html

Easy with mencoder (shipped with mplayer in Arch Linux):

$ mencoder -oac copy -ovc copy video1.avi video2.avi -o final.avi