postrm is in update-notifier-common 3.168.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | #!/bin/sh
set -e
dpkg-maintscript-helper rm_conffile /etc/update-motd.d/20-cpu-checker \
0.110.1ubuntu1 -- "$@"
|