postrm is in liblockfile1 1.09-6ubuntu1.
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 7 8 9 | #!/bin/sh
set -e
if [ "$1" = "remove" ]
then
ldconfig
fi
|