postrm is in mono-runtime-common 4.2.1.102+dfsg2-7ubuntu4.
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 | #!/bin/sh -e
if [ "$1" = "remove" ]; then
ldconfig
fi
|