prerm is in libfile-rename-perl 0.20-1.
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 -e
if [ "$1" != upgrade ]
then
update-alternatives --remove rename /usr/bin/file-rename
fi
|