postrm is in zsh-static 4.3.17-1ubuntu1.
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
case "$1" in
(remove)
remove-shell /bin/zsh-static
;;
esac
|