preinst is in mythbuntu-lightdm-theme 1.01.
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 10 11 12 13 14 15 16 17 18 | #! /bin/sh
set -e
case "$1" in
upgrade)
if dpkg --compare-versions "$2" lt "1.01"; then
d=/etc/lightdm
f=lightdm-gtk-greeter.conf
[ -L $d/$f ] && rm $d/$f
dpkg-divert --package mythbuntu-default-settings --remove \
--rename --divert $d/$f.diverted $d/$f
rm -f $d/$f
ln -s /etc/alternatives/lightdm-gtk-greeter-config-derivative \
/etc/lightdm/lightdm-gtk-greeter.conf
fi
;;
esac
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