preinst is in update-notifier 0.119ubuntu8.1.
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 7 8 9 10 | #!/bin/sh
set -e
# check if we have a unmodified conf-file rm upgrade-notifer
if [ -f /etc/apt/apt.conf.d/99upgrade-notifier ]; then
    if echo "9aff569ff6afcff455d5ba59c0587bab  /etc/apt/apt.conf.d/99upgrade-notifier" | md5sum -c 2>/dev/null; then
        rm -f  /etc/apt/apt.conf.d/99upgrade-notifier
    fi
fi
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