postinst is in gnome-session-flashback 3.8.1-7.
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
if [ "$1" = configure ]; then
update-alternatives --install /usr/bin/x-session-manager \
x-session-manager /usr/lib/gnome-panel/gnome-session-flashback 40 \
--slave /usr/share/man/man1/x-session-manager.1.gz \
x-session-manager.1.gz /usr/share/man/man1/gnome-session.1.gz
fi
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