postinst is in fluxbox 1.3.5-2.
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 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44  | #!/bin/sh
#
#  fluxbox postinst
#
#  Shamelessly ripped off of Marcelo Magallon's script
#     by Brent Fulgham <bfulgham@debian.org>
#
# Ripped off blackbox's postinst, by Matt Hope <dopey@debian.org>
#
set -e
manager=/usr/bin/startfluxbox
add_wm_entry ()
{
    update-alternatives --install /usr/bin/x-window-manager \
        x-window-manager $manager 50 \
          --slave /usr/share/man/man1/x-window-manager.1.gz \
        x-window-manager.1.gz /usr/share/man/man1/fluxbox.1.gz
}
case "$1" in
    configure)
        add_wm_entry
    ;;
    abort-*)
        # how did we get here?  Force a non-zero exit code
        exit 1
    ;;
    *)
    ;;
esac
# Automatically added by dh_installmenu
inst=/etc/menu-methods/fluxbox
if [ -f $inst ]; then
	chmod a+x $inst
	if [ -x "`which update-menus 2>/dev/null`" ]; then
		update-menus
	fi
fi
# End automatically added section
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