postinst is in icewm-experimental 1.3.7-1ubuntu1.
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 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  | #! /bin/sh -e
case "$0" in
    *experimental.postinst)
    FLAVOR=-experimental
    PRIO=60
    PRIOS=11
    ;;
    *lite.postinst)
    FLAVOR=-lite
    PRIO=40
    PRIOS=9
    ;;
    *)
    FLAVOR=""
    PRIO=50
    PRIOS=10
    ;;
esac
# Automatically added by dh_installmenu
if [ "$1" = "configure" ] && [ -x "`which update-menus 2>/dev/null`" ]; then
	update-menus
fi
# End automatically added section
if [ "$1" = "configure" ]; then
   update-alternatives --install /usr/bin/x-window-manager \
   x-window-manager /usr/bin/icewm$FLAVOR $PRIO \
   --slave /usr/share/man/man1/x-window-manager.1.gz x-window-manager.1.gz \
   /usr/share/man/man1/icewm$FLAVOR.1x.gz
  update-alternatives --install /usr/bin/x-session-manager \
  x-session-manager /usr/bin/icewm-session$FLAVOR $PRIOS
fi
exit 0
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