Comment 9 for bug 1097053

workaround:

  It is possible to use "greeter-setup-script" and "session-setup-script" with lightdm . This allows a user ro run a scrip to pop-up a dialog via zenity to present a information dialog.

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$ cat /etc/lightdm/lightdm.conf
 ...
# greeter-setup-script = Script to run when starting a greeter (runs as root)
# session-setup-script = Script to run when starting a user session (runs as root)
session-setup-script = /usr/bin/motorola

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$ cat /usr/bin/motorola
#!/bin/sh
#
# This is a example of what can be done
zenity --info --text Hi
# kIll gnome/unity session
killall -9 gnome-session

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