Question

Is it possible to run a script/command when the xfce session stops ?

Answer 1

Change the /usr/bin/xfce4-session executable with a shell script which runs the original xfce4-session and your logout script if xfce4-session finished.

# mv /usr/bin/xfce4-session /usr/bin/xfce4-session.orig

The new /usr/bin/xfce4-session file:

#!/bin/bash
/usr/bin/xfce4-session.orig
echo "my logout script" > /tmp/testfile

Don’t forget to set the execute permissions:

# chmod a+x /usr/bin/xfce4-session

(Tested on Debian Squeeze.)