Execute script before shutting down

You could add your command to the file /etc/gdm/PostSession/Default before the exit 0 line.

You can do this by creating a script in /etc/rc0.d (for shutdown) and /etc/rc6.d (for reboot). To do this, run:

gksu gedit /etc/rc0.d/K01mycustomscript

Edit your shutdown script in the text editor then run:

sudo chmod +x /etc/rc0.d/K01mycustomscript

And if you want it to run on reboot also then run this:

sudo cp /etc/rc0.d/K01mycustomscript /etc/rc6.d/K01mycustomscript

Source: this forum thread

Sounds like a question for superuser.com to me. Anyway, after some Googling I found How to run a script on logout? which says to add the script to $HOME/.bash_logout.

If that doesn't work, add

$HOME/.bash_logout

to /etc/gdm/PostSession/Default so it executes the logout script properly.

Seeing as before shutdown the user is logged off anyway, this should cover both bases.