How to stop X server in Ubuntu

To stop X in ubuntu switch to a virtual terminal (Ctrl+Alt+F1), log in, and issue the following command:

(Gnome)
sudo stop gdm OR
sudo /etc/init.d/gdm stop OR
sudo service gdm stop

To start X again, issue:
sudo start gdm  OR
sudo /etc/init.d/gdm start OR
sudo service gdm start

(KDE)
sudo /etc/init.d/kdm stop
sudo /etc/init.d/kdm start

(Lightdm)
sudo stop lightdm OR
sudo service lightdm stop

Sometimes you may need to issue
Ctrl+Alt+F7 to go back to graphical interface.
Ctlr+Alt+F1 to go to terminal mode