Here is a quick tip to disable your login manager like GDM, KDM or XDM from starting up automatically upon system boot. Many times you need to boot your Linux system just to a console based mode and ideally you would like to start the X server (Graphics/Login manager) only when you need it.
Disable login/graphics manager
To disable the login manager on your Debian/Ubuntu system (which in turns prevents X server to start automatically) give the following command:
# update-rc.d -f gdm remove
Now next time when you reboot your system, it will just to a plain console mode and now if you want to start your graphics manager all you need to do is simply give the following command as root depending on the type of login manager that you have:
# /etc/init.d/gdm start
# /etc/init.d/kdm start
# /etc/init.d/xdm start
To switch back to console mode from graphics mode hit:
and now to switch back to graphics mode hit:
Enable/Restore login manager
Let’s say you want to restore your system just like it was before i.e. Linux will boot into graphics mode automatically as it normally does. To do that give the following command:
# update-rc.d gdm defaults
That’s it. Now when you reboot your system next time it will start one of your login manager.