Disable display manager on boot

On robots that is normally without a screen, there is no need to run a display manager and a graphic user interface.

This will save some memory and start less processes.

inspect /etc/init there is acnfguration entry or the installed display manager, e.g.

gdm.conf or
kdm.conf

To start the display manager and graphic user interface when needed only, make a manual override file for the display manager:

sudo bash
echo "manual" > /etc/init/gdm.override
echo "manual" > /etc/init/kdm.override
exit

After reboot, there will be a text login only. To (re)start the display manager (graphic login screen) use the command

sudo gdm     or
sudo kdm