LightDM

Revision 5 as of 2011-06-02 06:44:47

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What is LightDM?

LightDM is a display manager. The most user visible aspect of the display manager is the login screen, however it also manages the X servers and facilitates remote logins using the XDMCP protocol. It is proposed to be the display manager in Ubuntu 11.10 (Oneric) replacing GDM which has been the display manager since the beginning. See the blueprint for more details.

The switch will mean two main things for Ubuntu users:

  1. There will be a new login theme that better matches Unity. This is being designed and developed during the Oneric cycle. Until then a simple GTK login screen will be used.
  2. There will be configuration changes for system administrators. Your feedback on how LightDM compares to GDM is very important, please file bugs and help out if there are features that are important to you.

Testing LightDM

Oneric is scheduled to use LightDM by default in Alpha 2 but before that you can test by installing it from Universe:.

$ sudo apt-get install lightdm lightdm-greeter-example-gtk

If using Ubuntu 11.04 (Natty) then use the the LightDM PPA:

$ sudo apt-add-repository ppa:lightdm-team/pp
$ sudo apt-get install lightdm lightdm-greeter-example-gtk

When installing the package you will be prompted to choose your default display manager. You can either run LightDM by default or on demand.

Running LightDM by default

If you have LightDM as your default display manager then it will just be there when you boot! To switch the default run:

$ sudo dpkg-reconfigure lightdm

Running LightDM on demand

If you are using another display manager by default (e.g. GDM) then you will have to stop that and run LightDM on demand. Log out of any graphical sessions, go to a text terminal (alt-ctrl-F1) then switch display managers with the following commands:

$ sudo stop gdm
$ sudo start lightdm

Running LightDM in test mode

You can also run LightDM in a test mode to try out the greeters (you cannot log into any accounts except your own). You do not need root access to do this. Open a terminal and enter:

$ lightdm --test-mode

Debugging LightDM

LightDM writes verbose logs to /var/log/lightdm. Please check these if there appears to be a problem. If you are running it from the command line (e.g. in test mode) you can also get debugging information by running with the --debug flag.

What to do if things go wrong

If things go horribly wrong (we promise to try very hard to avoid this) you may need switch back to GDM. Some tricks to do this:

  • If you have no graphics, go to a text terminal using alt-ctrl-F1
  • Stop LightDM with sudo stop lightdm

  • Start GDM with sudo start gdm

  • Run sudo dpkg-reconfigure lightdm to set the default display manager

  • Edit /etc/X11/default-display-manager and set it to /usr/sbin/gdm if you can't run the above
  • Uninstall LightDM and GDM will replace it after a reboot
  • If things go really bad, you may need to remote login using SSH or by using grub to enter a recovery mode (It should never get this bad and if it does then blame X).

Reporting problems, feature requests

Use the Launchpad project and file bugs please.

Developing greeters

Install liblightdm-gobject-0-dev or liblightdm-qt-0-dev for the LightDM greeter API. Look at the LightDM source for example greeters in greeters/.