LightDM Display Manager Documentation

LightDM is a lightweight, cross-desktop display manager daemon. It manages the lifecycle of a graphical login session by:

• Running display servers (e.g., X).
• Running greeters to allow users to select user accounts and session types.
• Facilitating authentication via PAM.
• Launching session processes after successful authentication.
• Providing remote graphical login options (incoming via XDMCP and VNC; outgoing via XDMCP and pluggable mechanisms).

LightDM is designed to be cross-desktop, supporting various technologies including X, Wayland, and Mir.

You can test your LightDM configuration and display setup scripts using xephyr to run a nested X server.

1. Install the required testing tool:

   sudo apt install xserver-xephyr

2. Run the LightDM test command as the lightdm user:

   sudo -u lightdm lightdm --test-mode --debug

The core LightDM project does not include a greeter by default. To provide a graphical login interface, you must install a greeter compatible with your desktop environment. Popular options include:

• LightDM GTK+ Greeter: Moderate requirements (GTK+).
• LightDM KDE Greeter: For KDE environments (Qt).
• LXQt Greeter: For LXQt environments (Qt).
• Pantheon Greeter: For elementary OS (GTK+/Clutter).
• Unity Greeter: For Unity.
• WebKit2 Greeter: Allows theming using HTML/CSS/Javascript.
• No Greeter: For automatic login only.

LightDM uses the keyfile format for configuration. Settings are aggregated from multiple files in a specific order of precedence:

1. /usr/share/lightdm/lightdm.conf.d/*.conf (System defaults)
2. /etc/lightdm/lightdm.conf.d/*.conf (Administrator overrides)
3. /etc/lightdm/lightdm.conf (Administrator overrides)

Best Practice: To override system defaults, create a new file in /etc/lightdm/lightdm.conf.d/ rather than editing the main /etc/lightdm/lightdm.conf file directly. Most user-facing settings, such as session selection, should be placed under the [Seat:*] section to apply to all seats.

# Example: Overriding the default session in /etc/lightdm/lightdm.conf.d/50-myconfig.conf

[Seat:*]
user-session=mysession

LightDM can execute an external shell script to configure displays during the setup process. If you implement a custom display setup script, it must adhere to these requirements:

• Location: Must be located under /usr/share.
• Permissions: Must be owned by the user lightdm and group lightdm.
• Output: The script must not print or log to any destination that is not accessible to the lightdm user.

To use a script, add the display-setup-script key to the [Seat:*] section of your configuration.

[Seat:*]
display-setup-script=/usr/share/example_display_setup_script.sh