How to Change the Default Display Manager in Ubuntu (GDM3, LightDM, SDDM)

The Display Manager (often called the login manager) is the graphical program responsible for initiating your user session, prompting for account passwords, and launching your desktop environment (such as GNOME, XFCE, or KDE Plasma) upon system boot. While standard Ubuntu Desktop uses GDM3 (GNOME Display Manager) by default, users frequently install alternative desktop flavours or prefer lightweight display managers like LightDM (favoured for its low memory usage) or SDDM (the modern QML-based manager for KDE). Understanding how to reconfigure the default display manager using native Debian/Ubuntu package management tools ensures smooth desktop transitions without breaking display services.

Comparing Popular Ubuntu Display Managers

Depending on your hardware resources and desktop environment, different display managers offer distinct trade-offs:

• GDM3 (GNOME Display Manager): The default Ubuntu login manager. Provides tight integration with GNOME Shell, system lock screens, Wayland session management, and accessibility screen readers. However, it consumes slightly more memory in background states.
• LightDM (Lightweight Display Manager): A fast, memory-efficient, and cross-desktop display manager used in Xubuntu and Lubuntu. It supports extensive theming via greeters (such as the Slick Greeter) and operates exceptionally well on legacy hardware.
• SDDM (Simple Desktop Display Manager): The modern display manager standard for KDE Plasma and LXQt, powered by Qt and QML for highly animated, hardware-accelerated login screens.

How to Check Your Currently Active Display Manager

Before installing new packages or making configuration changes, verify which display manager is currently controlling your graphical session:

cat /etc/X11/default-display-manager

The terminal will output the path to the active executable binary, such as /usr/sbin/gdm3 or /usr/sbin/lightdm. Alternatively, query the systemd display-manager alias:

systemctl status display-manager

Installing an Alternative Display Manager

To switch display managers, ensure the target package is installed on your Ubuntu system:

• To install LightDM:
  sudo apt update && sudo apt install -y lightdm
• To install SDDM:
  sudo apt update && sudo apt install -y sddm
• To install GDM3 (if currently running a different flavour):
  sudo apt update && sudo apt install -y gdm3

During installation, the Debian package manager (debconf) will often pause and present an ncurses dialogue box prompting you to choose the default display manager immediately.

How to Switch the Default Display Manager via dpkg-reconfigure

If you already have multiple display managers installed, you do not need to reinstall them to change which one handles your login screen:

1. Open your terminal and run the debconf reconfiguration utility:

   sudo dpkg-reconfigure gdm3

   (Note: You can pass either gdm3, lightdm, or sddm as the package argument; all target the same underlying display-manager debconf template.)
2. A blue terminal window titled Configuring gdm3 appears. Press Enter to proceed past the explanatory screen.
3. A list of all detected display managers currently installed on your system is displayed (e.g. gdm3, lightdm, sddm).
4. Use the Up and Down arrow keys to highlight your preferred manager, press Tab to select <Ok>, and press Enter.
5. Ubuntu updates the configuration file at /etc/X11/default-display-manager and re-links the systemd display-manager.service symlink automatically.

Applying the New Display Manager

To load the newly selected login screen, you can reboot your system:

sudo reboot

If you are connected via SSH or a secondary virtual console (TTY), you can restart the graphical service without a full system reboot by executing:

sudo systemctl restart display-manager

Warning: Restarting the display-manager service immediately terminates all running graphical applications and active desktop sessions, so ensure all open documents are saved before issuing this command.