How to change display manager for Ubuntu, Kubuntu or Debian

Login or display manager is an interface for users to login to their Linux system. There are multiple login managers to choose from and different Linux distribution choose different login manager for the system.

The default display/login manager for Ubuntu/GNOME is GDM. Kubuntu however uses SDDM as the default display/login manager.

If you have multiple display manager installed on your system, you can change the one used when logging in to your system.

1. Launch terminal.
2. Get current display manager.

   $ systemctl status display-manager
   ● gdm.service - GNOME Display Manager
        Loaded: loaded (/lib/systemd/system/gdm.service; static; vendor preset: enabled)
        Active: active (running) since Sat 2020-08-15 07:49:29 +08; 1 day 13h ago
       Process: 11141 ExecReload=/usr/share/gdm/generate-config (code=exited, status=0/SUCCESS)
       Process: 11143 ExecReload=/bin/kill -SIGHUP $MAINPID (code=exited, status=0/SUCCESS)
      Main PID: 919 (gdm3)
         Tasks: 3 (limit: 4624)
        Memory: 7.5M
        CGroup: /system.slice/gdm.service
                └─919 /usr/sbin/gdm3
   
   Ogos 15 07:49:29 host gdm-launch-environment][935]: pam_unix(gdm-launch-environment:session): session opened for user gdm by (uid=0)
   Ogos 15 07:49:45 host gdm-password][1453]: pam_unix(gdm-password:auth): Couldn't open /etc/securetty: No such file or directory
   Ogos 15 07:49:46 host gdm-password][1453]: pam_unix(gdm-password:auth): Couldn't open /etc/securetty: No such file or directory
   Ogos 15 07:49:46 host gdm-password][1453]: gkr-pam: unable to locate daemon control file
   Ogos 15 07:49:46 host gdm-password][1453]: gkr-pam: stashed password to try later in open session
   Ogos 15 07:49:46 host gdm-password][1453]: pam_unix(gdm-password:session): session opened for user user by (uid=0)
   Ogos 15 07:49:47 host gdm-password][1453]: gkr-pam: gnome-keyring-daemon started properly and unlocked keyring
   Ogos 15 07:49:55 host gdm3[919]: Child process -998 was already dead.
   Ogos 16 21:37:24 host systemd[1]: Reloading GNOME Display Manager.
   Ogos 16 21:37:24 host systemd[1]: Reloaded GNOME Display Manager.

3. Use dpkg-reconfigure to configure default desktop manager.

   $ sudo dpkg-reconfigure gdm3
   [sudo] password for user: 

4. Use <tab> key to navigate to the Ok button and press [ENTER].
5. Use arrow key to select your preferred display manager, use <tab> key to navigate to the Ok button and press [ENTER].
6. Reboot your system to start using the newly selected display manager.