How to change the default display manager in Debian, Ubuntu or Linux Mint (switch to GDM, LightDM, SDDM or LXDM)

When installing a display manager on a Debian-based Linux distribution, you will be asked which display manager to use by default, but for some users, it may not be clear how to change the display manager in the future.

GDM3 (Gnome Display Manager) in Ubuntu 18.10

The display manager or login manager is a program that manages the graphic display server and handles user login. This article explains how to change the display manager from / to GDM (GDM and GDM3), LightDM, SDDM or LXDM. The following instructions apply to any Linux distribution based on Debian and Ubuntu, including Linux Mint, basic OS, etc.
This is useful if your current display manager is having problems and you want to use another display manager, or if you have installed a desktop environment using another display manager by default and want to switch to that environment. on.How to change the default display manager in Debian, Ubuntu or Linux Mint (switch to GDM, LightDM, SDDM or LXDM)

To change the default display manager on Debian, Ubuntu, Linux Mint, basic OS and any Linux distribution based on Debian or Ubuntu, we will use dpkg-reconfigure, This is a tool provided by debconf, you can reconfigure the installed packages by asking configuration questions, just like when installing the package for the first time.
Therefore, to change the default display manager, use the following command:

sudo dpkg-reconfigure some_installed_display_manager

In this command, replace some_installed_display_manager For example, one of the display managers is installed on the system gdm3, lightdm, sddm, lxdmWait. After running this command, it will list all installed display managers, allowing you to choose which display manager to use. If only one display manager is installed, this command will not work.
After selecting the new default display manager, restart the system.
For example, if both LightDM and GDM3 are installed on your system, the command to switch to GDM3 or LightDM may be:

sudo dpkg-reconfigure gdm3

with:

sudo dpkg-reconfigure lightdm

If you want to switch to LightDM or GDM3, either of these two commands will work, because both commands are installed in this example.
For older GDMs (used for a while-for example before Ubuntu 16.04), please use gdm Instead gdm3 In the command.
Similarly, if SDDM or LXDM is installed, you can use the following command to change the display manager:

sudo dpkg-reconfigure sddm

Or

sudo dpkg-reconfigure lxdm

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