Tag Archives: gdm

Restart graphical user interface in Linux

To help you better understand how to restart the GUI in Linux, here are some background information about how the GUI works in Linux:

If you are familiar with Windows, you know that the graphical Windows interface is tightly integrated into Windows. If you start a Windows computer, you automatically start the graphical user interface (GUI). First the Windows logo is twirling around, and then you are presented with the login screen. Only recent server versions of Windows have the option to be installed without graphical user interface. It is called "Server Core", and even in this mode Windows displays a very basic graphical surface that has only a terminal window. This desktopless installation option was a big deal for Windows, as the GUI is so tightly integrated into the operating system. Also there was no adequate command line interface to control and configure all Windows functions. Therefore Microsoft put great effort into making the PowerShell just as "powerful" as the graphical Windows interface.

Linux follows a completely different approach. By default if you start Linux, only the basic command line interface is started. You are presented with a text login screen, and after you login you see the command prompt of your shell in text mode.

The GUI in Linux is built on top of this minimal system. It is made up of a couple of processes that are launched by the root user and the user that is currently logged in. The starting point for the GUI is a program called the "display manager". The display manager starts the display server and presents you with a graphical login screen. After successful login it starts your desktop environment and your programs like web browser or email client.

So here is the chain of events that start your Linux desktop:

 NameDescriptionExamples
1display managerStarts x-server and displays graphical login screen. After successful login, the desktop environment will be startedGDM, LightDM, KDM, SDDM, etc.
2desktop environmentStarts its window manager and runs user programs.Gnome, Unity, KDE, etc.
3window managerDetermines the look-and-feel of your GUI and is responsible for any 2D / 3D effects.Mutter, Compiz, KWin, etc.

The display server is the actual component that is responsible for drawing pixels on the screen and communicates with graphics card, mouse and keyboard under a graphical environment. Therefore it needs to be present both before and after graphical login. Notice that this is not the case for the desktop environment nor the window manager. They are only started after successful login. As of today the by far most popular display server is the x-server (also known as X11 or x.org server), although it will probably be replaced soon by other display servers like Wayland or Mir. When it comes to 2D- and 3D effects, this is performed by the window manager.

While there is a tight integration of desktop environment and window manager, you can use any display manager to start your favorite desktop environment. Usually there is an option on the login screen to tell the display manager which desktop environment to start after successful login. That might be useful if you accidentally installed and activated another login manager. Let's say you are running Gnome with the GDM display manager. After installing a KDE program, the dependencies of the program package pulled in the whole QT libraries along with the KDM display manager. The next time you login you are presented with the KDM display manager. This is actually no big deal. Just select the GNOME desktop environment from the options menu of KDM. After typing in username and password GNOME will be started.

Here is a list of desktop environments with their default display manager:

Desktop EnvironmentDefault Display Manager
UnityLightDM
GNOMEGDM
KDEKDM (obsolete) / SDDM

How does Linux know which display manager to start if there are more than one installed?
The file /etc/X11/default-display-manager contains the full path of the default display manager to start.

Back to the original question: How do you restart the graphical user interface of Linux without rebooting the whole computer? The answer is easy: Just restart the display manager.

As we have seen earlier, the display manager is actually the starting point for the whole GUI. By shutting down the display manager, all graphical processes will be stopped too. Here is an example how to stop SDDM (run as root):

$ sudo killall sddm

SDDM will be restarted automatically. With other display managers you may have to start them manually as root.

There is also a way to bypass the display manager. If you are already logged in as a regular user in text mode, you can also start the desktop environment manually. E.g. for KDE you would type in:

$ /usr/bin/startkde

Always make sure to start the desktop environment as a non-privileged user, not as root. Otherwise all programs will be running with root privileges, which is something you should avoid at all costs. If you want to start individual programs with root privileges (e.g. WireShark), there are tools like "kdesudo" that launch a program under the root user.

Important things to note:

  • Unlike Windows there is a strict separation of the graphical user interface and the basic system in Linux.
  • The display manager is started by the root user. It is the entry point for the graphical user interface.
  • The desktop environment is run by a non-privileged user, typically the user that logs into the display manager.
  • In general you can use any display manager to start any desktop environment. You can use GDM to start KDE, or SDDM to start GNOME.
  • To restart the graphical user interface, you need to restart the display manager.
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