Ubuntu – 18.04 `sudo dpkg-reconfigure lightdm` falls back to gdm3 after unity greeter

18.04desktop-environmentsgdmlightdmunity

Situation

  • Ubuntu 18.04.4 LTS
  • ubuntu-desktop 1.417.4
  • ubuntu-unity-desktop 0.1
  • lightdm 1.26.0-0ubuntu1

Action

Starting off from a GNOME enviroment, I have run sudo dpkg-reconfigure lightdm, chosen lightdm as default, exited with no error message, and rebooted.
Indeed, cat /etc/X11/default-display-manager gives /usr/sbin/lightdm.

Issue

The new greeter is lightdm/Unity-style, but the eventual desktop environment is GMD again (all works in it).
This happens all the time.
The switch to lightdm has been stable, though, in another laptop with the same versions as above.
So, this is not impossible.

Questions

  • Which settings make it so that lightdm/Unity does not proceed beyond the greeter?
  • What should I look into and edit?

Similar cases

Research

  • Looking into /var/log/lightdm/lightdm.log the part where things go gdm is

    [+143.56s] DEBUG: Greeter closed communication channel

    [+143.56s] DEBUG: Session pid=1246: Exited with return value 0
    …

    [+143.56s] DEBUG: Seat seat0: Greeter stopped, running session
    …

    [+143.56s] DEBUG: Session pid=1531: Running command /usr/sbin/lightdm-session env GNOME_SHELL_SESSION_MODE=ubuntu gnome-session –session=ubuntu
    …

    [+143.56s] DEBUG: Session pid=1531: Logging to .xsession-error

  • the line in boldface is contained in /usr/share/xsessions/ubuntu.desktop

    [Desktop Entry]

    Name=Ubuntu

    Comment=This session logs you into Ubuntu

    Exec=env GNOME_SHELL_SESSION_MODE=ubuntu gnome-session –session=ubuntu

    TryExec=gnome-shell

    Type=Application

    DesktopNames=ubuntu:GNOME

    X-Ubuntu-Gettext-Domain=gnome-session-3.0

Best Answer

lightdm and gdm3 are display managers from which you can choose the sessions. You still need to customize your session.

example of a lightdm greeter

From wikipedia:

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example of gdm3 greeter

From techrepublic

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how to choose the sessions

For the lightdm greeter, there is an round icon at the top right corner of the box named "test".
When you click on it, it will show the installed sessions and you can choose which one you would like to use: for example, ubuntu, unity (the one the OP wanted), gnome, etc...

For the gdm3 greeter, there is a cog wheel next to the text "sign in" that works to the same effect.

Also look at https://itsfoss.com/use-unity-ubuntu-17-10/ for a short tutorial or at What happens under the covers to log me in and start up Unity or another Graphical User Interface? for a description of the all process from scratch

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