No Display Manager in Kubuntu 16.04 After Upgrade From Kubuntu 15.10

[Clay Weber]

On Wednesday, June 1, 2016 7:38:14 AM EDT Stephen Morris wrote:
> Hi,
> 
>      After receiving notification that there was an upgrade for Kubuntu
> available (not that I actually have Kubuntu installed, I have Ubuntu
> installed with KDE installed as an addon), I ran a sudo
> do-release-upgrade to undertake the upgrade. This process downloaded
> between 2500 - 2600 packages.
> 
>      After the upgrade, when I boot into the os, the boot process wants
> to start SDDM but the start fails, and every re-attempt to start it
> fails. Using systemctl to attempt to start the service manually fails
> with no meaningful messages, and systemctl status sddm.service doesn't
> show any meaningful messages either. If I issue the command, systemctl
> start lightdm.service, that starts the DM quite happily.
> 
>      I have checked /etc/X11/default-display-manager and that had the
> command /usr/bin/lightdm as the display manager to launch, but that
> location in not the location where lightdm is installed. I have replaced
> this command with /usr/sbin/lightdm, but on a reboot the boot process
> still tries to unsuccessfully load SDDM. I have also noticed that the
> location of SDDM, unlike Lightdm, is actually /usr/bin/sddm.
> 
>      Is anyone able to shed some light on why the 16.04 boot wants to
> always start SDDM even though the default display manager is set to
> lightdm, and also why sddm cannot actually start? Could it be that the
> sddm.service process is looking for sddm in /usr/sbin when it is
> actually in /usr/bin, and if so, would a symlink in /usr/sbin back to
> /usr/bin/sddm rectify the issue?
> 
> regards,
> 
> Steve
My guess, not knowing exactly how you installed Plamsa desktop as an "addon" 
would be that though sddm was installed it may not have been registered or 
activated or whatever with systemd perhaps?  "meaningless" output might 
actually be useful here.

$ sudo systemctl start sddm
shows nothing?

You can try this to register sddm in systemd:

$ sudo systemctl enable sddm.service -f

You can also Try running 

$ sudo dpkg-reconfigure lightdm  ## or sddm

this should bring up a dialog to choose which login manager to use as default.
You should be able to select lightdm which is the default for Ubuntu anyway.


I am not sure if the  /etc/X11/default-display-manager specifies what happens 
when you use the start x manually rather than what the init starts up. 

I have no idea where lightdm should go, but I doubt symlinking probably will 
do nothing useful.

-- 
Clay Weber