How to get LightDM automatically started at startup after disabling it (in Ubuntu MATE)?

I am working with Ubuntu MATE (16.04) on a Raspberry Pi 3. I disabled LightDM with

sudo systemctl disable lightdm

On the next boot LightDM did not show up again and instead I just got a terminal asking me for my credentials, what was exactly what I wanted. When I am log in and type

startx

the grapical sessions shows up, so far so good.

Now I want to get LightDM back after boot and so I tried reenabling it with

sudo systemctl enable lightdm

The output is:

Synchronizing state of lightdm.service with SysV init with /lib/systemd/systemd-sysv-install...
Executing /lib/systemd/systemd-sysv-install enable lightdm

But when I now reboot my system, LightDM does not show up again. What went wrong?

In addition to that, when I just log in in terminal and type

sudo systemctl start lightdm

LightDM is started normally, i.e. the graphical session is started and the login screen is shown. How can I get LightDM started automatically after boot again?

Solution 1:

An enabled service should have a symlink to the unit file in /lib/systemd/system in /etc/systemd/system. The systemctl enable command should create that symlink, but in this case it fails for some reason. The correct symlink is:

$ ll /etc/systemd/system/display-manager.service
lrwxrwxrwx 1 root root 35 okt  7 00:20 /etc/systemd/system/display-manager.service -> /lib/systemd/system/lightdm.service

You can fix it by recreating the symlink yourself:

sudo ln -s /lib/systemd/system/lightdm.service /etc/systemd/system/display-manager.service

Tested in Ubuntu MATE 16.04.

Solution 2:

Reconfiguring LightDM worked for me (xubuntu 18.04):

sudo dpkg-reconfigure lightdm

Solution 3:

You have to run sudo systemctl set-default graphical.target because if multi-user.target is set as default you can not use the GUI.