Changing the default desktop on Ubuntu 16.04

14 Dec 2017

In my office at university, I have a desktop machine managed by the IT department, that runs a customised version of Ubuntu 16.04. I have sudo and can install whatever packages I like, but not necessarily without causing problems with the customisations made by IT.

A while back I decided to install i3 instead of using the default desktop, Unity. This worked nicely for a while, and I eventually decided to switch back to Unity. I uninstalled i3 and the other packages I used alongside it. Then I tried to log into my user account on the lightdm login screen, and was greeted by a scary message, saying ‘Failed to execute login command’.

Uh-oh. It seems like the system was able to change my default desktop to i3 when I installed it, but not change it back when I removed it.

After much googling, I found several potential solutions to this:

• Edit ~/.dmrc to list ubuntu as the user’s session

• Edit /etc/lightdm/lightdm.conf to read:

    [SeatDefaults]
    user-session=ubuntu
    ...
  

• Run the following commands:

    sudo dpkg-reconfigure ubuntu-session
    sudo dpkg-reconfigure ubuntu-desktop
    sudo apt install --reinstall ubuntu-session ubuntu-desktop
  

I did all of this, and still got the error. Those commands may have been necessary, I don’t know. What I do know is that after editing /var/lib/AccountsService/users/my-username to say XSession=ubuntu, I was able to log in again properly.

Happy days.