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Is the Login handled by a desktop manager? If so, what happens if I install another desktop manager (which one will provide the login?) and how to control which one is used?

Whenever I login it looks like the system switches between desktop managers or something: Sometimes I see a background set some time ago, sometimes with monitors in wrong order (left and right monitor switched), then login shows up, but frozen for some seconds with messed up backgound, then refresh and for example background is fixed, and when I finally login using some desktop manager (e.g. cinnamon) then the display has some other graphical bugs which look like switchting back and forth before the desktop manager is finally loaded. (To be honest: I never had a Ubuntu experience without some graphic bugs every now and then in years)

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In short:

  • to have working graphical UI you need X server and window manager
  • you can set up X to just start window manager, you choose, without presenting login screen, but then you will be logged in as the user, that runs X (so in most cases iirc root)
  • if you want to be able to log in as different user you need Display Manager
  • (disclaimer: I haven't dabble into it for some time, so don't know if what I say applies to Unity and Cinnamon) most of desktop environments provide their own display manager (XFCE - xdm, KDE/plasma - kdm, old Gnome - GDM)
  • if you use display manager, it's where you choose what session to run, and only after you provide correct credentials, chosen desktop environment is started
  • it is possible to use KDM and choose XFCE session and it will work ... well, in most cases at least, that is why, most desktop managers have their own DM, which they prefer and will install it by default (it boils down to each window manager/desktop environment requiring some set of variables to be set, or certain services to be started, before user can interact with the actual UI, that's why it's better to use native DM for given environment)
  • if you want to try out different desktop environment, it's generally better to install a distro that comes with it by default (f.ex. Kubuntu, Xubuntu, Ubuntu Gnome, etc.) that to install meta packages like xubuntu-desktop or kubuntu-desktop iirc
  • there are also stand-alone login managers (f.ex SLIM - simple login manager)

Glossary:

  • Window manager, Desktop Environment, Desktop manager - I'm using those interchangeable, I know, that from purist point of view they are not the same, but here, generally refer to your Desktop - be it Unity, KDE, Gnome, Fluxbox, or whatever else :)
  • Display Manager, Login manager - KDM, GDM, XDM, SLiM, etc
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