It's not my problem but someone has posted this in an other non-English forum and the problem made me very curious as I would love to know how linux handles each aspect

The user has installed Ubuntu 11.10, also he installed some GNOME, KDE and two more DEs
He has enabled the root account and set a password for it
Then he tried to log into the system using different DEs
Each time he tries that he ends up with the default unity DE!
http://i41.tinypic.com/2l95i8m.jpg

Any idea how linux handles the process? I am not interested in solving the problem, I am more interested in the mechanism and the priorities
Thanks

Open the login menu, there should be an icon near your name, currently a gear, and choose anyone from them.

That's right, this icon should modify the dmrc file in the home directory ie
Well the above does not work with the root user!!! why? any idea how to fix that? is it a bug?

Ubuntu was never designed to have root account enabled, therefore it is Not A Bug.

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Like I said above, I more interested the mechanism I couldn't care less about it being a bug or a feature

I think this feature got something to do with the permissions! maybe that icon has no enough privileges to edit the dmrc file inside the root home directory! Is that possible?

No, such is not the case. While logging into the system, from there also you can choose Desktop Environment. Right?

How do I switch to a certain DE while I am logged in an other one?

Just restart your machine and hop in on the login window, search there you'll find definitely.

Why restart the machine?
Just log out and you should be greeted with the Login prompt and DE chooser.

Guys r u talking about the same icon? that gear?

Yes, or something like it. There is usually a way to change your DE.

Ohhhh, right right!

This is getting crazier! and now I am obsessed with finding the solution
I have manually modified the dmrc file at /root to
However I ended up with unity again! I would love to know what component is checking on the root and stopping it from loading any DEs but the default one!
The rest of the users can easily switch between different DEs

There is a button on the gdm screen to allow you to select the session (It is the gear in 11.10). It will be the gear icon in 11.10, but I do recall one that had a little text box in a different version of Ubuntu. Select your session from that. You can even set that as default.

I don't think you are following me! maybe its my clumsy English!
Basically now matter what DE I choose the system logs me into unity, that simple! any ideas?