Ubuntu Diskless 14.04 LTS : Ubuntu/Unity session start by "Xsession" script (in fact : $HOME/.xsession).

[Vincent FARGET]

Hello,

I use Ubuntu Trusty (14.04 LTS) in Diskless mode (system mounted in
read-only by NFSv4).
(https://help.ubuntu.com/community/DisklessUbuntuHowto)

I use LightDM as the DM.
I have added a "session" (file ".desktop" in the "/usr/share/xsession/")
named "user", where the value of the "Exec" attribute is (the script) :
"/etc/X11/Xsession".
This "user" session is the one selected by default each time Ubuntu
Diskless boot.
In my personal directory, I have a file (executable bash script)
".xsession" in which I put the binary corresponding to the last session
chosen (KDE, GNOME, GNOME Flashback (Compiz), Flashback GNOME
(Metacity), LXDE , MATE, Openbox, Xfce and Ubuntu/Unity).

In summary, my personal file ".xsession" performs the same as the value
of the "Exec" attribute mentioned in the files ".desktop" of each of the
sessions mentioned above.

Everything is ok EXCEPT for Ubuntu/Unity where "/usr/bin/gnome-session
--session=ubuntu" does not work when call from the ".xsession".

If I choose the session "Ubuntu" at the LightDM login greeter, Unity
opens well (fully with the side icon bar and applets on the top right),
but if I leave the "user" session (at the LightDM login greeter) and if
my ".xsession" contains "/usr/bin/gnome-session --session=ubuntu", then
I have a desktop without Ubuntu/Unity (no lateral bar icon, no applet:
date, keyboard, sound settings, stop, etc ...).

Questions:
Is it normal that I do not have the same things in the two situations
described just above ?
If so, what do I miss in my ".xsession" ?

Thank you in advance for your help.
Best regards.
-- 
-
M. FARGET Vincent