What it mean "User defined session" in window manager

Question

I had this problem: Unable to log into Ubuntu

So I reinstalled gnome, unity, lighdm and gdm. Unfortunately I don't understand much in this field (Xorg, I mean).

Now I have login screen, probably of gdm and I have much options in drop down list in bottom left corner, such as: "User defined session", "Ubuntu", "Gnome" and so on.
When I try "User defined session", window manager hangs on the login screen, but other options are working. So my questions are:

• How I can determine which window manager I use?
• What it mean "User define session" and where are its configuration files?
• Which logs I can monitor for find what the problem with login to "User defined session"?

Answer 1

Usually the so called "Display Manager" (*dm) launches a session. There are predefined sessions (eg, a 'kde' session, a 'gnome' session, a 'xfce' session etc). They all are defined by people who know whats best for you. There might be a point in time when YOU know better. That's what the 'user defined session' is good for: A way for you, the end user, to be in 100% absolute control about what your session consists of, what programs are launched, where, when, why etc.

As soon as you provide a file called ~/.Xsession, make it executable (chmod +x ~/.Xsession) and launch your wanted apps (do not forget the 'window manager') then you have your own, user defined session.

When your 'user defined session' does not work this usually means either you don't have the .Xsession file or it is not executable or it finishes really fast (eg, by not launching a window manager). Sometimes you might find help in the /var/log/Xdm.log or gdm or whatever display manager you are using.