11.10 and above

gnome-session-quit

11.04 and previous versions

dbus-send --session --type=method_call --print-reply --dest=org.gnome.SessionManager /org/gnome/SessionManager org.gnome.SessionManager.Logout uint32:1

(via DoR, see his answer to "Reboot without sudoer privileges?" for more dbus goodness!)

or alternatively, you can use

gnome-session-save --force-logout

--force-logout in contrast to just --logout will not ask the user to deal with unsaved documents and so on.

is this the easiest way? no simple one line command like sudo logout?? I will never remember all that.

Yes, there is a command called logout, but it concerns the Terminal. gnome-session-save is the program that actually quits the gnome-session, which you can of course kill, but that wouldn't qualify as logging out. :-)

Notice as well that these commands don't require you to be root.

You can always add an alias to your system if you want to have a shorter command.

Open ~/.bash_aliases with a text editor, or create it if it isn't there, and add something like this to it:

alias logout-gnome="gnome-session-save --force-logout"

(.bashrc is a script that is run every time a new virtual terminal is started up, you should set up all your permanent aliases there, see also: How to create a permanent "alias"?)

I had installed mate desktop and none of the menus worked, even the f-keys didn't work. I managed to get xterm through browsing with file manager.

The only thing that worked was :

sudo pkill -u username

11.10 and above

Here's my personal solution!

In the terminal, run:

gedit ~/.bash_aliases

And add:

alias log-out="gnome-session-quit"

to the file! Now you just have to run log-out!

Looks like gnome-session-save was renamed to gnome-session-quit for 11.10. Everything else in the main answer should still work with that single change.

http://mail.gnome.org/archives/desktop-devel-list/2011-February/msg00147.html

For gnome sessions, gnome-session-quit works well. By default it asks for confirmation and then logs you out (i.e., the --logout argument is assumed unless overridden with --power-off explicitly). You can also tell the command to not prompt for confirmation on logout:

--no-prompt
      End the session without user interaction. This only works with --logout.

gnome-session-quit is still valid in 12.04.

Since the OP didn't specify the window/desktop manager, and the gnome-session-quit might not work with all possibilities, here's a generic X-windows way to return to the login screen or chooser which I've used several times over the last several releases:

From a terminal (invoked with, variously, 'Alt-F2 + xterm, or Ctrl-T, or Crt-Alt-F1, or by right-clicking on the desktop in Nautilus and using the "open in terminal" shortcut, etc.) type

sudo pkill X

This is generally overkill for most situations. I tend to use it when I've got a hung process or when I've got to log out quickly. When I've done this, Ubuntu has politely returned me to the login screen by re-spawning X-windows.

I've not yet used the power-off feature of the gnome-session-quit, as I typically use shutdown -P now for that purpose.

On Xubuntu (or whenever using an Xfce session), you can logout via the terminal by using

xfce4-session-logout --logout

You must specify --logout or the logout confirmation screen will be displayed. This will work whether you have specified 'Xubuntu session' or 'Xfce session' when you logged in with Lightdm, as the session is still managed by the Xfce session manager.

In addition, another option will log you out as well:

xfce4-session-logout --fast  

If this command is issued the session is not saved, so it is normally best to use xfce4-session-logout --logout.

The ability to logout using the terminal will be very useful to you, as will the other options such as suspend that are available with xfce4-session-logout.

For more information, see man xfce4-session-logout or the Ubuntu manpages online.

You can use below command:

sudo service lightdm restart

For Ubuntu Mate use mate-session-save --force-logout. It is like gnome-session-quit.

lubuntu 12.10 LXDE running on a Samsung netbook (CPU: 1.66 GHZ Intel Atom; RAM: 2GB)

The below two commands logged me out immediately:

sudo pkill -u username
sudo service lightdm restart

In case you don't want to kill your session, you can go back to the login screen using:

dm-tool switch-to-greeter

If you select your user and enter your password, the session will be resumed, so it's not a full logout.