How can I logout from the GUI using CLI?

This command will log out whichout showing logout prompt and take you to the log in window gnome-session-quit --no-prompt
To power off you can use this command this will open a dialog box to conform power off
gnome-session-quit --power-off
You can get rid of dialog box using --no-prompt

If you are using Openbox then you can use openbox --exit which will exit the Openbox session and go back to the login manager, in my case LightDM.

In modern systemd Linux distros, the answers are all a little too complicated. The solution is one tool: loginctl.

In a good shell you even have autocompletion, so make use of Tab to see the options and parameters and it is quite intuitive. The command to search for is kill-session.

If you tab, you'll notice each session has an ID, but in my case it also showed the username and TTY (that is the Ctrl+Alt+number you type) and the seat.

Here is how it looks to me, e.g.:

$ loginctl kill-session 10  
10  -- 1000 rugk seat0 tty2

You can tab through the sessions to find the correct one.

Otherwise, if that does not work you can find the session ID by running loginctl list-sessions or just loginctl. You get something like this:

$ loginctl list-sessions                            
SESSION  UID USER SEAT  TTY 
    10 1000 rugk seat0 tty2

I guess it's quite obvious the first column contains the session ID you need to pass to loginctl kill-session.

This works very well if the GUI hangs and you need to force-kill it, which seems to be your use case.

If you want that to be explained in a more elaborate way here is how you can kill your own session if the GUI is not responding or you cannot use your keyboard.

As an alternative, you can terminate user sessions using the following, works well to log out users except for the root user- when doing maintenance for example.

loginctl | egrep -v "root|SESSION|listed" | awk '{print $1}' | xargs loginctl terminate-session

This is what works best for me (with xfce, lightdm and ssh):

$ DISPLAY=:0.0 xfce4-session-logout --logout

Or

$ sudo service lightdm restart

The real problem is that the DBUS session variables must be set and match the session you're trying to control.

I've created this script that does set the DBUS session variables from the gnome-session environment in case you want to logout other users/sessions:

How to restart Gnome-Shell from command line?

function logout() {
    local USERNAME
    export USERNAMES=( ) 
    while [ -n "$1" ]; do case "$1" in
        -* ) break ;;
        *) USERNAMES+=( "$1" ); shift ;;
    esac; done

    for USERNAME in "${USERNAMES[@]}"; do
        local SESSION_PID=$(pgrep -fu "$USERNAME" gnome-session|head -1)
        if [ -n "$SESSION_PID" ]; then
            (
                sudo -u "$USERNAME" cat "/proc/$SESSION_PID/environ" | xargs -0 -n 1 echo export
                echo "gnome-session-quit --logout $@"
            ) | sudo -u "$USERNAME" sh -;
        fi
    done
}

If you are in a xubuntu session or similar, must use xfce4-session-logout insted of gnome session commands, that's why you see warnings

Another way,

sudo pkill -u NameOftheUser

or

sudo pkill x

which kill all users.

To end all user processes and be sent back to the login screen, you can use:

kill -9 -1

Don't run it as root though, for reasons discussed here.

Please follow takkat's suggestion. The standard is Ctrl+Alt+Backspace.

You can also run:

$ sudo service lightdm restart

This can be done using the gnome-session-quit command. It needs the --force option to suppress the confirmation dialog that would appear without it.

Unlike applications run from an X terminal emulator, ending a session from a TTY requires you to append the DISPLAY variable to indicate which X display is running the session. Hence:

DISPLAY=:0 gnome-session-quit --force

assuming that you are running GNOME on :0, which is the case in normal situations.

• In Ubuntu 12.04LTS running GNOME, the command

  "DISPLAY=:0 gnome-session-quit --logout --no-prompt" 
  

  works. The "--force" argument doesn't exist in the current update level]

You can use the command killall gnome-session to log out. This will work for all GNOME sessions and if I remember correctly all GNOME-related ones. It takes you right back to LightDM so you can select a new DE or a new user. :)