How to lock the screen in XUbuntu 16.04

Here's the default Xubuntu setup:

• light-locker is set to autostart because it must be running in the background to work
• Keyboard shortcuts for Ctrl+Alt+Del and Ctrl+Alt+L are configured that call xflock4
• xflock4 calls light-locker to lock the screen

If you run xflock4 in a terminal and nothing happens, odds are light-locker isn't running. The solution more or less is to restore the default settings:

1. Remove light-locker-settings package. It's not necessary because screen lock settings are handled by Xfce Power Manager (as you can see in the screenshot you posted)

   sudo apt remove light-locker-settings
   

2. Make sure essential packages are installed (just to be safe)

   sudo apt install light-locker xfce4-power-manager xfce4-session
   

3. Reset light-locker settings

   sudo dpkg-reconfigure light-locker
   

4. Remove any autostart overrides

   rm ~/.config/autostart/light-locker.desktop
   

   (This can be removed because the light-locker package installs a default autostart file here: /etc/xdg/autostart/light-locker.desktop)

5. Reset the LockCommand setting in case it's been overrriden

   xfconf-query -c xfce4-session -p /general/LockCommand -r
   

   (xflock4 is just a shell script that will first look at LockCommand to see what command to run, falling back to light-locker if it isn't set)

6. Reset keyboard shortcuts

   • If you're not worried about losing any other custom keyboard shortcuts, you can just reset them all to default:

     xfconf-query -c xfce4-keyboard-shortcuts -p / -rR
     

   • Otherwise, these are the specific keyboard shortcuts Xubuntu sets by default for screen locking:

     xfconf-query -c xfce4-keyboard-shortcuts -p "/commands/custom/<Primary><Alt>Delete" -s "xflock4" -n -t string
     xfconf-query -c xfce4-keyboard-shortcuts -p "/commands/custom/<Primary><Alt>l" -s "xflock4" -n -t string
     xfconf-query -c xfce4-keyboard-shortcuts -p "/commands/default/<Primary><Alt>Delete" -s "xflock4" -n -t string
     xfconf-query -c xfce4-keyboard-shortcuts -p "/commands/default/<Primary><Alt>l" -s "xflock4" -n -t string
     

7. Log out and log back in. This will make sure light-locker autostart is working and also may be necessary for the keyboard shortcuts to refresh.

8. Make sure light-locker is running

   $ ps -e | grep light-locker
    1869 ?        00:00:00 light-locker
   

9. Make sure xflock4 locks the screen

   xflock4
   

10. Make sure the keyboard shortcuts to lock the screen work

    • Ctrl+Alt+Del
    • Ctrl+Alt+L

11. Lastly, if you want the screen to automatically lock after a certain time

    1. Open the menu > Settings > Power Manager > Security

    2. Automatically lock the session > When the screensaver is activated

    3. If you want to have a real screensaver, you'll need to install and configure xscreensaver or gnome-screensaver. Otherwise if you're okay with a blank screen, go to the Display tab in Power Manager and adjust the times as desired

To lock the screen in xubuntu:

Ctrl+Alt+L

Note: The time os the button matters, try it several times to get it right.

Image: see line xflock4 Ctrl+Alt+L