Screensaver

Using Gnome
The command that worked for me:

gnome-screensaver-command --lock

You can

gnome-control-center

and then scroll to Keyboard and click the Shortcuts tab. In my case, I went to Custom Shortcuts, and created one called "Lock my screen" and ran the command above with --lock. After I entered that, I clicked on the right side of the window and hit the keyboard sequence I wanted to use, to have the screen lock come on immediately, requiring a password to use the system.