The LightDM display manager provides dm-tool to allow us to lock the screen from the command line
$ dm-tool lock
On Ubuntu 13.10 (and others), running this command locks the screen
and displays the display manager’s login screen. If we use
Ctl-Alt-F7 to get back to our desktop we will see a lock
screen login prompt. Entering our password there or on the display
manager’s login screen will unlock our screen.
Issue Lubuntu 13.10  Whist the display manager
greeter is displayed,
Ctl-Alt-F7 takes us back to an unlocked
desktop! This can be quite a security issue until fixed. See a
A similar command will also lock the screen and switch to the display manager’s login screen.
$ dm-tool switch-to-greeter
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