Open Terminal and enter the following in a single line:
/System/Library/CoreServices/Menu\ Extras/User.menu/Contents/Resources/CGSession -suspend
There isn’t any confirmation, the desktop is immediately locked and the lock screen appears regardless of what is currently going on with the active user account.
The menu item being used is the same Fast User Switching menu that displays a user name in the upper right corner, and the lock screen shown is identical to what is summoned if one were to select “Login Window…” from that same menu.
You can lock a screen using a keyboard shortcut as well, but using the command line offers two obvious advantages; it can be included within scripts or entered from SSH to remotely lock a Mac.
Source : Lock the Mac Desktop from the Command Line