Lock the Mac Desktop from the Command Line
With the help of a buried menu item, we can lock the Mac OS X screen right from the Terminal. This does not log a user out, it just brings up the standard Mac OS X lock screen and login window, requiring a valid user and password before the Mac can be used again.
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.
For those wondering, 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.