lightdm and desktop session control by command line

Using linux desktop systems (lightdm login manager, openbox, lxde or unity desktops), how can I enter and exit sessions (eg. log in from lightdm and log out from the desktop session) by command line, maybe even over SSH?

For example, I like to log out from an LXDE session and then log into an Unity session on a machine that does not have mouse nor keyboard attached by SSH.