How do I switch desktop session users from a shell script?

I've got 2 users, foo and bar. Currently if I'm logged in as foo, I can select bar from the menu in the top-right of Unity, and since bar has no password set, I switch straight to bar. However, I want to have this same result when I run a shell script, rather than having to use the menu. What do I need to put in the shell script to trigger the user switch?