It's not completely done yet, right? Because it's not giving any feedback to the user (from our code).

On of the big issues with bringing WordPress to the command line arises here: the switch_theme function is not returning a state and might be echo-ing output directly. This is very annoying and we might need to think about a proper way to solve it or to rewrite certain functions in WordPress to have a better programming interface.

It's not completely done yet, right? Because it's not giving any feedback to the user (from our code).

Unlike wp theme status, you don't use wp theme activate to get some information, but to execute a certain task. As such, the command shouldn't output anything, unless something went wrong.

A good read on this subject: http://www.antoarts.com/designing-command-line-interfaces/

One of the big issues with bringing WordPress to the command line arises here: the switch_theme function is not returning a state and might be echo-ing output directly.

This particular function isn't supposed to echo anything. We could use output buffering to catch error notices and send them to STDERR instead of STDOUT, but I don't think it's necessary in this case.

One thing that should be checked is if the parent theme exists.

Ok, cool Had a lot of troubles with this when making the core command ;-)