The way I look at this, the UI would be only to speed things up / simplify things a little bit. It wouldn't try to do full error validation before execution. It

The way I look at this, the UI would be only to speed things up / simplify things a little bit. It wouldn't try to do full error validation before execution. It's the job of a command-line app to fail with an error if wrong combination of flags or inputs was provided, and the UI might display that error in a pretty generic way.