[fluoridation]

Not always. The shell may be facing a serial interface, or it could be spawned by a program and have no user interface at all.

[MangoToupe]

I probably wouldn't describe either as a human interacting with a shell, but nevertheless point taken.

[fluoridation]

I wouldn't either, which I why I didn't include that interaction in the definition I gave. A shell just implements basic system functionality, it doesn't necessarily function as a UI.