Good question, maybe I wasn't precise enough. My understanding is that it's likely a .sh script will run at some point, and that's essentially running using a shell. The shell is essentially a command-line interpreter and is interpreting scripts even if it's running in the background and not visible.
At least, that's my understanding... I think I'm right, but I'm not an expert on everything. The main point was that the shell is crucial to the system even if you never open a Terminal window and use it yourself. If you ripped the shell out of a Linux system, you'd have all kinds of problems.
If there's a Linux expert in here, feel free to correct me on whether the shell can actually be considered to be running in the background or not. I'll hapilly edit the post and fix things if that was a little mistake.