A Detective Puzzle! Solve at your own will~ Puzzle 5

Walking backwards had worked! The traps were set off and I had reached the front door. When I had opened the door, it wasn't locked at all. Strange. When I walked in, I was met with 5 halls, and each hall had  10 doors and each door had a different color. Deciding that since this "Mystery Murderer" was probably in one of the left halls [Since the right ones were all locked up], there were was only two halls left, and 20 doors. I quickly deducted my decision to take the right hall as I saw the various booby traps and the tigers hiding in the cages behind those shadows. I quickly took a left turn.

As I walked into the 1st hall, there were 10 doors in the hall. There looked all the same, black & white stripes. The only difference that made it was their door knobs. They were all shaped different. One was a circle, one was a triangle, another was square and so on. There was a note beside the first door. As I opened it, it was written as below:

" I come in all shapes and sizes. I am so shy until I have to hide all my faces. There are no corners where I am. Turn my knob and meet me!" 

Which shape knob do I have to turn now?