You need door knobs for closet doors in a hallway, but these doors don't necessarily need a knob on the interior, right? Closet doors are typically done in two different ways.  width= Ball Catch 1 - Ball Catch Closure - Many closets have a spring loaded ball in a cylinder at the top of the door that catches into a stike to hold the door closed. If you have this type of catch, you wouldn't need a bore hole in your door. In this case you can use a single dummy mounted to the surface of the door.                       Spring Loaded Latch 2- Spring Loaded Latch (more common) - If your closet door uses a spring loaded latch that catches into a strike on the jamb (just like any other door to the house or room) you will need to install a passage function knob or lever. They do not make a single sided working knob or lever (in residential grade) so you just use a passage set and the inside knob or lever just isn't ever used."