In UNIX, multiple layers of interaction occur between the computer hardware and the user.

The first important layer is the kernel, which runs on actual machine hardware and manages all the interaction with the hardware.

All applications and commands in UNIX interact with the kernel rather than the hardware directly, and they make up the second layer.

Command-interpreter program, i.e. the shell forms the third layer. It manages the interaction between applications and the available UNIX commands and the users. Most UNIX commands are separate programs distinct from the kernel.

A final layer, which may or may not be present on all system, is a windowing system. The windowing system usually interacts with the shell, but it can also interact directly with the applications.

The topmost “layer” is us - the user. We interact with the entire operating system using command interpreter i.e. the shell, or through a combination of the shell and the window system. The figure below gives a simple representation of the layers of UNIX.