How do you create a UNIX program?

The same way you create any other program, by designing it and then coding it. A shell program is no different to any other program in terms of how it is created. Its purpose is simply to provide access to the applications and utilities, perhaps organise them in some way, and to present key system components such as a clock or calendar, as well as system notifications from background services. In Microsoft Windows, for instance, Windows Explorer is the default shell program and this provides a desktop, taskbar, start menu and notification area. However, only the first instance of Explorer provides the shell. All other instances of Explorer simply open navigation Windows. But while you cannot replace Windows Explorer itself (it is an integral part of the operating system), you can replace the shell component.