A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting language, and is used by the operating system to control the execution of the system using shell scripts.

Linux has hundreds of different distributions. UNIX has variants (Linux is actually a UNIX variant based somewhat on Minix, which is a UNIX variant) but the proper versions of the UNIX system are much smaller in number.