A shell is a user interface which provides access to an operating system’s services. There are two basic types of shells: a “command-line interface” (CLI) and a “graphical user interface” (GUI). You are likely familiar with common GUI’s from Microsoft Windows and perhaps Apple’s macOS. This document will focus on a CLI known as bash, a shell for the Unix operating system by Brian Fox. The shell is an acronym for Bourne-again shell, named for the Bourne shell that it replaces.