In the beginning was the command line. Actually, before that were punch cards and paper tape. But at Multics and RSTS and DTSS came out, programmers and users needed a way to interface with the computer through the teletypes and other terminals that appeared in the early age of interactive computing. Those were often just a program that sat on a filesystem eventually as a daemon, listening for input on keyboards. This was one of the first things the team that built Unix needed, once they had a kernel that could compile. And from the very beginning it was independent of the operating system.

Due to the shell's independence from the underlying operating system, numerous shells have been developed during Unix’s history, albeit only a few have attained widespread use. Shells are also referred to as Command-Line Interpreters (or CLIs), processes commands a user sends from a teletype, then a terminal. This provided a simpler interface for common tasks, rather than interfacing with the underlying C programming. Over the years, a number of shells have come and gone. Some of the most basic and original commands came from Multics, but the shell as we know it today was introduced as the Thompson shell in the first versions of Unix.

Ken Thompson introduced the first Unix shell in 1971 with the Thompson Shell, the ancestor of the shell we still find in /bin/sh. The shell ran in the background and allowed for a concise syntax for redirecting the output of commands to one another. For example, pass the output to a file with > or read input from a file with