Shell (computing)

A graphical interface from the late 1980s, which features a TUI window for a man page, a shaped window (oclock) as well as several iconified windows. In the lower right we can see a terminal emulator running a Unix shell, in which the user can type commands as if they were sitting at a terminal.

In computing, a shell is a computer program that exposes an operating system's services to a human user or other programs. In general, operating system shells use either a command-line interface (CLI) or graphical user interface (GUI), depending on a computer's role and particular operation. It is named a shell because it is the outermost layer around the operating system.^[1]^[2]

^ "The Internet's fifth man", Brain scan, The Economist, London: Economist Group, December 13, 2013, Mr Pouzin created a program called RUNCOM that helped users automate tedious and repetitive commands. That program, which he described as a "shell" around the computer's whirring innards, gave inspiration—and a name—to an entire class of software tools, called command-line shells, that still lurk below the surface of modern operating systems.
^ Raymond, Eric S. (ed.). "shell". The Jargon File.

[Economist-1] "The Internet's fifth man", Brain scan, The Economist, London: Economist Group, December 13, 2013, Mr Pouzin created a program called RUNCOM that helped users automate tedious and repetitive commands. That program, which he described as a "shell" around the computer's whirring innards, gave inspiration—and a name—to an entire class of software tools, called command-line shells, that still lurk below the surface of modern operating systems.

[JargonFile-2] Raymond, Eric S. (ed.). "shell". The Jargon File.

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