Shell Definition

A shell is a program that provides the traditional, text-only user interface for Linux and other Unix-like operating systems. Its primary function is to read commands that are typed into a console (i.e., an all-text display mode) or terminal window (an all-text window) in a GUI (graphical user interface) and then execute (i.e., run) them.

The term shell derives its name from the fact that it is an outer layer of an operating system. A shell is an interface between the user and the internal parts of the operating system (at the very core of which is the kernel).

A user is in a shell (i.e., interacting with the shell) as soon as that user has logged into the system. A shell is the most fundamental way that a user can interact with the system, and the shell hides the details of the underlying operating system from the user.

A shell prompt, also referred to as a command prompt is a character or set of characters at the start of the command line that indicates that the shell is ready to receive commands. It usually is, or ends with, a dollar sign ($) for ordinary users and a pound sign (#) for the root (i.e., administrative) user. The term command line is sometimes used interchangeably with the shell prompt, because that is where the user enters commands. For example, instructions for performing some activity might say "Enter the following at the command line," which is the same as saying "Enter the following at the shell prompt." However, a command line is not a program but rather just the space to the right of a shell prompt.

Shells, although small in size, are sophisticated and powerful programs that are used for the following: program execution (i.e., launching), substitution of variables and of file names, input/output (I/O), redirection (i.e., sending the output from a program to a destination other than its default destination, including to be used as the input for another program), user environment control (e.g., changing the shell or the shell prompt), and serving as a programming language (i.e., a language that can be used to write shell scripts). Shells in Unix-like operating systems are unusual in that they are both an interactive command language (i.e., a language that a user can employ interactively to issue commands) and a programming language.

On systems with GUIs, many users rarely interact with the shell directly. However, GUIs are merely front ends for shells; that is, they are merely attractive interface layers that are built on top of a shell and which use the shell's commands. As shells are usually more powerful and feature-rich than GUIs, most intermediate and advanced users of Unix-like operating systems find them to be preferable to GUIs for many tasks.

A number of different shells have been developed for Unix-like operating systems. They share many similarities, but there are also some differences with regard to commands, syntax and functions that are important mainly for advanced users. Every Unix-like operating system has at least one built-in shell, and most have several.

sh (the Bourne Shell) is the original UNIX shell, and it is still in widespread use today. Written by Stephen Bourne at Bell Labs in 1974, it is a simple shell with a small size and few features, perhaps the fewest of any shell for a Unix-like operating system. Bell Labs was the research and development arm of AT&T (The American Telephone and Telegraph Company), the former U.S. telecommunications monopoly. The first version of UNIX was developed at Bell Labs in 1969.

Among the functions that most users have come to expect in a shell but which are missing in sh are file name completion, command editing, command history and ease of executing multiple background processes (also referred to as jobs). A process is an instance of an executing program; a background process operates generally unnoticed while users are working with foreground processes. File name completion is the completion by the system of the names of files that have only partially typed in by a user. Command history allows users to conveniently find and reissue previously issued commands. Every Unix-like system contains sh or another shell which incorporates its commands.

bash (Bourne-again shell) is the default shell on Linux. It also runs on nearly every other Unix-like operating system as well, and versions are also available for other operating systems including the Microsoft Windows systems. Bash is a superset of sh (i.e., commands that work in sh also work in bash, but the reverse is not always true), and it has many more commands than sh, making it a powerful tool for advanced users. But it is also intuitive and flexible, and thus it is probably the most suitable shell for beginners. bash was written for the GNU project (whose goal is to develop a complete, Unix-compatible, high performance and entirely free operating system), primarily by Brian Fox and Chet Ramey. Its name is a pun on the name of Steve Bourne.

ash (the Almquist shell) is a clone of sh that was written by Kenneth Almquist in 1989. It is the shell that is the most compliant with sh, and it does not have any additional functions that other interactive shells such as bash and tcsh offer. ash also features small memory requirements compared to the other sh-compliant shells and is thus well suited for systems with small memories, especially small embedded systems (i.e., systems built into other products). It is available on most commercial Unix-like systems.

csh (the C shell) has a syntax that resembles that of the highly popular C programming language (also developed at Bell Labs), and thus it is sometimes preferred by programmers. It was created in 1978 by Bill Joy (who also wrote the vi text editor and later co-founded Sun Microsystems) at the University of California at Berkeley (UCB).

ksh (the Korn shell) is a superset of sh developed by David Korn at Bell Labs in 1983. It contains many features of the C shell as well, including a command history, which was inspired by the requests of Bell Labs users. It also features built-in arithmetic evaluation and advanced scripting capabilities similar to those found in powerful programming languages such as awk, sed and perl.

tcsh (the TENEX C shell) is based on csh but also has programmable file name completion, command line editing, a command history mechanism and other features lacking in csh. It is named after the TENEX operating system, which inspired the author of tcsh. tcsh replaced the csh as the default shell on some BSD operating systems (i.e., FreeBSD and Darwin).

zsh (the Z shell) is similar to ksh, but it also includes many features from csh. That is, it attempts to combine the programmability and syntax of the Korn shell with useful features from the C shell (which has some disadvantages as a programming language). It was written by Paul Falstad around 1990.

The great flexibility of shells on Unix-like operating systems is further enhanced by the fact that it is extremely easy to change the current shell. The shell for any user can be switched temporarily, or that user's default shell can be changed.

Some other families of operating systems also have shells. For example, MS-DOS became the shell on earlier versions of Microsoft Windows. However, it was replaced with a shell emulator on later versions (e.g., Windows 2000 and Windows XP), apparently because of the Microsoft philosophy of attempting to make the GUI all-powerful and de-emphasizing the command line.

Created June 24, 2004. Last updated June 30, 2006.
Copyright © 2004 - 2006 The Linux Information Project. All Rights Reserved.