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A Unix shell is a command-line interpreter or shell that provides a traditional user interface for the Unix operating system and for Unix-like systems. Users direct the operation of the computer by entering commands as text for a command line interpreter to execute or by creating text scripts of one or more such commands.
The most influential Unix shells have been the Bourne shell and the C shell. The Bourne shell, sh, was written by Stephen Bourne at AT&T as the original Unix command line interpreter; it introduced the basic features common to all the Unix shells, including piping, here documents, command substitution, variables, control structures for condition-testing and looping and filename wildcarding. The language, including the use of a reversed keyword to mark the end of a block, was influenced by ALGOL 68.
The C shell, csh, was written by Bill Joy while a graduate student at University of California, Berkeley. The language, including the control structures and the expression grammar, was modeled on C. The C shell also introduced a large number of features for interactive work, including the history and editing mechanisms, aliases, directory stacks, tilde notation, cdpath, job control and path hashing.
Both shells have been used as coding base and model for many derivative and work-alike shells with extended feature sets.
The most generic sense of the term shell means any program that users employ to type commands. A shell hides the details of the underlying operating system with the shell interface and manages the technical details of the operating system kernel interface, which is the lowest-level, or 'inner-most' component of most operating systems. In Unix-like operating systems users typically have many choices of command-line interpreters for interactive sessions. When a user logs in to the system, a shell program is automatically executed. The login shell may be customized for each user, typically in the
passwd file, and can be customized via
passwd -e, or on some systems via the
chsh program. In addition, a user is typically allowed to execute another shell program interactively.
The Unix shell was unusual when it was introduced. It is both an interactive command language as well as a scripting programming language, and is used by the operating system as the facility to control (shell script) the execution of the system. Shells created for other operating systems than Unix, often provide similar functionality.
On systems with a windowing system, some users may never use the shell directly. On Unix systems, the shell is still the implementation language of system startup scripts, including the program that starts the windowing system, the programs that facilitate access to the Internet, and many other essential functions.
Bourne shell 
The Bourne shell was one of the major shells used in early versions of the Unix operating system and became a de facto standard. It was written by Stephen Bourne at Bell Labs and was first distributed with Version 7 Unix, circa 1977. Every Unix-like system has at least one shell compatible with the Bourne shell. The Bourne shell program name is sh and it is typically located in the Unix file system hierarchy at /bin/sh. On many systems, however, /bin/sh may be a symbolic link or hard link to a compatible, but more feature-rich shell than the Bourne shell. The POSIX standard specifies its standard shell as a strict subset of the Korn shell. From a user's perspective the Bourne shell was immediately recognized when active by its characteristic default command line prompt character, the dollar sign ($).
C shell 
The C shell was developed by Bill Joy for the Berkeley Software Distribution, a line of Unix operating systems derived from Unix and developed at the University of California, Berkeley. It was originally derived from the 6th Edition Unix shell (Thompson shell). Its syntax is modeled after the C programming language. It is used primarily for interactive terminal use, but less frequently for scripting and operating system control. C shell has many interactive commands.
Shell categories 
Bourne shell compatible 
- Bourne shell (sh): Written by Steve Bourne, while at Bell Labs. First distributed with Version 7 Unix, circa 1978, and enhanced over the years.
- Almquist shell (ash): Written as a BSD-licensed replacement for the Bourne Shell; often used in resource-constrained environments. The sh of FreeBSD, NetBSD (and their derivatives) are based on ash that has been enhanced to be POSIX conformant for the occasion.
- Bourne-Again shell (bash): Written as part of the GNU Project to provide a superset of Bourne Shell functionality. This shell can be found installed and is the default interactive shell for users on most GNU/Linux and Mac OS X systems.
- Debian Almquist shell (dash): A modern replacement for ash in Debian and Ubuntu.
- Korn shell (ksh): Written by David Korn, while at Bell Labs.
- Public domain Korn shell (pdksh): Descendant of ksh.
- MirBSD Korn shell (mksh): Descendant of the OpenBSD /bin/ksh and pdksh, developed as part of MirOS BSD.
- Z shell (zsh): A relatively modern shell that is backward compatible with bash.
- Busybox: Tiny utilities for small and embedded systems, include a shell.
C shell compatible 
- C shell (csh): Written by Bill Joy, while at the University of California, Berkeley. First distributed with BSD in 1978.
- TENEX C shell (tcsh)
- Hamilton C shell: Written by Nicole Hamilton, first distributed on OS/2 in 1988 and on Windows since 1992.
Others or exotic 
- es: A functional programming rc-compatible shell written in the mid-1990s.
- Friendly interactive shell (fish): First released in 2005.
- Perl Shell (psh): A shell for Unix-like and Windows operating systems, combining aspects of bash (and other Unix shells) with the power of the Perl scripting language.
- pysh: A special profile of the IPython project, tries to integrate a heavily enhanced Python shell and system shell into a seamless experience.
- rc: The default shell on Plan 9 from Bell Labs and Version 10 Unix written by Tom Duff. Ports have been made to various Unix-like operating systems.
- scsh, a Scheme Shell.
- wish: A windowing shell for Tcl/Tk.
- zoidberg: A modular Perl shell written, configured, and operated entirely in Perl.
A list of various shells may be found at www.freebsd.org.
- Thompson shell (sh): The first Unix shell, written by Ken Thompson at Bell Labs. Distributed with Versions 1 through 6 of Unix, from 1971 to 1975. Considered very rudimentary by modern standards and not used on current systems, though available as part of some Ancient UNIX Systems.
- PWB shell or Mashey shell (sh): A version of the Thompson shell, augmented by John Mashey and others, while at Bell Labs. Distributed with the Programmer's Workbench UNIX, circa 1975-1977.
Configuration files for shells 
Shells read configuration files on multiple circumstances which differ depending on the shell. These files usually contain commands for that particular shell and are executed when loaded. These files are usually used to set important variables like $PATH used to find executables, and others that control the behavior and appearance of the shell. This table shows the configuration files for popular shells:
- blank means a file is not read by a shell at all.
- "yes" means a file is always read by a shell upon startup.
- "login" means a file is read if the shell is a login shell.
- "n/login" means a file is read if the shell is not a login shell.
- "int." means a file is read if the shell is interactive.
See also 
- Comparison of command shells
- List of Unix programs
- Restricted shell
- Shell (computing)
- Shell account
- Shell script
- Shell shoveling
- Re: Late Bloomers Revisited USENET post to comp.lang.misc by Piercarlo "Peter" Grandi, Dept of CS, UCW Aberystwyth, UK, Dec 17, 1989.
- Harley Hahn, Harley Hahn's Guide to Unix and Linux.
- Hamilton C shell for Windows Release Notes 4.0, retrieved June 19, 2010.
- only if
- SCO Unix Group, SCO Unixware 7 documentation, 22 Apr 2004, retrieved 18 Oct 2012.
- in fact, the first readable of
~/.profile; and only
~/.profileif invoked as
Further reading 
- Ellie Quigley (2001). "Introduction to UNIX shells". Unix Shells by Example. Prentice Hall PTR. ISBN 013066538X. — a history of the various shells, and the uses of and responsibilities of a shell on Unix
- Introduction to the Unix shell history (Softpanorama)
- Sven Mascheck. The Traditional Bourne Shell Family History and Development. Version 2009-02-12; Last accessed March 6, 2009
- Bourne shell, or sh (interview with Steve Bourne, Computerworld, March 5, 2009)
- Linux Shell Scripting Tutorial - A Beginner's handbook
- Table of Different UNIX Shell