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shell (computing)

In computing, a shell is a piece of software that provides an interface for users. Typically, the term refers to an operating system shell which provides access to the services of a kernel. However, the term is also applied very loosely to applications and may include any software that is "built around" a particular component, such as web browsers and email clients that are "shells" for HTML rendering engines. The name 'shell' originates from shells being an outer layer of interface between the user and the innards of the operating system (the kernel).

Operating system shells generally fall into one of two categories: command line and graphical. Command line shells provide a command line interface (CLI) to the operating system, while graphical shells provide a graphical user interface (GUI). In either category the primary purpose of the shell is to invoke or "launch" another program; however, shells frequently have additional capabilities such as viewing the contents of directories.

The relative merits of CLI- and GUI-based shells are often debated. CLI proponents claim that certain operations can be performed much faster under CLI shells than under GUI shells (such as moving files, for example). However, GUI proponents advocate the comparative usability and simplicity of GUI shells. The best choice is often determined by the way in which a computer will be used. On a server mainly used for data transfers and processing with expert administration, a CLI is likely to be the best choice. On the other hand, a GUI would be more appropriate for a computer to be used for image or video editing and the development of the above data.

Unix shells

Notable historic or popular Unix shells include:

Bourne shell (sh)

* Almquist shell (ash)

** Debian Almquist shell (dash)

* Bourne-Again shell (bash)

* Friendly interactive shell (fish)

* Korn shell (ksh)

C shell (csh)

* TENEX C shell (tcsh)

Es shell (es)

esh (Unix) – Easy Shell

rc shell (rc) – shell for Plan 9 and Unix

runscript – The initial shell interpreter used to process startup scripts in Gentoo

scsh (Scheme Shell)

Stand-alone Shell (sash)

Z shell (zsh)

Non-Unix shells

4DOS, 4OS2, 4NT – shells for DOS, OS/2, and Windows NT written and sold by JP Software. The Take Command programs were compatible shells requiring and enjoying a GUI.

Amiga CLI/Amiga Shell which functioned as alternative to the AmigaOS GUI called Workbench

BASIC-PLUS – RSTS/E

Beemos (BEEMos) – A small project that provides settings, appliations, and feels like a separate OS that runs over Windows XP.

CANDE MCS– command line shell and text editor on the MCP operating system

CCP – console command processor of CP/M

cmd.exe – shell for OS/2, Windows CE and Windows NT-based operating systems

COMMAND.COM – shell for DOS

Commodore DOS Wedge – an extension to the Commodore 64's BASIC 2.0 that included shorthand for common disk operations

DCL – the standard shell for OpenVMS, deriving from versions created for earlier DEC operating systems

DDT – PDP-10 debugger from DEC used as a command shell for the MIT Incompatible Timesharing System

DROS – J2ME Platform based DOS like shell for smart phones.

EFI-SHELL – an open source Extensible Firmware Interface command shell

iSeries QSHELL – 'UNIX style shell' on the IBM OS/400

Macintosh Programmer's Workshop – old command line environment used for software development on the classic Mac OS

Microsoft BASIC – the primary operating environment for a number of older 8-bit computer systems

Rexx – IBM's scripting languages

Windows PowerShell – the object-oriented successor of cmd.exe (formerly known as Monad or the Microsoft Shell (MSH))

Windows Recovery Console – feature of the Windows 2000, Windows XP and Windows 2003 operating systems

YouShell - JavaScript based command processor of YouOS

Shells for programming languages

Noteworthy interactive versions of programming languages include:

The Read-eval-print loop commonly associated with Lisp, but used in other programming languages as well

BeanShell – Shell for Java

Firebug – JavaScript shell and debugging environment as Firefox plugin

GMLCMD – GML Shell

Interactive Ruby Shell – interactive version of Ruby

JavaScript shell – Several programs by this name allow interactive JavaScript

PHPsh – Shell for PHP

Python's standard interpreter can be invoked in a shell mode

On Microsoft Windows

Modern versions of the Microsoft Windows operating system use Windows Explorer as their shell. Explorer provides the familiar desktop environment, Start menu, and task bar, as well as the file management functions of the operating system. Older versions also include Program Manager which was the shell for the 3.x series of Microsoft Windows.

Many individuals and developers dissatisfied with the interface of Windows Explorer have developed software that either alters the functioning and appearance of the shell or replaces it entirely. WindowBlinds by StarDock is a good example of the former sort of application. LiteStep, GeoShell and BB4Win are good examples of the latter.

Microsoft Windows environments:

Aston shell

BB4Win

bbLean

Cairo Shell

Calmira

Chroma

Packard Bell Navigator

SharpE

WinStep

On the X Window System

Graphical (GUI) shells typically build on top of a windowing system. In case of the X Window System, there are both independent X window managers, and complete desktop environments which depend on a window manager.

X Window System environments (primarily for Unix-like operating systems):

independent X window managers, such as Blackbox and Fluxbox

desktop shells, such as Enlightenment DR17

full desktop environments, such as:

* CDE

* GNOME

* KDE

* Xfce