A shell is a piece of software that provides an interface for users 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.
Notable historic or popular Unix shells include:
see also: '/etc/shells' on a linux systemes
Noteworthy interactive versions of programming languages include:
Modern versions of the Microsoft Windows operating system use Windows Shell 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 Emerge Desktop are good examples of the latter.
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):