Shell

In, a shell is a for access to an 's services. In general, operating system shells use either a (CLI) or  (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.

CLI shells require the user to be familiar with commands and their calling, and to understand concepts about the shell-specific scripting language (for example ). They are also more easily operated via, and provide certain advantages to s.

Graphical shells place a low burden on beginning computer users, and are characterized as being easy to use. Since they also come with certain disadvantages, most GUI-enabled operating systems also provide CLI shells.

Overview
Operating systems provide various services to their users, including, management (running and terminating ), , and operating system monitoring and configuration.

Most operating system shells are not direct interfaces to the underlying kernel, even if a shell communicates with the user via s attached to the computer directly. Shells are actually special applications that use the kernel in just the same way as it is used by other application programs. A shell manages the user–system interaction by prompting users for input, interpreting their input, and then handling an output from the underlying operating system (much like a read–eval–print loop, REPL). Since the operating system shell is actually an application, it may easily be replaced with another similar application, for most operating systems.

In addition to shells running on local systems, there are different ways to make remote systems available to local users; such approaches are usually referred to as remote access or remote administration. Initially available on, which provided text-based UIs for each active user simultaneously by means of a  connected to the mainframe via serial line or , remote access has extended to  systems and Microsoft Windows. On Unix-like systems, Secure Shell protocol is usually used for text-based shells, while can be used for X Window System–based graphical user interfaces (GUIs). On Microsoft Windows, Remote Desktop Protocol can be used to provide GUI remote access, and since, can be used for text-based remote access via WMI, RPC, and WS-Management.

Most operating system shells 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). Other possibilities, although not so common, include voice user interface and various implementations of a text-based user interface (TUI) that are not CLI. The relative merits of CLI- and GUI-based shells are often debated.

Text (CLI) shells
A (CLI) is an operating system shell that uses  characters typed on a keyboard to provide instructions and data to the operating system, interactively. For example, a can send codes representing keystrokes to a command interpreter program running on the computer; the command interpreter parses the sequence of keystrokes and responds with an error message if it cannot recognize the sequence of characters, or it may carry out some other program action such as loading an application program, listing files, logging in a user and many others. Operating systems such as UNIX have a large variety of programs with different commands, syntax and capabilities. Some operating systems had only a single style of command interface; commodity operating systems such as came with a standard command interface but third-party interfaces were also often available, providing additional features or functions such as menuing or remote program execution.

Application programs may also implement a command-line interface. For example, in Unix-like systems, the program has a number of commands for controlling a link to a remote computer system. Since the commands to the program are made of the same keystrokes as the data being sent to a remote computer, some means of distinguishing the two are required. An can be defined, using either a special local keystroke that is never passed on but always interpreted by the local system. The program becomes modal, switching between interpreting commands from the keyboard or passing keystrokes on as data to be processed.

A feature of many command-line shells is the ability to save sequences of commands for re-use. A data file can contain sequences of commands which the CLI can be made to follow as if typed in by a user. Special features in the CLI may apply when it is carrying out these stored instructions. Such (script files) can be used repeatedly to automate routine operations such as initializing a set of programs when a system is restarted. Batch mode use of shells usually involves structures, conditionals, variables, and other elements of programming languages; some have the bare essentials needed for such a purpose, others are very sophisticated programming languages in and of themselves. Conversely, some programming languages can be used interactively from an operating system shell or in a purpose-built program.

The command-line shell may offer features such as, where the interpreter expands commands based on a few characters input by the user. A command-line interpreter may offer a history function, so that the user can recall earlier commands issued to the system and repeat them, possibly with some editing. Since all commands to the operating system had to be typed by the user, short command names and compact systems for representing program options were common. Short names were sometimes hard for a user to recall, and early systems lacked the storage resources to provide a detailed on-line user instruction guide.

Graphical (GUI) shells
Graphical shells (or desktop shells) provide means for manipulating programs based on (GUI), by allowing for operations such as opening, closing, moving and resizing, as well as switching  between windows. Graphical shells may be included with s or come separately, even as a set of loosely coupled utilities.

Most graphical user interfaces develop the, where data files are represented as if they were paper documents on a desk, and application programs similarly have graphical representations instead of being invoked by command names.

Unix-like systems
Graphical shells typically build on top of a. In the case of or, the shell consists of an  or a , respectively, as well as of one or multiple programs providing the functionality to start installed applications, to manage open windows and virtual desktops, and often to support a widget engine.

In the case of, could be thought of as the windowing system, and the shell consists of the , the , SystemUIServer, and.

Microsoft Windows
Modern versions of the operating system use the  as their shell. Windows Shell provides the familiar, , and , as well as a for accessing the file management functions of the operating system. Older versions also include, which was the shell for the 3.x series of Microsoft Windows, and which in fact shipped with later versions of Windows of both the 95 and NT types at least through Windows XP. The interfaces of Windows versions 1 and 2 were markedly different.

Desktop applications are also considered shells, as long as they use a third-party engine. Likewise, 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. by is a good example of the former sort of application. and are good examples of the latter.

Interoperability programmes and purpose-designed software lets Windows users use equivalents of many of the various Unix-based GUIs discussed below, as well as Macintosh. An equivalent of the OS/2 Presentation Manager for version 3.0 can run some OS/2 programmes under some conditions using the OS/2 in versions of Windows NT.

Other uses
"Shell" is also used loosely to describe application software that is "built around" a particular component, such as web browsers and email clients, in analogy to the shells found in nature. These are also sometimes referred to as "wrappers".

In s, a shell is a piece of software that is an "empty" expert system without the knowledge base for any particular application.