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In computing, a shell is a user interface for access to an operating system's services. In general, operating system shells use either a command-line interface (CLI) or graphical user interface (GUI), depending on a computer's role and particular operation. The design of a shell is guided by cognitive ergonomics and the goal is to achieve the best workflow possible for the intended tasks; the design can be constricted by the available computing power (for example, of the GPU) or the available amount of graphics memory. The design of a shell is also dictated by the employed computer periphery, such as computer keyboard, pointing device (a mouse with one button, or one with five buttons, or a 3D mouse) or touchscreen, which is the direct human–machine interface. CLI shells allow some operations to be performed faster in some situations, especially when a proper GUI has not been or cannot be created. However, they require the user to memorize commands and their calling syntax, and also to learn the shell-specific scripting language, for example bash script. CLIs are also easier to be operated via refreshable braille display and provide certain advantages to screen readers. Graphical shells have a low burden to start using a computer, and they are characterized as being simple and easy to use. With the widespread adoption of programs with GUIs, the use of graphical shells has gained greater adoption. Since graphical shells come with certain disadvantages (for example, lack of support for easy automation of operation sequences), most GUI-enabled operating systems also provide additional CLI shells. == Overview == Operating systems provide various services to their users, including file management, process management (running and terminating applications), batch processing, 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 peripheral devices attached to the computer directly. Shells are actually special applications that use the kernel API 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. 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 multi-user mainframes, which provided text-based UIs for each active user ''simultaneously'' by means of a text terminal connected to the mainframe via serial line or modem, remote access has extended to Unix-like systems and Microsoft Windows. On Unix-like systems, Secure Shell protocol is usually used for text-based shells, while SSH tunneling 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. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Shell (computing)」の詳細全文を読む スポンサード リンク
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