Shell (computing)

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.

Shell

Shell may refer to:

Architecture and design

Business

Chemistry and Physics

Computing

Shell corporation

A shell corporation is a company which serves as a vehicle for business transactions without itself having any significant assets or operations. Some shell companies may have had operations, but those may have shrunk due to unfavorable market conditions or company mismanagement. A shell corporation may also arise when a company's operations have been wound up, for example following a takeover, but the “shell” of the original company continues to exist.

Shell corporations are not in themselves illegal, and they do have legitimate business purposes. However, they are a main component of the underground economy, especially those based in tax havens. They may also be known as international business companies, personal investment companies, front companies, or "mailbox" companies.

Shell companies can also be used for tax avoidance. A classic tax avoidance operation is based on the buying and selling through tax haven shell companies to disguise true profits. The firm does its international operations through this shell corporation, thus not having to report to its country the sums involved, avoiding any taxes.

Unix shell

A Unix shell is a command-line interpreter or shell that provides a traditional Unix-like command line user interface. 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. Users typically interact with a Unix shell using a terminal emulator, however, direct operation via serial hardware connections, or networking session, are common for server systems. All Unix shells provide filename wildcarding, piping, here documents, command substitution, variables and control structures for condition-testing and iteration.

Concept

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 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 interactively, a shell program is automatically executed for the duration of the session. The type of shell, which may be customized for each user, is typically stored in the user's profile, for example in the local passwd file or in a distributed configuration system such as NIS or LDAP; however, the user may execute any other available shell interactively.