Unix Shell

The most generic sense of the term ''shell'' means ''any'' program that users use to type commands. Since in the Unix operating users can select which shell they want to use (which program should execute when they login), many shells have been developed. It is called a "shell" because it hides the details of the underlying Operating System behind the shell's interface (contrast " Kernel ", which refers to the lowest-level, or 'inner-most' component of an operating system). Similarly, Graphical User Interface s for Unix, such as GNOME and KDE , are sometimes called ''visual shells'' or ''graphical shells''. By itself, the term ''shell'' is usually associated with the command line. In Unix, any program can be the user's shell; users who want to use a different syntax for typing commands can specify a different program as their shell.

The term ''shell'' also refers to a particular program, namely the Bourne Shell , sh. The Bourne shell was the shell used in early versions of Unix and became a ''de facto'' standard; every Unix-like system has the equivalent of the Bourne shell. The Bourne shell program is located in the UNIX file hierarchy at /bin/sh. On some systems, such as BSD , /bin/sh is a Bourne shell or equivalent, but on other systems such as Linux , /bin/sh is likely to be a Link to a compatible, but more feature-rich shell, such as Bash . POSIX specifies the standard shell as a strict subset of the Korn Shell .

The Unix shell is unusual since it is both an interactive command language and the language used to script the system; it is a Scripting Programming Language .

On systems using a Windowing System , some users may never use the shell directly, though on Unix systems, the shell is still the implementation language of system startup scripts, including the program that starts the windowing system, the program that dials into the Internet, and many other essential functions.

On Windows, equivalents to Unix system scripts are called Batch Files , and have either a ".bat" or ".cmd" Extension .

Many regular users of a UNIX system still find a modern command line shell much more convenient for many tasks than any GUI Application .

UNIX SHELLS

Bourne shell compatible

Bourne Shell (sh) Written by Steve Bourne , while at Bell Labs . First distributed with Version 7 Unix , circa 1978 .

--- Almquist Shell (ash)

--- Bourne-Again Shell (bash)

--- Korn Shell (ksh) Written by David Korn , while at Bell Labs .

--- Z Shell (zsh)

C shell compatible

C Shell (csh) Written by Bill Joy , while at the University Of California, Berkeley . First distributed with BSD , circa 1979 .

--- TENEX C Shell (tcsh)

Other

Fish , first released in 2005.

Rc , the default shell on Plan 9 From Bell Labs and Version 10 Unix written by Tom Duff . Ports have been made to Inferno and Unix-like operating systems.

--- Es Shell (es) A Functional Programming rc-compatible shell written in the mid-1990s.

Scsh (Scheme Shell)

Historic

Thompson Shell (sh) The first Unix shell, written by Ken Thompson at Bell Labs . Distributed with Versions 1 through 6 of Unix, from 1971 to 1975 .

PWB Shell (sh) A version of the Thompson shell, augmented by John Mashey and others, while at Bell Labs . Distributed with the Programmer's Workbench UNIX , circa 1976 .

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Unix Shell