www.shellui.com

In computer science, shell refers to the software that provides interface to operate for users. Shell is also an important part of Unix/Linux system, as it is the main interface between the operating system and the external environment. Shell plays two roles in Unix/Linux:

Managing of interactions between OS and users: waiting for input, inteprete the input and output the result.
Providing methods of communications, which can be either interactive (like keyboard input) or non-interactive (like the shell script, a commbinations of commands).

Classification

There are generally two kinds of shell: command shell and graphical shell. The former is often implied when no other indication is given.

Command shell

Command shell is also called command line interface (CLI) shell. It is in fact a command intepretor, receives commands from users, then executes the relative program. Some popular shells are

Standard Bourne shell (sh) / C shell (csh) / ksh / bash for Unix / Linux
Command.com for MS-DOS
cmd.exe for Windows NT
PowerShell for Windows NT that supports .NET Framework

The Unix / Linux shell may look like traditional command line in Windows, however it can be used to fulfill powerful and complex functions. Users can also use shell language to write their onw prorams.

History

The first Unix shell, called "sh", is written by Ken Thompson in the inspiration from shell of Multic.

The Borune shell (sh) is written by Steve Bern in Bell Laboratory, published with Unix Version 7 in 1978. After that, a lot of command shell has emerged with the compatibility to Bourne shell:

Almquist shell (ash)
Bourne-Again shell (Bash)
Debian Almquist shell (dash)
Korn shell (ksh) by David Korn in Bell Laboratory
Z shell (Zsh)

The C shell (csh) is written by Bill Joey in UCLA, published with BSD in 1979. The TENEX C shell (tcsh) is a shell that is compatible to csh.

Some other kinds of shell:

fish: first published in 2005
rc shell (rc): shell used in 9 project system, written by Tom Duff in Bell Laboratory, and further ported to Unix and other operating system
es shell (es): a rc-compatible shell that supports functional programming, written in 90s in the 20th centry
scsh (Scheme Shell)

Some old shells that are no longer used:

Thompson shell (sh): The first Unix Shell, written by Ken Thompson in Bell Laboratory. Published in the first to six version of Unix from 1971 to 1975
PWB shell (sh): A version of Thompson shell enhanced by John Mashey and others in Bell Laboratory, powered with PWB Unix in 1976.

Graphical shell

Graphical shell is also called graphical user interface (GUI) shell. The most known GUI shells are Windows Desktop (Windows Explorer) and Linux shells.

Graphical shell consists of window manager and desktop environment.

Window manager (WM)

The window manager offers functions of managing windows, which can be independent from or integrated into a desktop environment. Some popular window manager in Linux are:

Compiz: Mixed window manager based on OpenGL
KWin: Mixed window manager based on OpenGL
I3: Windows manager using tile mode
Xmonad: Windows manager using tile mode
Enlightenment: Powerful window manager that offers many functions of desktop environment
Openbox: Highly customizable window manager of the next generation
Fvwm: Full featured window manager with minimum memory usage
Fluxbox: Lightweight window manager

Desktop environment (DE)

A desktop environment is a series of component that has a window manager for managing windows, meets full software requirements and provides modules like system settings. Some popular desktop environments are:

Deepin desktop environment (DDE): a nouveau desktop environment developed using Qt/C++ and Golang
Gnome Shell: The most popular desktop environment based on Gtk
KDE: Elegant and full featured desktop environment
Unity: Desktop environment developed for Ubuntu
Xfce: Light and practicle desktop environment
LXDE: Lightweight dekstop environment
MATE: Desktop environment derived from Gnome 2
Cinnamon：Desktop environment based on Gnome Shell
Razor-qt: Another desktop environment developed using QT