Shell in Linux/Unix

What is the Shell?

• A shell is any program that provides an interactive environment for users
• Whenever you login to a Unix system you are placed in a program called the shell. All of your work is done within the shell.
• The shell is your interface to the operating system. It acts as a command interpreter; it takes each command and passes it to the operating system. It then displays the results of this operation on your screen.
• There are several shells in widespread use. The most common ones are described below.
  There is a family of shells available on linux/unix:
  

  Shell	Symbol	Prompt	Description
  bourne	sh	$	Original Unix shell written by Steve Bourne of Bell Labs. Available on all UNIX systems. Does not have the interactive facilites provided by modern shells such as the C shell and Korn shell. The Bourne shell does provide an easy to use language with which you can write shell scripts.
  korn	ksh	$	Written by David Korn of bell labs. It is now provided as the standard shell on Unix systems. Provides all the features of the C and TC shells together with a shell programming language similar to that of the original Bourne shell.
  c shell	csh	%	Written at the University of California, Berkley. As it name indicates, it provides a C like language with which to write shell scripts.
  bourne again	bash	$	Public domain shell written by the Free Software Foundation under their GNU initiative. Ultimately it is intended to be a full implementation of the IEEE Posix Shell and Tools specification. Widely used within the academic commnity. Provides all the interactive features of the C shell (csh) and the Korn shell (ksh). Its programming language is compatible with the Bourne shell (sh).

• Many more public domain shells are available like ash, zsh ...

Purpose of shells

• Command line interpreter - Reading commands from users and executing them
• Programming languages - Processing a collection of commands in a file
• Job control language -
  
  • enables command sto be executed at a later time
  • allows commands to be run in the background
  • allows commands to be chained together
  • enables command input/output to be redirected