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Shell

Definition of the Shell
Shell is an interactive environment which provides an interface to an Operating System. It gathers input from user and execute the commands.
Bourne shell(sh)- 1977
The Bourne shell was introduced. The Bourne shell(sh), by Stephen Bourne at AT&T Bell Labs for V7 UNIX, remains a useful shell today (in some cases, as the default root shell). The Bourne shell was developed after working on an ALGOL68 compiler, so its grammar is more along the lines of Algorithmic Language (ALGOL) than other shells. The source code was developed in C.
The Bourne shell served two primary goals:
  • Executing UNIX/Linux commands for the operating system,i.e, command line interpreter
  • Writing reusable scripts that could be invoked through the shell,i.e, scripting
In addition to replacing the Thompson shell, the Bourne shell offered many other advantages over its predecessors such as control flows, loops, and variables into scripts, providing a more functional language to interact with the operating system.
The shell also permitted you to use shell scripts as filters, providing integrated support for handling signals but lacked the ability to define functions.
It introduced the world to a number of features that we use today, like command substitution and HERE documents to embed preserved string literals within a script.
The Bourne shell led to the development of the many shells that we use today.
C Shell 1978
The C shell(csh) was developed by Bill Joy with the objective of achieving a scripting language similar to C programming language. This was useful given that C was a primary language in use back then which also made it easier and faster to use.
Korn Shell (ksh)-1983
Developed by David Korn, the Korn Shell(ksh) combined features of both Bourne Shell and C Shell. It is backward-compatible with the Bourne Shell. It included features from the C Shell such as job control, command aliasing & command history.
TENEX C Shell(tcsh)-1983
The TENEX C Shell(tcsh) was introduced as a derivative of the C Shell but with a programmable command line completion and editing features added to it.
Bourne-Again Shell (bash)-1989
One of the most widely used shells today, the Bourne-Again Shell (bash) was written by Brian Fox for the GNU project as a pre-software replacement for the Bourne Shell. It showed all features from the Bourne shell but is much more efficient and easy to use.
It supported filename globbing, piping, command substitution, and control structures for conditional testing and iteration.
Bash stands for Bourne Again Shell and it is the default shell on many Linux distributions today. It is also a sh-compatible shell and offers practical improvements over sh for programming and interactive use which includes:
Command line editing
Job Control
Unlimited size command history
Shell Functions and Aliases
Unlimited size Indexed arrays
Integer arithmetic in any base from two to sixty-four

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