Linux shell:- Linux shell is a special-purpose program designed to read commands typed by a user and execute appropriate programs in response to those commands. Such a program is sometimes known as a command interpreter. The shell acts as an interface between the user and the kernel. When a user logs in, the login program checks the username and password, and then starts another program called the shell. The shell is a command line interpreter (CLI). It interprets the commands the user types in and arranges for them to be carried out. The commands are themselves programs: when they terminate, the shell gives the user another prompt (% on our systems).
Important Linux shell in market
A number of important shells have appeared over time:
Bourne shell (sh):-
This is the oldest of the widely used shells, and was written by Steve Bourne. It was the standard shell for Seventh Edition UNIX. The Bourne shell contains many of the features familiar in all shells: I/O redirection, pipelines, filename generation (globbing), variables, manipulation of environment variables, command substitution, background command execution, and functions. All later UNIX implementations include the Bourne shell in addition to any other shells they might provide.
C shell (csh):-
This shell was written by Bill Joy at the University of California at Berkeley. The name derives from the resemblance of many of the flow-control constructs of this shell to those of the C programming language. The C shell provided several useful interactive features unavailable in the Bourne shell, including command history, command-line editing, job control, and aliases. The C shell was not backward compatible with the Bourne shell. Although the standard interactive shell on BSD was the C shell, shell scripts (described in a moment) were usually written for the Bourne shell, so as to be portable across all UNIX implementations.
Korn shell (ksh):
This shell was written as the successor to the Bourne shell by David Korn at AT&T Bell Laboratories. While maintaining backward compatibility with the Bourne shell, it also incorporated interactive features similar to those provided by the C shell.
Bourne again shell (bash):
This shell is the GNU project’s reimplementation of the Bourne shell. It supplies interactive features similar to those available in the C and Korn shells. The principal authors of bash are Brian Fox and Chet Ramey. Bash is probably the most widely used shell on Linux. (On Linux, the Bourne shell, sh, is actually provided by bash emulating sh as closely as possible.)
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