OS means Operating System, UNIX means a system which allows the users to get connected with the computer. Shell programming is a program that allows the user to work with the computer differently as required.
The Computer program that distributes the resources as per the requirement, or the need of the user coordinates all needs internally is called the operating system or the Kernel
The user communicates with the Operating System or Kernel through a program which is known as shell programming. The Shell is like a command an interpreter that translates your commands, which you have entered. These commands are then converted to a language which the Kernel understands.
UNIX program was that, which was developed in 1969 by a group of employees namely Ken Thompson, Dennis Ritche, Douglas, MacIlroy and Joe Ossanna at Bell Labs owned by AT&T Company.
There are various UNIX variants available in the market like Solaris, AIX, HP UNIX, and BSD etc, Linux is a flavour of UNIX which is easily available in the market.
The UNIX Computers is one such product which can be used by many, at the same time hence it is called multiuser. The user can run multiple programs at the same time with UNIX therefore it gives a multitasking atmosphere to the user.
This is a process which takes your requests, once your request is taken up the command is carried out; it interprets the way you want to program. The shell uses standard syntax for all your commands; there are various shells like C Shell, Bourne Shell and Korn Shell are some of the important and famous shells easily available with most of the UNIX Variants.