SHELL is a PROGRAM that acts as the interface between the user and the operating system. It is a command-line interface to the Unix operating system. It collects data from you and runs programs depending on that data. The output of the program is displayed once it has completed its execution.

Shell allows us to run commands, programs, and shell SCRIPTS. Kernel maintains resources between processes and regulates all-important computer operations. It also restricts hardware access, organizes all performing utilities, and manages resources between processes. Only kernel users have access to the operating system's utilities.

Types of SHELLS:

• The C Shell: It included aliases and command history, among other features. It has features like built-in math and C-like EXPRESSION syntax that make programming easier.
• The Bourne Shell: It's the very first UNIX shell. It is more convenient and faster. It lacks interactive features such as the ability to recall past commands. It also has no built-in support for ARITHMETIC and logical expressions. It is the default shell for the Solaris operating system.
• The Korn Shell: The Bourne shell is a subset of it. As a result, everything in the Bourne shell is supported. It includes interactive elements. Built-in arithmetic and C-like arrays, functions, and string manipulation facilities are among the features. It is more efficient than a C shell. It works with scripts written in the C shell.
• The GNU Bourne-Again Shell: It can be used with the Bourne shell. It incorporates Korn and Bourne shell features.