Officially trademarked as UNIX, is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Douglas McIlroy, and Joe Ossanna.
There are various command interpreters available today. These interpreters are known as shells which handles user inputs.
Following are some well known shells available today.
C Shell (/bin/csh)
Korn Shell (/bin/ksh)
Bourne Shell (/bin/sh)
To know which Shell you are working on, type echo $SHELL. C Shell will response /bin/csh, Korn Shell will response /bin/ksh and Bourne Shell will response /bin/sh.
Unix is a case sensitive operating system. This means uppercase and lowercase text are not same, i.e.Hello and hello are not same in Unix.