By Norman Matloff
Purpose - The information here is intended to be a review for those who have had a bit of prior exposure to Unix, and as a quick introduction to Unix for those who have never seen it before. (Some of the material may be new even to those with some prior exposure to Unix.)
Shells - A shell is a program that inputs Unix commands from the keyboard and relays them to the Unix system for execution. Shells typically include various shortcuts for users to use in stating their commands, and also a programming feature, in which users can make programs out of sets of their commands.
The first popular Unix shell was the Bourne shell, named sh. It still is in common use, though more often in a modernized version called the Bourne Again Shell, bash.