What Is a Shell?
On a UNIX system, anyone wishing to log in and make use of anything other than an installed application will have to become familiar with using a shell. A shell is a program that sits between you (the user) and the UNIX kernel. You can tell the computer what to do by typing commands into the shell and it will run them for you. You can even write your own programs that can be run (interpreted) by the shell. These are generally called “shell scripts.”
To make things difficult (or to make things easy—depending on how you look at it), there are a number of shells to choose from. Some have similar features to others, some are very different, and some also have rather strange names.
Whichever shell you use, they all have some things ...