The program that interacts with users is called a shell. The most common shell is called the C shell because parts of it work a lot like the C programming language. Some of the things described here are actually features of the C shell and will not work in other shells such as the Bourne shell. If you have never thought about shells, then you are almost certainly using a C shell. If you are sophisticated enough to run a shell other than the C Shell, you should know what kind of features are shell-dependent.