Unix in a Nutshell, 4th Edition

Introduction to the Shell

Today’s microwave ovens, and many other household appliances (ovens, washing machines, dishwashers), let you use simple push-buttons to instruct them what to do. They provide a simple user interface to a possibly complicated internal system.

The shell is the user interface to Unix, and by the same token, several shells are available in Unix. Most systems provide more than one for you to choose from. Each shell has different features, but all of them affect how commands will be interpreted and provide tools to create your Unix environment.

The original shells were developed before the time of Graphical User Interfaces (GUIs), and at first glance, appear harder to use than GUI interfaces. The truth, though, is that they aren’t harder to use, they are harder to learn. However, once you’ve mastered them, you’ll find that you can accomplish an infinite variety of tasks that just cannot be managed with a GUI.

The shell is simply a program that allows the system to understand your commands. (That’s why the shell is often called a command interpreter.) For many users, the shell works invisibly--"behind the scenes.” Your only concern is that the system does what you tell it to do; you don’t care about the inner workings. In our microwave analogy, this is comparable to pressing the START button. Most of us don’t care whether the user interface communicates with an embedded microcomputer, or drives analog electronics, as long as the popcorn is ready in time for the movie, ...