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Chapter 1. Korn Shell Basics

You've used your computer for simple tasks, such as invoking your favorite application programs, reading your electronic mail, and perhaps examining and printing files. You know that your machine runs the Unix operating system, or maybe you know it under some other name, like Solaris, HP-UX, AIX, or SunOS. (Or you may be using a system such as GNU/Linux or one of the 4.4-BSD-derived systems that is not based on the original Unix source code.) But apart from that, you may not have given too much thought to what goes on inside the machine when you type in a command and hit ENTER.

It is true that several layers of events take place whenever you enter a command, but we're going to consider only the top layer, known as the shell. Generally speaking, a shell is any user interface to the Unix operating system, i.e., any program that takes input from the user, translates it into instructions that the operating system can understand, and conveys the operating system's output back to the user.

There are various types of user interface. The Korn shell belongs to the most common category, known as character-based user interfaces. These interfaces accept lines of textual commands that the user types; they usually produce text-based output. Other types of interface include the now-common graphical user interfaces (GUI), which add the ability to display arbitrary graphics (not just typewriter characters) and to accept input from mice and other pointing devices, touch-screen interfaces (such as those you see on some automatic teller machines), and so on.

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