2.1 The Bourne Shell: /bin/sh

The shell is one of the most important parts of a Unix system. A shell is a program that runs commands, like the ones that users enter into a terminal window. These commands can be other programs or built-in features of the shell. The shell also serves as a small programming environment. Unix programmers often break common tasks into smaller components and use the shell to manage tasks and piece things together.

Many important parts of the system are actually shell scripts—text files that contain a sequence of shell commands. If you’ve worked with MS-DOS previously, you can think of shell scripts as very powerful .BAT files. Because they’re important, Chapter 11 is devoted entirely to shell scripts.

As you progress through this book and gain practice, you’ll add to your knowledge of manipulating commands using the shell. One of the best things about the shell is that if you make a mistake, you can easily see what you typed to find out what went wrong, and then try again.

There are many different Unix shells, but all derive features from the Bourne shell (/bin/sh), a standard shell developed at Bell Labs for early versions of Unix. Every Unix system needs a version of the Bourne shell in order to function correctly, as you will see throughout this book.

Linux uses an enhanced version of the Bourne shell called bash or the “Bourne-again” shell. The bash shell is the default shell on most Linux distributions, and /bin/sh is normally a link to bash on a Linux system. You should use the bash shell when running the examples in this book.

note

You may not have bash as your default shell if you’re using this chapter as a guide for a Unix account at an organization where you’re not the system administrator. You can change your shell with chsh or ask your system administrator for help.