bash Cookbook, 2nd Edition

Chapter 1. Beginning bash

What’s a shell, and why should you care about it?

Any recent computer operating system (by recent, we mean since about 1970) has some sort of user interface—some way of specifying commands for the operating system to execute. But in lots of operating systems, that command interface was really built in and there was only one way to talk to the computer. Furthermore, an operating system’s command interface would let you execute commands, but that was about all. After all, what else was there to do?

The Unix operating system popularized the notion of separating the shell (the part of the system that lets you type commands) from everything else: the input/output system, the scheduler, memory management, and all of the other things the operating system takes care of for you (and that most users don’t want to care about). The shell was just one more program; it was a program whose job was executing other programs on behalf of users.

But that was the beginning of a revolution. The shell was just another program that ran on Unix; if you didn’t like the standard one, you could create your own. So by the end of Unix’s first decade, there were at least two competing shells: the Bourne shell, sh (which was a descendant of the original Thompson shell), plus the C shell, csh. By the end of Unix’s second decade, there were a few more alternatives: the Korn shell, ksh, and the first versions of the bash shell. By the end of Unix’s third decade, there were probably a ...