As mentioned before, Linux is a multitasking, multiuser operating system. Multitasking is very useful, and once you understand it, you'll use it all of the time. Before long, you'll run programs in the background, switch between tasks, and pipeline programs together to achieve complicated results with a single command.

Many of the features we'll cover in this section are features provided by the shell itself. Be careful not to confuse Linux (the actual operating system) with a shell--a shell is just an interface to the underlying system. The shell provides functionality inaddition to Linux itself.

A shell is not only an interpreter for the interactive commands you type at the prompt, but also a powerful programming language. It lets you to write shell scripts , to ``batch'' several shell commands together in a file. If you know MS-DOS you'll recognize the similarity to ``batch files''. Shell scripts are a very powerful tool, that will let you automate and expand your use of Linux.

There are several types of shells in the Linux world. The two major types are the ``Bourne shell'' and the ``C shell''. The Bourne shell uses a command syntax like the original shell on early UNIX systems, like System III. The name of the Bourne shell on most Linux systems is /bin/sh (where sh stands for ``shell''). The C shell (not to be confused with sea shell) uses a different syntax, somewhat like the programming language C, and on most Linux systems is named /bin/csh.

Under Linux, several variations of these shells are available. The two most commonly used are the Bourne Again Shell, or ``Bash'' ( /bin/bash ), and ``Tcsh'' ( /bin/tcsh ). bash is a form of the Bourne shell that includes many of the advanced features found in the C shell. Because bash supports a superset of the Bourne shell syntax, shell scripts written in the standard Bourne shell should work with bash . If you prefer to use the C shell syntax, Linux supports tcsh , which is an expanded version of the original C shell.

The type of shell you decide to use is mostly a religious issue. Some folks prefer the Bourne shell syntax with the advanced features of bash , and some prefer the more structured C shell syntax. As far as normal commands such as cp and ls are concerned, the shell you use doesn't matter--the syntax is the same. Only when you start to write shell scripts or use advanced features of a shell do the differences between shell types begin to matter.

As we discuss the features of the various shells, we'll note differences between Bourne and C shells. However, for the purposes of this manual most of those differences are minimal. (If you're really curious at this point, read the man pages for bash and tcsh ).