Shell Programming

There is much more to a shell than meets the eye. The shell can do much more than the command-line interpreter everyone is used to using. UNIX shells actually provide a powerful interpretive programming language as well.

In this chapter, we'll cover ksh shell programming. I chose ksh because most ksh programming techniques work with the Bourne shell as well. There is a follow-on to ksh programming at the end of the chapter for csh because the csh employs some different programming techniques than the ksh .

We'll cover the most important ksh programming techniques. The climax of this chapter is a fairly sophisticated shell program to remove files and place them in a directory, rather than just permanently removing files from the system with rm . This program employs all the shell programming techiques covered in the chapter. This shell program, called trash , with some minor modifications, can be run in the Bourne shell as well.

The shell is one of the most powerful and veratile features on any UNIX system. If you can't find the right command to accomplish a task, you can probably build it quite easily using a shell script.

The best way to learn shell programming is by example. There are many examples given in this chapter. Some serve no purpose other than to demonstrate the current topic. Most, however, are useful tools or parts of tools that you can easily expand and adapt into your environment. The examples provide easy-to-understand prompts and output messages. Most examples show what is needed to provide the functionality we are after. They do not do a great deal of error checking. From my experience, however, it only takes a few minutes to get a shell program to do what you want; it can take hours to handle every situation and every error condition. Therefore, these programs are not very dressed up (perhaps a sport coat versus a tuxedo ). I'm giving you what you need to know to build some useful tools for your environment. I hope that you will have enough knowledge and interest by the time we get to the end of this chapter to learn and do more.

Most of the examples in this chapter were performed on a Solaris system; however, shells are nearly identical going from one system to another, so you should be able to get the programs in this chapter running on your system quickly.