2.2. Sample Scripts: Comparing the Major Shells

At the end of each section pertaining to a specific shell, you will find a small program to illustrate how to write a complete script. At first glance, the programs for each shell look very similar. They are. And they all do the same thing. The main difference is the syntax. After you have worked with these shells for some time, you will quickly adapt to the differences and start formulating your own opinions about which shell is your favorite. A detailed comparison of differences among the C/TC, Bourne, Bash, and Korn shells is found in Appendix B.

Before Getting Started

You must have a good handle on UNIX/Linux commands. If you do not know the basic commands, you cannot do much with shell programming. The next three chapters will teach you how to use some of the major UNIX/Linux commands, and Appendix A in the back of the book, gives you a list of the most common commands (also called utilities).

The Purpose

The sample scripts provided at the end of each section send a mail message to a list of users, inviting each of them to a party. The place and time of the party are set in variables. The list of guests is selected from a file called guests. The existence of the guest file is checked and if it does not exist, the program will exit. A list of foods is stored in a word list (array). A loop is used to iterate through the list of guests. Each user will receive an e-mail invitation telling him or her the time and place of the party and asking him or her to bring an item from the food list. A conditional is used to check for a user named root, and if he is on the guest list, he will be excluded; that is, he will not be sent an e-mail invitation. The loop will continue until the guest list is empty. Each time through the loop, a food item is removed from the list, so that each guest will be asked to bring a different food. If, however, there are more users than foods, the list is reset. This is handled with a standard loop control statement.