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29 – Flow Control: Looping With while / until
In the previous chapter, we developed a menu-driven program to produce various kinds of system information. The program works, but it still has a significant usability problem. It only executes a single choice and then terminates. Even worse, if an invalid selection is made, the program terminates with an error, without giving the user an opportunity to try again. It would be better if we could somehow construct the program so that it could re- peat the menu display and selection over and over, until the user chooses to exit the pro- gram.
In this chapter, we will look at a programming concept called looping, which can be used to make portions of programs repeat. The shell provides three compound commands for looping. We will look at two of them in this chapter, and the third in a later one.