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Verifying Input
A general rule of good programming is that if a program accepts input, it must be able to deal with anything it receives. This usually means that input must be carefully screened, to ensure that only valid input is accepted for further processing. We saw an example of this in the previous chapter when we studied the read command. One script contained the following test to verify a menu selection:
[[ $REPLY =~ ^[0-3]$ ]]
[[ $REPLY =~ ^[0-3]$ ]]
This test is very specific. It will only return a zero exit status if the string entered by the user is a numeral in the range of zero to three. Nothing else will be accepted. Sometimes these kinds of tests can be very challenging to write, but the effort is necessary to produce a high quality script.
Design Is A Function Of Time
When I was a college student studying industrial design, a wise professor stated that the amount of design on a project was determined by the amount of time given to the designer. If you were given five minutes to design a device “that kills flies,” you designed a flyswatter. If you were given five months, you might come up with a laser-guided “anti-fly system” instead.
The same principle applies to programming. Sometimes a “quick-and-dirty” script will do if it’s only going to be used once and only used by the programmer. That kind of script is common and should be developed quickly to make the effort economical. Such scripts don’t need a lot of comments and defensive checks. On the other hand, if a script is intended for production use, that is, a script that will be used over and over for an important task or by multiple users, it needs much more careful development.