Basic Debugging Aids

What sort of functionality do you need to debug a program? At the most empirical level, you need a way of determining what is causing your program to behave badly, and where the problem is in the code. You usually start with an obvious what (such as an error message, inappropriate output, infinite loop, etc.), try to work backwards until you find a what that is closer to the actual problem (e.g., a variable with a bad value, a bad option to a command), and eventually arrive at the exact where in your program. Then you can worry about how to fix it.

Notice that these steps represent a process of starting with obvious information and ending up with often obscure facts gleaned through deduction and intuition. Debugging aids make it easier to deduce and intuit by providing relevant information easily or even automatically, preferably without modifying your code.

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The simplest debugging aid (for any language) is the output statement, echo, in the shell's case. Indeed, old-time programmers debugged their FORTRAN code by inserting WRITE cards into their decks. You can debug by putting lots of echo statements in your code (and removing them later), but you will have to spend lots of time narrowing down not only what exact information you want but also where you need to see it. You will also probably have to wade through lots and lots of output to find the information you really want.