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Learn how to effectively test null or undefined variables in GNU Make using simple, clear examples. Find solutions to common pitfalls and improve your Makefile skills.
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This video is based on the question https://stackoverflow.com/q/75043642/ asked by the user 'aimass' ( https://stackoverflow.com/u/2397947/ ) and on the answer https://stackoverflow.com/a/75043738/ provided by the user 'MadScientist' ( https://stackoverflow.com/u/939557/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Null or Undefined Variables in GNU Make

In the world of automation using GNU Make, one often encounters the need to check the state of variables. Specifically, you might want to verify if either of your variables are null or undefined. This is a common requirement for making sure that your build processes run smoothly without unexpected prompts. Let's dive into the problem you've encountered and explore a more effective approach to achieve your goal.

The Problem

The issue arises when attempting to check if either VAR1 or VAR2 is null or undefined using the $(and ...) function. The original logic is as follows:

[[See Video to Reveal this Text or Code Snippet]]

At first glance, this looks correct as it seems to follow a logical flow: if either variable is not defined, a message is printed to prompt the user to define them. However, if both variables contain values, the condition fails to evaluate correctly.

What You're Missing

The core misunderstanding stems from how GNU Make evaluates the values following $(and ...). Here’s what you need to know:

If both VAR1 and VAR2 contain values, the expression $(and VAR1 VAR2) only returns the last value, not a representation that states they are defined.

The comparison in the ifeq statement to "" (an actual empty string) will never be true unless both variables are empty. This is because GNU Make interprets quoted strings as actual characters, and not as empty spaces.

Key Points to Remember

An empty string in GNU Make is not represented as two double-quote characters (""); rather, it’s simply an absence of content.

The string comparison to an empty value using "" will be false if anything other than an empty variable is involved.

The Solution - Correcting the If Statement

To properly check if either VAR1 or VAR2 is null or undefined, you should adjust the logic in your Makefile. Instead of comparing against "", it is more effective to compare against an absent value directly:

Here’s the revised code:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Changes:

Empty Comparison: The updated ifeq statement checks if the result of $(and $(VAR1),$(VAR2)) is empty (which indicates that at least one of the variables is null or undefined).

Simplicity: This straightforward approach eliminates confusion regarding the evaluation of strings within GNU Make.

Conclusion

By correctly setting up the condition to check for null or undefined variables, you can ensure that your Makefile behaves as intended without unnecessary prompts or confusion. Always remember to consider how GNU Make interprets empty values and string comparisons when designing your build scripts.

Now that you have the right approach, you can confidently manage variable states in your GNU Make projects. Happy coding!