Bug: Makefile Fails With '@unset' – Not Recognized Across All Environments

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Introduction

When working with Makefiles, it's essential to ensure that variables are unset correctly to avoid conflicts and unexpected behavior. However, using the @unset command can lead to issues, especially when working across different environments. In this article, we'll explore the problem of @unset not being recognized in Makefiles and provide a recommended fix.

What Were You Trying to Achieve?

I wanted to unset a Makefile variable (GRPC_TLS or GRPC_MTLS) when the other is enabled, to ensure mutual exclusivity before running Docker Compose via the make run target.

What Are the Expected Results?

When GRPC_MTLS=true, the @unset GRPC_TLS command should remove the variable. The make run command should complete without errors, with only one of GRPC_TLS or GRPC_MTLS being active.

What Are the Received Results?

When running make run, I get the following error:

make: unset: No such file or directory
make: *** [check_mtls] Error 1

This error is due to @unset being used, which is:

  • Not available in Windows environments (e.g., CMD or PowerShell)
  • Executed in a subshell in Unix-based systems (Linux/macOS), making it ineffective across lines

Steps To Reproduce

Run:

make run

Error occurs:

make: unset: No such file or directory
make: *** [check_mtls] Error 1

In What Environment Did You Encounter the Issue?

  • OS: macOS Ventura / Ubuntu 22.04 / Windows 10
  • Make: GNU Make 4.3+
  • Shell: Bash 5.x / CMD / PowerShell / Git Bash

Additional Information You Deem Important

  • unset is a shell builtin (not an external command), and does not persist in Makefile since each line is executed in a separate shell.
  • On Windows, unset is not recognized in CMD or PowerShell.
  • Recommended fix: avoid @unset; use $(eval GRPC_TLS=) instead:
$(eval GRPC_TLS=)

Or enforce logic using conditional $(error ...) to catch mutual conflict.

Why @unset Fails in Makefiles

When using @unset in a Makefile, it's executed in a subshell, which means it doesn't persist across lines. This is because each line in a Makefile is executed in a separate shell. As a result, the unset command is not recognized, leading to the error.

Why @unset Fails in Windows Environments

In Windows environments, unset is not recognized in CMD or PowerShell. This is because unset is a shell builtin command, and Windows shells don't support it.

Recommended Fix: Avoid @unset

To avoid the issue with @unset, you can use the $(eval) function to unset the variable. This function evaluates the expression and sets the variable to the specified value.

$(eval GRPC_TLS=)

Alternatively, you can use conditional $(error ...) to catch mutual conflict and enforce the logic.

Conclusion

In conclusion, using @unset in Makefiles can lead to issues, especially when working across different environments. To avoid this problem, you can use the $(eval) function to unset variables or enforce logic using conditional $(error ...) to catch mutual conflict. By following these recommendations, you can ensure that your Makefiles work correctly across all environments.

Best Practices for Working with Makefiles

When working with Makefiles, it's essential to follow best practices to ensure that your files are correct and efficient. Here are some tips to keep in mind:

  • Use the $(eval) function to unset variables instead of @unset.
  • Enforce logic using conditional $(error ...) to catch mutual conflict.
  • Avoid using @unset in Makefiles.
  • Test your Makefiles in different environments to ensure they work correctly.
  • Use version control to track changes to your Makefiles.

Q: What is the issue with using '@unset' in Makefiles?

A: The issue with using @unset in Makefiles is that it's executed in a subshell, which means it doesn't persist across lines. This is because each line in a Makefile is executed in a separate shell. As a result, the unset command is not recognized, leading to the error.

Q: Why is '@unset' not recognized in Windows environments?

A: In Windows environments, unset is not recognized in CMD or PowerShell. This is because unset is a shell builtin command, and Windows shells don't support it.

Q: What is the recommended fix for this issue?

A: The recommended fix is to avoid using @unset and instead use the $(eval) function to unset variables. This function evaluates the expression and sets the variable to the specified value.

Q: How do I use the $(eval) function to unset variables?

A: To use the $(eval) function to unset variables, you can use the following syntax:

$(eval GRPC_TLS=)

This will unset the GRPC_TLS variable.

Q: Can I use conditional $(error ...) to catch mutual conflict?

A: Yes, you can use conditional $(error ...) to catch mutual conflict and enforce the logic. This function evaluates the expression and displays an error message if the condition is not met.

Q: How do I use conditional $(error ...) to catch mutual conflict?

A: To use conditional $(error ...), you can use the following syntax:

ifeq ($(GRPC_TLS), true)
  $(error GRPC_TLS and GRPC_MTLS cannot be true at the same time)
endif

This will display an error message if both GRPC_TLS and GRPC_MTLS are true.

Q: Why is it essential to test Makefiles in different environments?

A: It's essential to test Makefiles in different environments to ensure they work correctly. This is because Makefiles can behave differently in different environments, and testing them in different environments can help you catch any issues early on.

Q: How can I test Makefiles in different environments?

A: To test Makefiles in different environments, you can use the following methods:

  • Use a virtual machine or a container to test your Makefile in a different environment.
  • Use a cross-compilation tool to compile your Makefile in a different environment.
  • Test your Makefile in a different shell or terminal emulator.

Q: What are some best practices for working with Makefiles?

A: Some best practices for working with Makefiles include:

  • Using the $(eval) function to unset variables instead of @unset.
  • Enforcing logic using conditional $(error ...) to catch mutual conflict.
  • Avoiding using @unset in Makefiles.
  • Testing your Makefiles in different environments to ensure they work correctly.
  • Using version control to track changes to your Make.

By following these best practices, you can ensure that your Makefiles are correct, efficient, and work correctly across all environments.