I have found that Terraform does not have constants or functions to determine the operating system it is running on. You can work around this limitation with some knowledge of the target platforms you are running on. The most common use case is discerning between Windows and Unix-based systems to execute shell scripts, for example.

Ideally, you do not have to do this, but sometimes, you, your colleagues, and your CI/CD pipeline do not utilize a homogeneous environment.

One almost 100% certain fact is that Windows addresses storage devices with drive letters. You can leverage this to detect a Windows host by checking the project’s root path and storing the result in a variable.

locals {
  is_windows = length(regexall("^[a-z]:", lower(abspath(path.root)))) > 0
}

output "absolute_path" {
    value = abspath(path.root)
}

output "operating_system" {
    value = local.is_windows ? "Windows" : "Linux"
}

The output values are for demonstration purposes only. All you need is the regex for potential drive letters and the absolute path of the directory. Any path would do, actually.

The regexall function returns a list of all matches, and if the path starts with a drive letter, the resulting list contains more than zero elements, which you can check with the length function.

You could also check for “/home” to detect a Linux-based system or “/Users” for a macOS computer. In those instances, the source code must always be located somewhere in a user’s directory during execution. That may not be the case in a CI/CD pipeline, so keep that in mind. Here is the result on Windows.

And here on Linux.

You can find the source code on GitHub.

I hope this was useful.

Thank you for reading