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Terraform

Terraform: ${path.module} Resolves to "." Not Module Path

By Ondřej Dolanský on 12/22/2024

Learn why Terraform's ${path.module} variable always references the current working directory instead of the module's location, and how this understanding affects your infrastructure code.

Terraform: ${path.module} Resolves to "." Not Module Path

Table of Contents

Introduction

The ${path.module} expression in Terraform is a powerful tool for dynamically referencing paths within your modules. It returns the directory path of the module currently being evaluated, which can be incredibly useful for tasks like referencing files within the same module or constructing dynamic file paths. However, understanding its behavior in different contexts, especially with nested modules and constructs like count and for_each, is crucial to avoid unexpected outcomes.

Step-by-Step Guide

The ${path.module} expression in Terraform provides the directory path of the currently evaluated module. It's crucial to understand how this works, especially when dealing with nested modules.

Let's break down some common scenarios:

Scenario 1: Using ${path.module} within a module's resource

# module/my-module/main.tf
resource "local_file" "example" {
  filename = "${path.module}/output.txt"
  content  = "Hello from ${path.module}"
}

In this case, if you call this module using module "my-instance" { source = "../module/my-module" }, the ${path.module} will resolve to the path where my-module is located, not the root of your project or the calling module's location.

Scenario 2: Referencing files within the same module

# module/my-module/variables.tf
variable "input" {
  default = file("${path.module}/defaults.tfvars")
}

Here, ${path.module} is a convenient way to reference the defaults.tfvars file within the same module directory.

Scenario 3: Potential issues with count and for_each

When using count or for_each to create multiple instances of a module, be mindful that ${path.module} will point to the same module directory for each instance. This might lead to unexpected behavior if you're trying to create unique files or resources based on ${path.module}.

Scenario 4: Absolute paths are not supported

Terraform doesn't support absolute file paths with ${path.module}. It's designed to work within the context of your Terraform project structure.

Key takeaways:

  • ${path.module} provides the directory of the currently evaluated module.
  • It's particularly useful for referencing files within the same module.
  • Be cautious when using it with count or for_each to avoid unintended consequences.
  • Stick to relative paths within your project structure.

Code Example

This code repository provides practical examples of how to use the ${path.module} expression in Terraform. It demonstrates how ${path.module} resolves to the directory of the current module, which can be useful for referencing files or constructing paths relative to the module. The examples cover scenarios like creating files within a module directory, setting default variable values from files within the module, and potential pitfalls when using ${path.module} with constructs like count and for_each. The repository aims to provide a clear understanding of ${path.module} and its effective usage in Terraform projects.

This repository demonstrates the usage of ${path.module} in Terraform with practical examples.

Project Structure

terraform-path-module-example/
├── modules
│   └── my-module
│       ├── main.tf
│       ├── variables.tf
│       └── defaults.tfvars
└── main.tf

modules/my-module/main.tf

resource "local_file" "example" {
  filename = "${path.module}/output.txt"
  content  = "Hello from ${path.module}"
}

This resource creates a file named output.txt inside the my-module directory. The content of the file will be "Hello from [path to my-module directory]".

modules/my-module/variables.tf

variable "input" {
  default = file("${path.module}/defaults.tfvars")
}

This variable definition sets a default value for the input variable by reading the content of the defaults.tfvars file located in the same directory as the module.

modules/my-module/defaults.tfvars

sample_variable = "This is a default value"

This file contains default values for variables used within the module.

main.tf

module "my-instance" {
  source = "./modules/my-module"
}

This code snippet calls the my-module module and demonstrates how ${path.module} resolves to the module's directory, not the root of the project.

Scenario 3: Potential issues with count and for_each (Illustrative Example)

# modules/my-module/main.tf
resource "local_file" "example" {
  count = var.instance_count
  filename = "${path.module}/output-${count.index}.txt"
  content  = "Hello from instance ${count.index} at ${path.module}"
}

In this example, even though we are creating multiple instances of the local_file resource using count, they will all end up in the same directory (my-module) because ${path.module} resolves to the module's directory, not individual instances.

This example demonstrates a potential pitfall and highlights the need for careful consideration when using ${path.module} with constructs like count and for_each.

This repository provides a practical guide to understanding and using ${path.module} effectively in your Terraform projects. Remember to adapt these examples to your specific use case and project structure.

Additional Notes

These notes expand on the core concepts and provide practical insights for using ${path.module} effectively:

Understanding the Context:

  • Module-Centric: The most important thing to remember is that ${path.module} is always evaluated from the perspective of the module where it's used, not the root module or any calling module.
  • Dynamic Resolution: The path is calculated during Terraform's runtime, not at the time you write your code. This means the actual path represented by ${path.module} might not be immediately obvious just by looking at your directory structure.

Best Practices and Considerations:

  • Modularity and Reusability: Leverage ${path.module} to make your modules self-contained and reusable. By referencing files relative to the module's location, you ensure the module can be easily moved or copied within your project without breaking file paths.
  • Variable Defaults: As shown in the examples, ${path.module} is extremely useful for loading default variable values from files within your module. This promotes clean separation of configuration and code.
  • Templating with count and for_each: When using ${path.module} within resources created with count or for_each, carefully consider if you need unique paths for each instance. If so, you'll need to incorporate the loop index or key into your file/directory naming scheme.
  • Alternatives for Root-Relative Paths: If you absolutely need to reference files from the root of your Terraform project, consider using the terraform.workspace data source or environment variables to construct the path. However, this can make your modules less portable.
  • Security Implications: Be mindful of the files you reference with ${path.module}, especially if your modules are shared or publicly available. Avoid exposing sensitive information through file paths.

Debugging and Troubleshooting:

  • Outputting the Path: If you're unsure about the resolved path, use output or terraform console to print the value of ${path.module}. This can help you understand how Terraform is interpreting the path in your specific context.
  • Inspecting State Files: Terraform's state files store the resolved paths. Examining these files can provide insights into how ${path.module} was evaluated during previous runs.

By understanding these nuances and following best practices, you can harness the power of ${path.module} to write cleaner, more organized, and reusable Terraform code.

Summary

Feature Description Potential Issues
${path.module} Expression Provides the directory path of the currently evaluated Terraform module. None inherently, but its behavior in certain contexts requires attention.
Usage within a Module's Resource Resolves to the module's own directory, not the calling module's location.
Referencing Files within the Same Module Convenient for accessing files relative to the module's location.
Behavior with count and for_each ${path.module} points to the same module directory for each instance created by count or for_each. May lead to unexpected behavior when trying to create unique resources based on ${path.module}.
Absolute Path Support Not supported; use relative paths within your project.

Conclusion

By understanding these nuances and following best practices, you can harness the power of ${path.module} to write cleaner, more organized, and reusable Terraform code. Remember that ${path.module} always refers to the directory of the currently evaluated module, not the root module or any calling module. Use it strategically for referencing files within your modules, setting default variable values, and constructing dynamic file paths. However, exercise caution when using it with count or for_each to avoid unintended consequences. By mastering ${path.module}, you can enhance the modularity, portability, and maintainability of your Terraform projects.

References

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