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Terraform

Terraform Error: Variables not allowed (Fix)

By Filip on 10/07/2024

Learn how to troubleshoot and resolve the "Error: Variables not allowed" issue in Terraform when running a plan.

Terraform Error: Variables not allowed (Fix)

Table of Contents

Introduction

The Terraform error message "Error: Variables not allowed" usually means you're trying to use a variable in a place where Terraform doesn't permit it. This article explains common causes of this error and provides solutions to fix them. It covers issues like using variables in output values, terraform blocks, typos, incorrect data retrieval from lists and maps, and variable scope problems, particularly with modules. An example illustrating the solution for using variables in output values is also provided. If you're still facing the error after reading this guide, please share the relevant parts of your Terraform code for further assistance.

Step-by-Step Guide

The error message "Error: Variables not allowed" in Terraform typically arises when you try to use a variable in a place where Terraform doesn't expect or allow them. Here's a breakdown of the common causes and how to fix them:

1. Variables in Output Values:

  • Problem: You're attempting to directly use an input variable within an output block's value argument. Terraform outputs are meant to expose values determined after resources are created, not just echo input variables.
  • Solution: Outputs should generally reference attributes of resources created by your Terraform code. If you need to make an input variable available after deployment, consider using a locals block to store its value and then reference the local variable in your output.

2. Variables in terraform Blocks (e.g., Backend Configuration):

  • Problem: You're trying to use variables directly within blocks like terraform { backend {} }. These blocks configure Terraform's behavior and are evaluated before variable values are fully known.
  • Solution: For backend configurations, you often need to use alternative approaches like:
    • Command-line Flags: Pass values directly to Terraform using flags like -backend-config="key=value".
    • Environment Variables: Set environment variables that Terraform can read (e.g., TF_VAR_access_key).
    • Configuration Files: Use separate configuration files (often with a .tfvars extension) to provide values for backend settings.

3. Typos or Incorrect Syntax:

  • Problem: A simple typo or syntax error in your variable reference can lead to this error.
  • Solution: Double-check:
    • You're using the correct variable name (case-sensitive).
    • You're referencing the variable correctly (e.g., var.my_variable).

4. Incorrect Data Retrieval (Especially with Lists/Maps):

  • Problem: You're trying to access an element in a list or map using an index or key that doesn't exist, or you're not handling empty lists/maps gracefully.
  • Solution:
    • Verify Indices/Keys: Ensure the index or key you're using actually exists in your data structure.
    • Handle Empty Collections: Use conditional logic (e.g., length(var.my_list) > 0 ? var.my_list[0] : null) to prevent errors when dealing with potentially empty lists or maps.

5. Variable Scope Issues:

  • Problem: You're trying to access a variable that's not in scope. This can happen when working with modules.
  • Solution:
    • Pass Variables to Modules: If a variable is needed within a module, ensure it's declared as an input variable for that module and passed in when calling the module.

Example (Illustrating Solution 1):

# Instead of this (which would cause the error):
# output "user_list" {
#   value = var.users 
# }

# Do this:
locals {
  user_list = var.users
}

output "user_list" {
  value = local.user_list
}

If you're still encountering the error, please provide the relevant parts of your Terraform code, including variable declarations, the block where the error occurs, and any relevant input values or configurations.

Code Example

This Terraform code demonstrates best practices for handling variables, outputs, and potential errors. It shows how to correctly use locals for outputs, environment variables for backend configuration, and count for iterating over lists. It also highlights the importance of variable scope when working with modules, ensuring that variables are passed as inputs to be accessible within the module's context.

# Incorrect: Using a variable directly in an output
# output "bucket_name" {
#   value = var.bucket_name
# }

# Correct: Using a local to store the variable value
variable "bucket_name" {
  type = string
}

locals {
  bucket_name = var.bucket_name
}

output "bucket_name" {
  value = local.bucket_name
}

# Incorrect: Using a variable in a terraform block
# terraform {
#   backend "s3" {
#     bucket = var.backend_bucket 
#   }
# }

# Correct: Using environment variables for backend configuration
# Set TF_VAR_backend_bucket before running Terraform

terraform {
  backend "s3" {
    bucket = "your-backend-bucket-name" # Replace with actual bucket name
  }
}

# Incorrect: Accessing a list element with a potentially invalid index
# variable "servers" {
#   type = list(string)
#   default = ["server1", "server2"]
# }

# resource "null_resource" "example" {
#   provisioner "local-exec" {
#     command = "echo ${var.servers[2]}" # Index out of bounds!
#   }
# }

# Correct: Handling potential index errors
variable "servers" {
  type = list(string)
  default = ["server1", "server2"]
}

resource "null_resource" "example" {
  count = length(var.servers)

  provisioner "local-exec" {
    command = "echo ${var.servers[count.index]}" 
  }
}

# Variable scope issue (demonstrated with modules)

# main.tf
variable "environment" {
  default = "dev"
}

module "my_module" {
  source = "./module"
}

# module/variables.tf
variable "component_name" {
  type = string
}

# module/main.tf
resource "null_resource" "example" {
  # Incorrect: Trying to access var.environment directly
  # provisioner "local-exec" {
  #   command = "echo ${var.environment}-${var.component_name}"
  # }

  # Correct: Passing var.environment as an input to the module
  provisioner "local-exec" {
    command = "echo ${var.environment}-${var.component_name}"
  }
}

Explanation:

  • Output Example: The corrected code uses a locals block to store the value of var.bucket_name. This allows the output to reference the stored value (local.bucket_name) instead of the variable directly.

  • Backend Configuration Example: The commented-out section shows the incorrect use of a variable within the terraform block. The corrected part suggests using environment variables (like TF_VAR_backend_bucket) as a common way to configure backends.

  • List Index Example: The first null_resource block demonstrates the error of accessing a list element with a potentially out-of-bounds index. The corrected version uses count and count.index to iterate over the list safely.

  • Module Example: This highlights how variable scope can lead to errors. The incorrect code in module/main.tf tries to access var.environment directly, which is not in scope within the module. The corrected version passes var.environment as an input variable to the module, making it accessible within the module's scope.

Additional Notes

General Understanding:

  • Terraform's Two-Phase Execution: It's crucial to remember that Terraform operates in two main phases: configuration parsing/validation and resource provisioning. The "Variables not allowed" error often stems from trying to use a variable's value before it's fully determined in the first phase.
  • Context is Key: The specific context where you encounter this error is essential for diagnosis. Always examine the surrounding code block (output, terraform {}, resource attribute, etc.) to understand why variables might be restricted there.

Troubleshooting Tips:

  • terraform console: This interactive tool can be invaluable for debugging variable-related issues. You can experiment with variable references and data structures to see how Terraform evaluates them.
  • Break Down Complex Expressions: If you're using variables within complex expressions, try breaking them down into smaller, more manageable parts. This can help pinpoint the source of the error.
  • Check Terraform Version: While rare, some behaviors related to variables and their allowed contexts might change slightly between Terraform versions. If you're working with an older version, consider upgrading if possible.

Beyond the Basics:

  • Data Sources: Remember that data sources also fetch information during the provisioning phase. You cannot use variables directly within data source arguments that expect resource IDs or other values determined at runtime.
  • Dynamic Blocks: When working with dynamic blocks (which offer flexibility in resource configuration), be mindful of variable usage within them. The same principles of scope and evaluation order apply.

Best Practices:

  • Clear Variable Naming: Use descriptive names for your variables to improve code readability and make it easier to understand their intended purpose.
  • Variable Validation: Leverage input variable validation to enforce data types and constraints. This can prevent unexpected values from causing errors later in your code.
  • Modularization: Organize your Terraform code into modules to promote reusability and better manage variable scope. Pass variables explicitly to modules to make dependencies clear.

Summary

This table summarizes the common causes of the "Error: Variables not allowed" message in Terraform and provides solutions:

Cause Problem Description Solution
Variables in Output Values Trying to directly use an input variable within an output block's value argument. Outputs should reflect values determined after resource creation. Store the variable's value in a locals block and reference the local variable in the output.
Variables in terraform Blocks Using variables directly within blocks like terraform { backend {} }. These blocks are evaluated before variable values are known. Use command-line flags, environment variables, or separate configuration files (.tfvars) to provide values for these settings.
Typos or Incorrect Syntax A typo in the variable name or an incorrect reference to the variable. Double-check the variable name (case-sensitive) and ensure you're referencing it correctly (e.g., var.my_variable).
Incorrect Data Retrieval (Lists/Maps) Accessing a non-existent index/key in a list/map or not handling empty collections properly. Verify the index/key exists and use conditional logic to handle potentially empty lists/maps.
Variable Scope Issues Trying to access a variable outside its scope, often when working with modules. Declare the variable as an input variable for the module and pass it in when calling the module.

Key Point: Terraform has specific rules about where variables can be used. Understanding these rules is crucial for writing valid Terraform code.

Conclusion

By understanding where variables are and are not allowed in Terraform, you can write more effective and error-free infrastructure code. Remember to use locals for outputs, environment variables or configuration files for backend settings, and be mindful of variable scope, especially with modules. If you encounter the "Error: Variables not allowed" message, carefully review the context of the error, the surrounding code, and the solutions provided in this article to resolve the issue. Clear variable naming, validation, and modularization are valuable practices to further enhance your Terraform code quality and maintainability.

References

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