Variables Within Variables in Terraform

Terraform's variable system is powerful and flexible, allowing you to reference variables within other variables. This capability is useful for creating dynamic configurations that adapt to different environments or use cases. In this guide, you'll learn how to use variables within variables effectively.

Why Use Variables Within Variables?

• Dynamic Configurations: Adjust settings based on other variables.
• Reusability: Create templates that work across multiple environments.
• Simplified Management: Reduce duplication and centralize configuration logic.

Example Use Cases

• Deriving resource names from a base name variable.
• Constructing ARNs dynamically.
• Setting default values based on other variables.

Using Variables Within Variables

Let's walk through an example of how to use variables within variables in Terraform. This example will demonstrate how to define base variables and then create derived variables using the locals block.

Step 1: Define Input Variables

Start by defining the base variables that will be used to construct other variables.

variable "environment" {
  description = "The environment to deploy to (e.g., dev, staging, prod)."
  type        = string
}

variable "base_name" {
  description = "The base name for resources."
  type        = string
}

Step 2: Use Locals for Derived Variables

Use the locals block to define variables that depend on input variables. Locals are a great way to encapsulate logic and keep your configuration clean.

locals {
  resource_name = "${var.base_name}-${var.environment}"
  bucket_arn    = "arn:aws:s3:::${local.resource_name}"
}

Step 3: Reference Derived Variables

You can now use the derived variables in your resource definitions.

resource "aws_s3_bucket" "example" {
  bucket = local.resource_name
  acl    = "private"

  tags = {
    Environment = var.environment
    Name        = local.resource_name
  }
}

output "bucket_arn" {
  value = local.bucket_arn
}

Best Practices

• Use Descriptive Names: Name your variables and locals clearly to indicate their purpose.
• Avoid Over-Nesting: Keep variable references simple to improve readability.
• Validate Inputs: Use validation blocks to enforce constraints on input variables.
• Document Your Variables: Add descriptions to all variables for better maintainability.

Example Directory Structure

Here's an example of how your Terraform files might look:

my-terraform-project/
├── main.tf
├── variables.tf
├── outputs.tf

By using variables within variables, you can create dynamic and reusable Terraform configurations that are easier to manage and adapt to different environments.