Day 5: Terraform Variables-#30DaysOfAWSTerraform

Terraform variables are inputs that allow to customize infrastructure without modifying the main configuration. Think of them like function arguments — you write your Terraform code once but change the values whenever needed.

Why variables matter:

• Avoid hard-coding resource values

• Make Terraform configurations reusable

• Improve code organization

• Support multiple environments (dev, test, prod)

Types of Variables in Terraform

Terraform uses variables to define values that can be passed into your configuration, making your code reusable and configurable. They are broadly categorized in two ways: by purpose (how they are used) and by value (what type of data they hold).

1. Based on Purpose (How they are used)

Input Variables

These are the most common type. They allow you to define parameters for your configuration, letting users customize the deployment without editing the core code.

Example code:

variable "environment" {
    default = "dev"
    type = string
}

#Create S3 Bucket
resource "aws_s3_bucket" "first_demo_bucket"{
  bucket = "tech-demo-mathanki-bucket-1-${aws_vpc.demo_vpc.id}"

  tags = {
    Name        = "${var.environment}-my-bucket-1.0"
    Environment = var.environment
  }
}

access the variable value using the var. prefix.

Terraform gives multiple flexible ways to assign values to input variables. Each approach is useful in a different scenario.

1. Default values

variable "environment" {
    default = "dev"
    type = string
}

when run terraform plan without passing anything it set the environment=dev

When to use:

• When you want a safe fallback value

• When running Terraform in development or tutorials

• When the variable is optional

2. terraform.tfvars File (Auto-loaded)

This file is automatically read by Terraform if present in the working directory.

How it works:

• Terraform automatically loads terraform.tfvars

• It overrides default values

When to use:

• To store environment-specific values

• To keep variable values separate from code

• Very common in production setups (e.g., dev.tfvars, prod.tfvars)

3. Command Line Flags (-var)

Pass variable values directly when running Terraform commands.

How it works:

• CLI variables override both defaults and tfvars files

When to use:

• Quick testing

• CI/CD pipelines

• When you want temporary values without editing files

Avoid using this for sensitive values like passwords — it may get logged in terminal history.

4. Environment Variables (TF_VAR_)

Terraform automatically loads environment variables prefixed with TF_VAR_

How it works

• The variable TF_VAR_environment maps to var.environment

• Overrides default & tfvars unless CLI -var is used

When to use:

• Passing secrets from a secure environment (Azure DevOps, GitHub Actions, Jenkins, AWS SSM)

• Preventing credentials from being stored in files

• CI/CD environments

Most secure method

Useful when integrating Terraform with:

• HashiCorp Vault

• AWS Secrets Manager

• GitHub Actions Secrets

• Docker & Kubernetes environments

Priority Order (Highest → Lowest)

Terraform applies values in this order:

1. CLI -var (Highest priority)

2. Environment variables (TF_VAR_)

3. .tfvars files

4. Default values in variables.tf (Lowest priority)

Locals (Local Values)

To define named expressions within a module. They help you avoid repeating the same complex logic or string concatenation multiple times, improving readability and maintainability. Locals are not exposed to the user like Input or Output variables.

Example code:

locals {
  bucket_name="${var.bucket-name}-${var.environment}"
  vpc_name="${var.environment}-vpc"
}

#Create S3 Bucket
resource "aws_s3_bucket" "first_demo_bucket"{
  bucket = local.bucket_name

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

access the value using the local. prefix.

Output(Output Values)

To return specific, important attributes of the created infrastructure to the command line user or to other Terraform configurations that depend on this module.

Example code:

output "vpc_id" {
    value = aws_vpc.demo_vpc.id
}

output "ec2_id" {
    value = aws_instance.web_server.id
}

After running terraform apply, these values are displayed in the terminal:

Once the resources are created, the output values are available for display using the following command:

terraform output

Conclusion

Terraform variables are the foundational elements that inject flexibility and reusability into your Infrastructure as Code (IaC). By skillfully leveraging input variables (for external configuration), local variables (for internal computation and reuse), and output variables (for exposing critical resource data), you transform static configurations into dynamic, adaptable blueprints

Reference

Terraform Variables in AWS — Input vs Output vs Local Variables