Terraform Variables & Outputs

🌱 Introduction to Terraform Variables & Outputs

In Terraform, variables and outputs are the foundation for building reusable, scalable, and modular infrastructure. Variables allow you to parameterize your code and make it dynamic, while outputs let you extract and share important information after the infrastructure is provisioned. In this blog, we'll break down how to use both effectively.

📦 Why Use Variables?

Variables are useful when you want to avoid hardcoding values like region names, instance sizes, or environment-specific data. They enhance code reusability and allow you to manage changes with minimal effort.

🔧 Defining Variables

Variables can be defined in a separate variables.tf file using the variable block. You can specify a default value or leave it open for user input.

variable "region" {
  description = "The AWS region to deploy into"
  type        = string
  default     = "us-west-2"
}

variable "instance_type" {
  description = "EC2 instance type"
  type        = string
}

📂 Providing Variable Values

You can pass variable values in multiple ways:

• Using the -var flag
• Through a terraform.tfvars file
• Environment variables

terraform apply -var="instance_type=t2.micro"

region        = "us-east-1"
instance_type = "t3.medium"

🧮 Variable Types

Terraform supports various types including string, number, bool, list, map, and even custom object types.

variable "tags" {
  type = map(string)
  default = {
    Environment = "dev"
    Owner       = "admin"
  }
}

🔄 Using Locals for Internal Values

locals allow you to define internal variables within a module, useful for derived or computed values.

locals {
  full_name = "${var.project_name}-${var.environment}"
}

🚀 Introduction to Outputs

Outputs let you display useful information after Terraform has applied your configuration. They are commonly used to extract values like public IPs, resource IDs, or module exports.

📤 Declaring Output Values

Use the output block in your Terraform code to expose values.

output "instance_public_ip" {
  description = "Public IP of the deployed EC2 instance"
  value       = aws_instance.my_ec2.public_ip
}

🔗 Sharing Data Between Modules

Outputs are key for passing values between modules. One module’s output can serve as the input for another.

// In module A
output "vpc_id" {
  value = aws_vpc.main.id
}

// In main.tf
module "vpc" {
  source = "./modules/vpc"
}

module "ec2" {
  source = "./modules/ec2"
  vpc_id = module.vpc.vpc_id
}

📋 Naming Conventions & Documentation

Use clear, descriptive names for both variables and outputs. Document them using the description argument so collaborators understand their purpose without guessing.

🧪 Best Practices

• Group your variables in logical files like variables.tf.
• Use terraform.tfvars to manage environment-specific values.
• Secure sensitive variables using sensitive = true to prevent them from showing in logs.
• Validate input using validation blocks in Terraform 0.13+.

variable "environment" {
  type        = string
  description = "Deployment environment"
  validation {
    condition     = contains(["dev", "staging", "prod"], var.environment)
    error_message = "Environment must be one of dev, staging, or prod."
  }
}

✅ Conclusion

Mastering Terraform Variables & Outputs is essential for writing clean, efficient, and maintainable IaC. By abstracting configuration values and exposing outputs smartly, you enable better reuse, automation, and team collaboration. Whether you're building a small project or managing enterprise-scale infrastructure, these concepts are fundamental to success.