Mastering Terraform Variables in AWS: Input, Output, and Local Variables

Day 5 of 30 Days of AWS Terraform – Learn how to use variables effectively to make your Terraform code reusable, maintainable, and environment-agnostic.

Why Do We Need Variables in Terraform?

In real-world scenarios, you'll manage hundreds or thousands of resources like S3 buckets, VPCs, and EC2 instances.

Problems without variables:

• Repetition of the same values across multiple files

• Typos creating inconsistent setups

• Hard to maintain when you need to update values everywhere

The fix is to define values once using variables and reference them everywhere.

Terraform Variable Types Flow:

Flow 2: Detailed View

Terraform Variable Types

1. Input Variables

Input variables accept values from the user and are used across the configuration.

Basic example:

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

---

2. Local Variables

Local variables help you create reusable or computed values inside the same module.

Example:

// locals
# locals {
#     // can use like this also 
#  env = var.environment
#  vpc_name = "${local.env}-VPC"
#  instance_name = "${local.env}-Instance"
# }


locals {
  vpc_name = "${var.environment}-VPC"
  instance_name = "${var.environment}-Instance"
}

Useful for naming patterns or reusable strings.

---

3. Output Variables

Outputs show values after terraform apply or pass values to other modules.

Example:

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

output "instance_id" {
    value = aws_instance.example.id
}

---

Variable Precedence

Terraform follows this priority order:

1. Default value

2. Environment variables

3. terraform.tfvars

4. CLI -var flag

---

Complete Working Example (Your Code Added)

Your full code with variables, locals, S3 backend, VPC, EC2, and outputs is inserted here.

terraform {

    backend "s3" {
        bucket = "anjali-gupta-bucket-76"
        key    = "terraform.tfstate"
        region = "eu-north-1"
        use_lockfile = true
    }

  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 6.0"
    }
  }
}

provider "aws" {
  region = "eu-north-1"
}

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

variable "region" {
  type        = string
  default     = "eu-north-1"
}

locals {
  vpc_name = "${var.environment}-VPC"
  instance_name = "${var.environment}-Instance"
}

resource "aws_s3_bucket" "first_bucket" {
  bucket = "anjali-gupta-bucket-76"
  region = var.region

  tags = {
    Name        = "My bucket"
    Environment = var.environment
  }
}

resource "aws_vpc" "sample" {
    cidr_block = "10.0.1.0/24"
    region = var.region
    tags = {
      Environment = var.environment
      Name = local.vpc_name
    } 
}

resource "aws_instance" "example" {
  ami           = "resolve:ssm:/aws/service/ami-amazon-linux-latest/al2023-ami-kernel-default-x86_64"
  instance_type = "t2.micro"
  region        = var.region

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

output "instance_id" {
    value = aws_instance.example.id
}

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

---

Commands to Run

terraform init
terraform plan
terraform apply --auto-approve
terraform output
terraform destroy --auto-approve

---

Best Practices Checklist

Use input variables for environment-specific values
Use local variables for naming conventions
Define outputs for important resource IDs
Store defaults in terraform.tfvars
Use CLI -var for sensitive values
Destroy test resources after practice

Common Errors & Solutions

Video:

Key Takeaway

Variables transform Terraform from script to infrastructure factory. Define once, deploy everywhere. Environment promotion? Just swap tfvars files. 💪

Practice challenge: Create input vars for region/environment, locals for resource names, outputs for IDs. Run precedence tests!

Huge thanks to Piyush Sachdeva for the crystal-clear hands-on demo!

#30DaysOfAWSTerraform Day 5 complete!