terraform/
├── main.tf           # Main resources
├── variables.tf      # Input variables
├── outputs.tf        # Output values
├── providers.tf      # Provider config
├── versions.tf       # Version constraints
├── terraform.tfvars  # Variable values
└── modules/
    └── vpc/
        ├── main.tf
        ├── variables.tf
        └── outputs.tf

# versions.tf
terraform {
  required_version = ">= 1.5"

  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
    }
  }

  backend "s3" {
    bucket         = "terraform-state"
    key            = "prod/terraform.tfstate"
    region         = "us-east-1"
    dynamodb_table = "terraform-locks"
    encrypt        = true
  }
}

provider "aws" {
  region = var.aws_region

  default_tags {
    tags = {
      Environment = var.environment
      ManagedBy   = "terraform"
    }
  }
}

# variables.tf
variable "environment" {
  description = "Environment name"
  type        = string

  validation {
    condition     = contains(["dev", "staging", "prod"], var.environment)
    error_message = "Environment must be dev, staging, or prod."
  }
}

variable "instance_config" {
  description = "EC2 instance configuration"
  type = object({
    instance_type = string
    ami_id        = string
    volume_size   = number
  })

  default = {
    instance_type = "t3.micro"
    ami_id        = "ami-12345678"
    volume_size   = 20
  }
}

# main.tf
resource "aws_vpc" "main" {
  cidr_block           = var.vpc_cidr
  enable_dns_hostnames = true

  tags = {
    Name = "${var.project}-vpc"
  }
}

resource "aws_subnet" "private" {
  count             = length(var.availability_zones)
  vpc_id            = aws_vpc.main.id
  cidr_block        = cidrsubnet(var.vpc_cidr, 8, count.index)
  availability_zone = var.availability_zones[count.index]

  tags = {
    Name = "${var.project}-private-${count.index + 1}"
  }
}

# Using a module
module "vpc" {
  source = "./modules/vpc"

  project     = var.project
  environment = var.environment
  cidr_block  = "10.0.0.0/16"
}

# Module outputs
output "vpc_id" {
  value = module.vpc.vpc_id
}

data "aws_ami" "amazon_linux" {
  most_recent = true
  owners      = ["amazon"]

  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }
}

resource "aws_instance" "web" {
  ami           = data.aws_ami.amazon_linux.id
  instance_type = var.instance_type
}

# Initialize
terraform init

# Format
terraform fmt -recursive

# Validate
terraform validate

# Plan
terraform plan -out=tfplan

# Apply
terraform apply tfplan

# Destroy
terraform destroy