Terraform¶

# Initialize working directory (first time setup)
terraform init # (1)!

# Validate configuration syntax
terraform validate # (2)!

# Format code (like prettier for HCL)
terraform fmt -recursive # (3)!

# Plan changes (dry run - ALWAYS do this)
terraform plan -out=tfplan # (4)!

# Apply changes (create/update infrastructure)
terraform apply tfplan # (5)!

# Destroy everything (dangerous!)
terraform destroy # (6)!

# Show current state
terraform show # (7)!

# List resources in state
terraform state list # (8)!

# Import existing resource into state
terraform import aws_instance.example i-1234567890abcdef0 # (9)!

# Refresh state from real infrastructure
terraform refresh # (10)!

# Output values from state
terraform output # (11)!

# Workspace management (like git branches for infrastructure)
terraform workspace list
terraform workspace new staging # (12)!
terraform workspace select production

# Provider configuration (AWS example)
terraform {
  required_version = ">= 1.9.0" # (1)!

  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0" # (2)!
    }
  }

  backend "s3" { # (3)!
    bucket         = "my-terraform-state"
    key            = "prod/terraform.tfstate"
    region         = "us-east-1"
    dynamodb_table = "terraform-locks"
    encrypt        = true
  }
}

provider "aws" {
  region = var.aws_region

  default_tags { # (4)!
    tags = {
      Environment = var.environment
      ManagedBy   = "Terraform"
      Project     = var.project_name
    }
  }
}

# Variables (inputs)
variable "environment" {
  description = "Environment name (dev, staging, prod)"
  type        = string
  validation { # (5)!
    condition     = contains(["dev", "staging", "prod"], var.environment)
    error_message = "Environment must be dev, staging, or prod."
  }
}

variable "instance_type" {
  description = "EC2 instance type"
  type        = string
  default     = "t3.micro" # (6)!
}

# Data sources (query existing resources)
data "aws_ami" "ubuntu" { # (7)!
  most_recent = true
  owners      = ["099720109477"] # Canonical

  filter {
    name   = "name"
    values = ["ubuntu/images/hvm-ssd/ubuntu-jammy-22.04-amd64-server-*"]
  }
}

# Resources (create new infrastructure)
resource "aws_instance" "web" { # (8)!
  ami           = data.aws_ami.ubuntu.id
  instance_type = var.instance_type

  tags = {
    Name = "${var.environment}-web-server"
  }

  lifecycle { # (9)!
    create_before_destroy = true
    prevent_destroy       = false
  }
}

# Outputs (export values)
output "instance_id" { # (10)!
  description = "EC2 instance ID"
  value       = aws_instance.web.id
}

output "public_ip" {
  description = "Public IP address"
  value       = aws_instance.web.public_ip
  sensitive   = false # (11)!
}

# Module usage (reusable infrastructure components)
module "vpc" { # (1)!
  source  = "terraform-aws-modules/vpc/aws"
  version = "5.5.0"

  name = "${var.environment}-vpc"
  cidr = "10.0.0.0/16"

  azs             = ["us-east-1a", "us-east-1b", "us-east-1c"]
  private_subnets = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24"]
  public_subnets  = ["10.0.101.0/24", "10.0.102.0/24", "10.0.103.0/24"]

  enable_nat_gateway = true
  single_nat_gateway = var.environment != "prod" # (2)!

  tags = var.common_tags
}

# Local values (computed within module)
locals { # (3)!
  common_tags = {
    Environment = var.environment
    ManagedBy   = "Terraform"
    CostCenter  = "engineering"
  }

  db_name = "${var.project_name}-${var.environment}-db"
}

# Count meta-argument (create multiple similar resources)
resource "aws_instance" "app" { # (4)!
  count = var.instance_count

  ami           = data.aws_ami.ubuntu.id
  instance_type = var.instance_type

  tags = {
    Name = "${var.environment}-app-${count.index}"
  }
}

# For_each meta-argument (better than count for maps)
resource "aws_iam_user" "developers" { # (5)!
  for_each = toset(var.developer_names)

  name = each.value
  path = "/developers/"

  tags = {
    Team = "Engineering"
  }
}