# Initialize (download providers)
terraform init

# Preview changes
terraform plan
terraform plan -out=tfplan          # Save plan

# Apply changes
terraform apply
terraform apply tfplan              # Apply saved plan
terraform apply -auto-approve       # Skip confirmation

# Destroy resources
terraform destroy
terraform destroy -target=aws_instance.web

# Format code
terraform fmt
terraform fmt -recursive

# Validate configuration
terraform validate

# Show current state
terraform show
terraform state list
terraform state show aws_instance.web

# main.tf
terraform {
  required_version = ">= 1.5"
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
    }
  }

  backend "s3" {
    bucket = "my-terraform-state"
    key    = "prod/terraform.tfstate"
    region = "us-east-1"
  }
}

provider "aws" {
  region = var.region
}

# variables.tf
variable "region" {
  description = "AWS region"
  type        = string
  default     = "us-east-1"
}

variable "instance_type" {
  description = "EC2 instance type"
  type        = string
  default     = "t3.micro"
}

variable "tags" {
  description = "Resource tags"
  type        = map(string)
  default = {
    Environment = "dev"
    ManagedBy   = "terraform"
  }
}

variable "allowed_cidrs" {
  description = "Allowed CIDR blocks"
  type        = list(string)
  default     = ["0.0.0.0/0"]
}

# Pass variables
terraform plan -var="region=us-west-2"
terraform plan -var-file="production.tfvars"

# Auto-loaded files: terraform.tfvars, *.auto.tfvars

# outputs.tf
output "instance_ip" {
  description = "Public IP of the instance"
  value       = aws_instance.web.public_ip
}

output "bucket_arn" {
  description = "S3 bucket ARN"
  value       = aws_s3_bucket.main.arn
  sensitive   = true
}

terraform output
terraform output instance_ip
terraform output -json

# EC2 Instance
resource "aws_instance" "web" {
  ami           = "ami-0abcdef1234567890"
  instance_type = var.instance_type

  tags = merge(var.tags, {
    Name = "web-server"
  })

  lifecycle {
    create_before_destroy = true
    prevent_destroy       = true
    ignore_changes        = [tags["UpdatedAt"]]
  }
}

# S3 Bucket
resource "aws_s3_bucket" "main" {
  bucket = "my-app-${var.environment}"
}

# Security Group
resource "aws_security_group" "web" {
  name        = "web-sg"
  description = "Web server security group"
  vpc_id      = var.vpc_id

  ingress {
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = var.allowed_cidrs
  }

  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

# Look up existing resources
data "aws_ami" "ubuntu" {
  most_recent = true
  owners      = ["099720109477"]  # Canonical

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

data "aws_vpc" "default" {
  default = true
}

# Use in resource
resource "aws_instance" "web" {
  ami = data.aws_ami.ubuntu.id
}

# Using module
module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "5.0.0"

  name = "my-vpc"
  cidr = "10.0.0.0/16"

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

  enable_nat_gateway = true
}

# Local module
module "web_server" {
  source = "./modules/web-server"

  instance_type = "t3.small"
  subnet_id     = module.vpc.public_subnets[0]
}

# Import existing resource
terraform import aws_instance.web i-1234567890abcdef0

# Move resource in state
terraform state mv aws_instance.old aws_instance.new

# Remove from state (without destroying)
terraform state rm aws_instance.web

# Pull remote state
terraform state pull > state.json

# Replace provider
terraform state replace-provider hashicorp/aws registry.terraform.io/hashicorp/aws

# List workspaces
terraform workspace list

# Create/switch workspace
terraform workspace new staging
terraform workspace select production

# Use in config
resource "aws_instance" "web" {
  instance_type = terraform.workspace == "production" ? "t3.large" : "t3.micro"
}

# Conditional resource
resource "aws_instance" "bastion" {
  count = var.enable_bastion ? 1 : 0
  # ...
}

# For each
resource "aws_iam_user" "users" {
  for_each = toset(var.user_names)
  name     = each.value
}

# Dynamic blocks
resource "aws_security_group" "main" {
  dynamic "ingress" {
    for_each = var.ingress_rules
    content {
      from_port   = ingress.value.port
      to_port     = ingress.value.port
      protocol    = "tcp"
      cidr_blocks = ingress.value.cidrs
    }
  }
}

# Local values
locals {
  common_tags = {
    Environment = var.environment
    Project     = var.project_name
    ManagedBy   = "terraform"
  }
}