Terraform Language Patterns Cheat Sheet

variable "environment" {
  type        = string
  description = "Deployment environment"
}

# Declare a typed string variable.

variable "subnet" {
  type = object({
    cidr = string
    az   = string
  })
}

# Declare a typed object variable.

locals {
  common_tags = {
    app         = "web"
    environment = var.environment
  }
}

# Compute reusable values in locals.

output "db_password" {
  value     = random_password.db.result
  sensitive = true
}

# Mark an output sensitive to reduce accidental display.

resource "aws_instance" "web" {
  count = 2
  ami   = var.ami_id
  instance_type = "t3.micro"
}

# Create multiple similar resources with count.

resource "aws_s3_bucket" "logs" {
  for_each = toset(["app","elb","audit"])
  bucket   = "${var.prefix}-${each.key}"
}

# Create resources keyed by map/set members.

resource "aws_instance" "web" {
  # ...
  depends_on = [aws_internet_gateway.this]
}

# Force resource ordering when dependency is not inferred from arguments.

resource "aws_s3_bucket" "logs" {
  bucket = var.bucket_name
  lifecycle {
    prevent_destroy = true
    ignore_changes  = [tags]
  }
}

# Control replace/delete behavior and ignored drift.

instance_type = var.environment == "prod" ? "t3.large" : "t3.micro"

# Choose values conditionally.

locals {
  private_subnet_ids = [for s in aws_subnet.private : s.id]
}

# Transform collections with a for expression.

locals {
  tags = merge(local.common_tags, { owner = "platform" })
}

# Combine multiple maps into one.

locals {
  instance_arn = try(aws_instance.web.arn, null)
  has_name     = can(var.config.name)
}

# Handle optional values and guard against invalid access.

user_data = templatefile("${path.module}/user-data.sh.tftpl", {
  environment = var.environment
})

# Render a file template with variables.

dynamic "ingress" {
  for_each = var.ingress_rules
  content {
    from_port   = ingress.value.from_port
    to_port     = ingress.value.to_port
    protocol    = ingress.value.protocol
    cidr_blocks = ingress.value.cidr_blocks
  }
}

# Generate repeated nested blocks from input data.

data "aws_ami" "ubuntu" {
  most_recent = true
  owners      = ["099720109477"]

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

# Query existing infrastructure data for use in resources.