Terraform HCL Syntax: A Practical Guide

What Is HCL?

HashiCorp Configuration Language (HCL) is the language Terraform uses for configuration files. It's designed to be human-readable while being machine-parseable.

Basic Structure

HCL files use blocks, arguments, and expressions:

hcl
# Block type "resource", labels "aws_instance" and "web"
resource "aws_instance" "web" {
  # Arguments
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  # Nested block
  tags = {
    Name = "WebServer"
  }
}

Variables

hcl
variable "instance_type" {
  description = "EC2 instance type"
  type        = string
  default     = "t2.micro"
}

variable "allowed_ports" {
  type    = list(number)
  default = [80, 443]
}

variable "tags" {
  type = map(string)
  default = {
    Environment = "dev"
    Team        = "platform"
  }
}

Reference with var.instance_type.

Outputs

hcl
output "instance_ip" {
  description = "Public IP of the web server"
  value       = aws_instance.web.public_ip
}

Expressions

hcl
# Conditional
instance_type = var.environment == "prod" ? "t2.large" : "t2.micro"

# String interpolation
name = "server-${var.environment}-${count.index}"

# For expressions
upper_names = [for name in var.names : upper(name)]

Built-in Functions

hcl
# String functions
name = lower("MyServer")           # "myserver"
id   = substr(var.long_id, 0, 8)   # first 8 chars

# Collection functions
first = element(var.list, 0)
merged = merge(var.tags, { Name = "web" })

# File functions
key = file("~/.ssh/id_rsa.pub")

Locals

hcl
locals {
  common_tags = {
    Project     = var.project_name
    Environment = var.environment
    ManagedBy   = "terraform"
  }
}

resource "aws_instance" "web" {
  tags = local.common_tags
}

Learn More

Master HCL from scratch in our Terraform for Beginners course — practical exercises with real AWS infrastructure.