What is HCL?

HCL stands for HashiCorp Configuration Language. It is the language you write Terraform configurations in. Every .tf file in a Terraform project is written in HCL. The goal of HCL is to sit in the sweet spot between JSON (machine-friendly but hard to read) and a full programming language (powerful but complex for configuration).

HCL is also used by other HashiCorp tools — Packer, Vault, Consul, and Nomad all use HCL for their configuration files.

HCL Syntax Basics

ROUTEROS

# This is a comment
 
# A block has a type, optional labels, and a body
resource "aws_s3_bucket" "order_images" {   # type=resource, labels="aws_s3_bucket" "order_images"
  bucket = "swiggy-order-images-prod"        # argument: name = value
  
  tags = {                                   # map value
    Environment = "production"
    Team        = "platform"
  }
}
 
# String interpolation
locals {
  bucket_name = "swiggy-${var.environment}-images"   # ${} for interpolation
}
 
# Multiline string (heredoc)
resource "aws_instance" "web" {
  user_data = <<-EOF
    #!/bin/bash
    echo "Hello from ${var.environment}" > /tmp/init.txt
    apt-get update -y
  EOF
}

HCL vs JSON

Terraform accepts both HCL (.tf) and JSON (.tf.json). HCL is preferred for human-written files:

NGINX

# HCL — readable, supports comments
resource "aws_s3_bucket" "example" {
  bucket = "my-bucket"
  tags   = { Environment = "prod" }
}

JSON

// JSON equivalent — verbose, no comments
{
  "resource": {
    "aws_s3_bucket": {
      "example": {
        "bucket": "my-bucket",
        "tags": { "Environment": "prod" }
      }
    }
  }
}

Types in HCL

GAMS

variable "examples" {
  # Primitive types
  # string  = "ap-south-1"
  # number  = 3
  # bool    = true
 
  # Collection types
  # list(string)  = ["a", "b", "c"]
  # map(string)   = { key = "value" }
  # set(string)   = toset(["a", "b"])
 
  # Structural types
  # object({ name = string, count = number })
  # tuple([string, number, bool])
}

Expressions and Functions

MAXIMA

locals {
  # Conditional expression (ternary)
  instance_type = var.environment == "prod" ? "t3.large" : "t3.small"
 
  # Built-in functions
  upper_env  = upper(var.environment)           # "PROD"
  joined     = join("-", ["zerodha", "prod"])   # "zerodha-prod"
  has_items  = length(var.subnet_ids) > 0       # true/false
 
  # For expression — transform a list
  upper_tags = [for tag in var.tags : upper(tag)]
 
  # For expression — transform a map
  tag_map = { for k, v in var.raw_tags : k => lower(v) }
}

Dynamic Blocks

NGINX

# Instead of repeating ingress blocks manually:
resource "aws_security_group" "app" {
  name = "app-sg"
 
  # Generate multiple ingress rules from a variable
  dynamic "ingress" {
    for_each = var.allowed_ports   # e.g., [80, 443, 8080]
    content {
      from_port   = ingress.value
      to_port     = ingress.value
      protocol    = "tcp"
      cidr_blocks = ["0.0.0.0/0"]
    }
  }
}

HCL Formatting

Bash

# Format all .tf files to standard style
terraform fmt
 
# Check formatting without changing files (use in CI)
terraform fmt -check
 
# Format recursively through subdirectories
terraform fmt -recursive

PLACEMENT PRO TIP
**Tip:** Run `terraform fmt` before every commit. Add `terraform fmt -check` to your CI pipeline to reject unformatted code. This keeps the codebase consistent across engineers.

REMEMBER THIS
**Remember:** HCL is not a general-purpose programming language. It has no loops in the traditional sense (use `for_each` and `for` expressions), no if/else blocks (use ternary), and no functions you can define. For complex logic, you need to think in HCL's declarative model.