Day 09: HCL (HashiCorp Configuration Language) — The Terraform Syntax Guide I Wish I Had.

Terraform uses HCL—a clean, declarative language—to describe infrastructure. Once you understand HCL’s building blocks, every Terraform repo starts to make sense.

HCL at a glance

• Blocks: TYPE "LABEL" "LABEL" { ... }

• Arguments: key = value

• Expressions: references (var.region, local.tags), functions (merge(), format()), conditionals, loops.

• Types: string, number, bool, list(...), set(...), map(...), object({...}), tuple([...])

• Comments: #, //, or /* ... */

---

1) terraform block

Declares provider dependencies, Terraform version, and (optionally) backend.

terraform {
  required_version = ">= 1.6.0"

  required_providers {
    aws = { source = "hashicorp/aws",   version = "6.9.0" }
    random = { source = "hashicorp/random", version = "3.7.2" }
  }

  # backend "s3" { ... }  # optional: store tfstate remotely
}

Why: pins versions for reproducibility.

---

2) provider block

Configures how Terraform talks to a cloud.

provider "aws" {
  region = var.region
  # alias = "west"  # use aliases to configure multiple regions/accounts
}

Pro tip: use env vars (AWS_PROFILE, AWS_ACCESS_KEY_ID…) for credentials.

---

3) variable blocks

Inputs you pass to your configuration.

variable "region" {
  description = "AWS region"
  type        = string
  default     = "ap-south-1"
}

variable "bucket_name_prefix" {
  description = "Prefix for S3 bucket name"
  type        = string
  validation {
    condition     = length(var.bucket_name_prefix) >= 3
    error_message = "Prefix must be at least 3 chars."
  }
}

Tips:

• sensitive = true hides values in CLI output.

• TF_VAR_region=ap-south-1 terraform apply can set vars via env.

---

4) locals block

Reusable computed values.

locals {
  name_prefix = "hcl101"
  common_tags = {
    Project = "Terraform-HCL"
    Owner   = "Abdul Raheem"
    Env     = "dev"
  }
}

Why: keeps code DRY.

---

5) data blocks (read existing things)

Query cloud for read-only info (no creation).

data "aws_ami" "amazon_linux" {
  most_recent = true
  owners      = ["amazon"]

  filter { name = "name", values = ["amzn2-ami-hvm-*-x86_64-gp2"] }
  filter { name = "virtualization-type", values = ["hvm"] }
}

Why: no hardcoded IDs; always fetch latest AMI, VPC, SG, etc.

---

6) resource blocks (create/update things)

Declaratively manage infrastructure.

resource "random_id" "suffix" {
  byte_length = 2
}

resource "aws_s3_bucket" "demo" {
  bucket = "${var.bucket_name_prefix}-${random_id.suffix.hex}"

  tags = merge(local.common_tags, { Name = "${local.name_prefix}-bucket" })

  lifecycle {
    prevent_destroy = false
    ignore_changes  = []   # specify attributes to ignore if needed
  }
}

Meta-arguments you should know:

• count / for_each → multiple resources

• depends_on → explicit dependency

• lifecycle → create_before_destroy, prevent_destroy, ignore_changes

• (Provisioners exist, but prefer cloud-native bootstrapping via user data)

---

7) output blocks

Expose values after apply.

output "bucket_name" {
  value       = aws_s3_bucket.demo.bucket
  description = "Created S3 bucket name"
}

output "ec2_public_ip" {
  value     = aws_instance.demo.public_ip
  sensitive = false
}

---

8) module blocks

Package and reuse infra (from local path, Git, or Registry).

# Example (disabled by default). Set count=1 to use.
module "vpc" {
  count  = 0
  source = "terraform-aws-modules/vpc/aws"
  version = "~> 5.0"

  name = "${local.name_prefix}-vpc"
  cidr = "10.1.0.0/16"

  azs             = ["ap-south-1a","ap-south-1b"]
  public_subnets  = ["10.1.1.0/24","10.1.2.0/24"]
  enable_dns_hostnames = true
  tags = local.common_tags
}

Module anatomy:

• Inputs → variables the module expects

• Resources → what it creates

• Outputs → what it returns

• Versioning → always pin version for reproducibility

---

9) Expressions you’ll use daily

• Conditionals: var.env == "prod" ? "m5.large" : "t3.micro"

• For-expressions: { for k, v in var.tags : upper(k) => v }

• Functions: merge(), coalesce(), join(), format(), file(), length()

• Dynamic blocks (generate nested blocks in loops)

---

✨ Mini Demo (complete main.tf below)

• Variables → region, bucket_name_prefix

• Locals → tags & name prefix

• Data → latest Amazon Linux AMI

• Resources → random suffix, S3 bucket, EC2 with Nginx via user data

• Outputs → bucket name & public IP

(See GitHub section for the ready-to-run file.)

---

Key takeaways

• HCL is block + arguments + expressions.

• Use variables, locals, data to keep configs clean and dynamic.

• Modules make infra reusable and production-grade.

• Prefer user data/cloud-init over provisioners.

• Pin versions and use backends for teamwork.

---

Follow my journey

• GitHub (learning repo): https://github.com/<your-username>/terraform-learning-journey

• X / Twitter: https://x.com/Abdulraheem183

• Terraform series: https://abdulraheem.hashnode.dev/series/terraform-with-aws