Terraform: Basic HCL

Basic HCL

HashiCorp Configuration Language (HCL) is a domain-specific language used for defining infrastructure as code, primarily in tools like Terraform. Here are the basics of HCL syntax:

Comments

You can use either # or // for single-line comments, and /* … */ for multi-line comments.

# This is a single-line comment
// This is also a single-line comment

/*
This is a
multi-line comment
*/

Blocks

Blocks are the fundamental building structures in HCL. They start with a keyword followed by a name and enclosed in {}.

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
}

Attributes

Attributes are key-value pairs defined within blocks. They can be strings, numbers, lists, or maps.

variable "instance_count" {
  description = "Number of instances"
  type        = number
  default     = 1
}

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  count         = var.instance_count
}

Strings

Strings can be defined using double quotes. Interpolation is done with ${}.

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  tags = {
    Name = "example-${var.environment}"
  }
}

Numbers

Numbers can be integers or floats.

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  count         = 2
}

Lists

Lists are enclosed in square brackets [] and elements are separated by commas.

variable "availability_zones" {
  type    = list(string)
  default = ["us-west-1a", "us-west-1b"]
}

Maps

Maps are enclosed in curly braces {} and key-value pairs are separated by commas.

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

Conditionals

Conditionals in HCL are written using the ternary operator.

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  count         = var.enable_instance ? 1 : 0
}

Functions

HCL support a variety of built-in functions.

output "instance_ids" {
  value = join(", ", aws_instance.example.*.id)
}

Locals

Locals are used to define reusable expressions.

locals {
  instance_name = "example-${var.environment}"
}

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  tags = {
    Name = local.instance_name
  }
}