HCL and Configuration

HashiCorp Configuration Language (HCL) is the language used to write Terraform configuration files. This lesson covers HCL syntax, block structure, arguments, expressions, functions, and comments.

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HCL Basics

HCL files use the .tf extension. The language is designed to be:

• Human-readable — easy to write and review
• Machine-friendly — parseable and generatable
• Declarative — describes the desired end state

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Block Structure

HCL is built around blocks. Every block has a type, optional labels, and a body:

block_type "label_1" "label_2" {
  argument = value
}

Common Block Types

Block Type	Labels	Purpose
terraform	none	Terraform settings (providers, backend)
provider	provider name	Provider configuration
resource	type, name	Infrastructure resource
data	type, name	Data source (read-only)
variable	name	Input variable
output	name	Output value
locals	none	Local values
module	name	Module call

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Arguments and Attributes

Arguments assign values inside blocks:

resource "aws_instance" "web" {
  ami           = "ami-0c55b159cbfafe1f0"   # string
  instance_type = "t3.micro"                # string
  count         = 2                         # number
  monitoring    = true                      # boolean
}

Attributes are values exposed by a resource after creation (e.g., aws_instance.web.id).

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Data Types

Type	Example	Description
string	"hello"	Text
number	42, 3.14	Integer or float
bool	true, false	Boolean
list	["a", "b", "c"]	Ordered collection
set	toset(["a", "b"])	Unordered unique collection
map	{ key = "value" }	Key-value pairs
object	{ name = string, age = number }	Typed key-value pairs
tuple	[string, number]	Typed ordered collection
null	null	Absence of a value

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String Interpolation and Templates

Use ${ } for interpolation within strings:

resource "aws_instance" "web" {
  tags = {
    Name = "web-${var.environment}"
  }
}

Heredoc Strings

For multi-line strings:

resource "aws_iam_policy" "example" {
  policy = <<-EOF
    {
      "Version": "2012-10-17",
      "Statement": [{
        "Effect": "Allow",
        "Action": "s3:GetObject",
        "Resource": "*"
      }]
    }
  EOF
}

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Expressions

References