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HashiCorp Configuration Language  ›  Lesson

Blocks, Labels and Arguments

Basic Syntax 12 minEvery HCL block has a type, zero or more quoted labels and a body in braces, and the body holds arguments written name = value plus any nested blocks.

Blocks, Labels and Arguments

HCL has only a handful of shapes, and one of them — the block — is everywhere. Once you can take a block apart at a glance, every Terraform file becomes readable.

The rule

Type, labels, braces: arguments set values, nested blocks group them.

Here is Crumb's web server:

resource "aws_instance" "web" {
  ami           = "ami-0123456789abcdef0"
  instance_type = "t3.micro"

  root_block_device {
    volume_size = 20
  }
}
  • Block type — resource. It says what kind of block this is.
  • Labels — the quoted words after the type. A resource takes two: the resource type aws_instance and your own local name web.
  • Body — everything inside { }.
  • Arguments — name = value, like instance_type = "t3.micro". Each argument name appears once per block.
  • Nested blocks — root_block_device { ... } groups related settings. No = sign, and some nested blocks may repeat.

Different block types take different numbers of labels:

Block Labels Example
terraform 0 terraform { ... }
provider 1 provider "aws" { ... }
variable 1 variable "environment" { ... }
resource 2 resource "aws_instance" "web" { ... }

Value syntax: strings in double quotes, numbers bare (20), booleans true/false, lists ["a", "b"], maps { team = "web" }. Aligning the = signs is a style that terraform fmt applies for you.

The shortcut

Read every block left to right as "a (type) called (labels) with (body)". Then scan the body for =: a line with = is an argument; a name followed straight by { is a nested block.

Reading the web server blockStep 1 of 4

  1. Type

    resource: this block describes a piece of infrastructure.

A common slip is writing root_block_device = { volume_size = 20 }. That is an argument holding a map, which is a different thing, and most providers reject it for a field defined as a block.

Drills

Answer fast — trust the rule.

quick drill

In resource "aws_instance" "web" { ... }, what is web?

quick drill

How many labels does a provider block take?

quick drill

Which line is a nested block?

quick drill

Which value is a map?

Remember

  • A block is type, labels and a body in braces.
  • Shortcut: = means argument; name then { means nested block.
  • Trap: name = { } is an argument holding a map, not a nested block.

Practice — level up

0/1 solved

Write your answer, then compare it with a rubric and a model answer.

ExplainTake a block apart

Using Crumb's aws_instance web server block, name each part: the block type, each label and what it means, one argument, and one nested block. Then explain the difference between an argument and a nested block.