A note of Terraform HCL basic syntax

[yunkon-kim]

Directories and moduels

A module is a collection of .tf and/or .tf.json files kept together in a directory.

A Terraform module only consists of the top-level configuration files in a directory; nested directories are treated as completely separate modules, and are not automatically included in the configuration.

(see Directories and Modules in Files and Directories

Blocks

A block is a container of other content:

resource "aws_instance" "example" {
  ami = "abc123"

  network_interface {
    # ...
  }
}

A block has a type (resource in this example). Each block type defines how many labels must follow the type keyword. The resource block type expects two labels, which are aws_instance and example in the example above. A particular block type may have any number of required labels, or it may require none as with the nested network_interface block type.

After the block type keyword and any labels, the block body is delimited by the { and } characters. Within the block body, further arguments and blocks may be nested, creating a hierarchy of blocks and their associated arguments.

(see Block in Configuration Syntax)

Identifiers

Argument names, block type names, and the names of most Terraform-specific constructs like resources, input variables, etc. are all identifiers.

Identifiers can contain letters, digits, underscores (_), and hyphens (-). The first character of an identifier must not be a digit, to avoid ambiguity with literal numbers.

For complete identifier rules, Terraform implements the Unicode identifier syntax, extended to include the ASCII hyphen character -.

(see Identifiers in Configuration Syntax

Resource Syntax

A resource block declares a resource of a specific type with a specific local name. Terraform uses the name when referring to the resource in the same module, but it has no meaning outside that module's scope.

In the following example, the aws_instance resource type is named web. The resource type and name must be unique within a module because they serve as an identifier for a given resource.

resource "aws_instance" "web" {
ami = "ami-a1b2c3d4"
instance_type = "t2.micro"
}

Within the block body (between { and }) are the configuration arguments for the resource itself. The arguments often depend on the > resource type. In this example, both ami and instance_type are special arguments for the aws_instance resource type.

(see Resource Syntax in Resource Blocks)