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layout	page_title	sidebar_current	description
docs	Configuration Syntax	docs-config-syntax	The Terraform language has its own syntax, intended to combine declarative structure with expressions in a way that is easy for humans to read and understand.

Configuration Syntax

Other pages in this section have described various configuration constructs that can appear in the Terraform language. This page describes the lower-level syntax of the language in more detail, revealing the building blocks that those constructs are built from.

This page describes the native syntax of the Terraform language, which is a rich language designed to be easy for humans to read and write. The constructs in the Terraform language can also be expressed in JSON syntax, which is harder for humans to read and edit but easier to generate and parse programmatically.

This low-level syntax of the Terraform language is defined in terms of a syntax called HCL, which is also used by configuration languages in other applications, and in particular other HashiCorp products. It is not necessary to know all of the details of HCL syntax in order to use Terraform, and so this page summarizes the most important details. If you are interested, you can find a full definition of HCL syntax in the HCL native syntax specification.

Attributes and Blocks

The Terraform language syntax is built around two key syntax constructs: attributes and blocks.

An attribute assigns a value to a particular name:

image_id = "abc123"

The identifier before the equals sign is the attribute name, and after the equals sign is the attribute's value. The semantics applied to each attribute name define what value types are valid, but many attributes accept arbitrary expressions, which allow the value to either be specified literally or generated from other values programmatically.

A block is a container for 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 shown here expects two labels, which are "aws_instance" and "example" in this case. 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 attributes and blocks may be nested, creating a heirarchy of blocks and their associated attributes.

Unfortunately, the low-level syntax described here uses the noun "attribute" to mean something slightly different to how it is used by the main Terraform language. Elsewhere in this documentation, "attribute" usually refers to a named value exported by an object that can be accessed in an expression, such as the "id" portion of the expression aws_instance.example.id. To reduce confusion, other documentation uses the term "argument" to refer to the syntax-level idea of an attribute.

Style Conventions

The Terraform parser allows you some flexibility in how you lay out the elements in your configuration files, but the Terraform language also has some idiomatic style conventions which we recommend users should always follow for consistency between files and modules written by different teams. Automatic source code formatting tools may apply these conventions automatically.

Indent two spaces for each nesting level.
When multiple attributes with single-line values appear on consecutive lines at the same nesting level, align their equals signs:
```
ami           = "abc123"
instance_type = "t2.micro"
```
When both attributes and blocks appear together inside a block body, place all of the attributes together at the top and then place nested blocks below them. Use one blank line to separate the attributes from the blocks.
Use empty lines to separate logical groups of attributes within a block.

For blocks that contain both arguments and "meta-arguments" (as defined by the Terraform language semantics), list meta-argument attributes first and separate them from other attributes with one blank line. Place meta-argument blocks last and separate them from other blocks with one blank line.

resource "aws_instance" "example" {
  count = 2 # meta-argument attribute first

  ami           = "abc123"
  instance_type = "t2.micro"

  network_interface {
    # ...
  }

  lifecycle { # meta-argument block last
    create_before_destroy = true
  }
}

Top-level blocks should always be separated from one another by one blank line. Nested blocks should also be separated by blank lines, except when grouping together related blocks of the same type.
Avoid separating multiple blocks of the same type with other blocks of a different type, unless the block types are defined by semantics to form a family. (For example: root_block_device, ebs_block_device and ephemeral_block_device on aws_instance form a family of block types describing AWS block devices, and can therefore be grouped together and mixed.)

Identifiers

Attribute names, block type names, and the names of most Terraform-specific constructs like resources, input variables, etc. are all identifiers. The Terraform language implements the Unicode identifier syntax, extended to also include the ASCII hyphen character -.

In practice, this means that identifiers can contain letters, digits, underscores, and hyphens. To avoid ambiguity with literal numbers, the first character of an identifier must not be a digit.

Comments

The Terraform language supports three different syntaxes for comments:

# begins a single-line comment, ending at the end of the line
// also begins a single-line comment, as an alternative to #.
/* and */ are start and end delimiters for a comment that might span over multiple lines.

The # single-line comment style is the default comment style and should be used in most cases. Automatic configuration formatting tools may automatically transform // comments into # comments, since the double-slash style is not idiomatic.

Character Encoding and Line Endings

Terraform configuration files must always be UTF-8 encoded. While the delimiters of the language are all ASCII characters, Terraform accepts non-ASCII characters in identifiers, comments, and string values.

Terraform accepts configuration files with either Unix-style line endings (LF only) or Windows-style line endings (CR then LF), but the idiomatic style is to use the Unix convention, and so automatic configuration formatting tools may automatically transform CRLF endings to LF.

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