130 lines
5.3 KiB
Markdown
130 lines
5.3 KiB
Markdown
---
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layout: "language"
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page_title: "Syntax - Configuration Language"
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sidebar_current: "docs-config-syntax"
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description: |-
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Key constructs of the native Terraform language syntax, including identifiers, arguments, blocks, and comments.
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---
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# Configuration Syntax
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Other pages in this section have described various configuration constructs
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that can appear in the Terraform language. This page describes the lower-level
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syntax of the language in more detail, revealing the building blocks that
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those constructs are built from.
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This page describes the _native syntax_ of the Terraform language, which is
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a rich language designed to be relatively easy for humans to read and write.
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The constructs in the Terraform language can also be expressed in
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[JSON syntax](/docs/language/syntax/json.html), which is harder for humans
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to read and edit but easier to generate and parse programmatically.
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This low-level syntax of the Terraform language is defined in terms of a
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syntax called _HCL_, which is also used by configuration languages in
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other applications, and in particular other HashiCorp products.
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It is not necessary to know all of the details of HCL syntax in
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order to use Terraform, and so this page summarizes the most important
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details. If you are interested, you can find a full definition of HCL
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syntax in
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[the HCL native syntax specification](https://github.com/hashicorp/hcl/blob/main/hclsyntax/spec.md).
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## Arguments and Blocks
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The Terraform language syntax is built around two key syntax constructs:
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arguments and blocks.
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### Arguments
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An _argument_ assigns a value to a particular name:
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```hcl
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image_id = "abc123"
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```
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The identifier before the equals sign is the _argument name_, and the expression
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after the equals sign is the argument's value.
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The context where the argument appears determines what value types are valid
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(for example, each resource type has a schema that defines the types of its
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arguments), but many arguments accept arbitrary
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[expressions](/docs/language/expressions/index.html), which allow the value to
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either be specified literally or generated from other values programmatically.
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-> **Note:** Terraform's configuration language is based on a more general
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language called HCL, and HCL's documentation usually uses the word "attribute"
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instead of "argument." These words are similar enough to be interchangeable in
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this context, and experienced Terraform users might use either term in casual
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conversation. But because Terraform also interacts with several _other_ things
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called "attributes" (in particular, Terraform resources have attributes like
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`id` that can be referenced from expressions but can't be assigned values in
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configuration), we've chosen to use "argument" in the Terraform documentation
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when referring to this syntax construct.
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### Blocks
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A _block_ is a container for other content:
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```hcl
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resource "aws_instance" "example" {
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ami = "abc123"
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network_interface {
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# ...
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}
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}
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```
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A block has a _type_ (`resource` in this example). Each block type defines
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how many _labels_ must follow the type keyword. The `resource` block type
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expects two labels, which are `aws_instance` and `example` in the example above.
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A particular block type may have any number of required labels, or it may
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require none as with the nested `network_interface` block type.
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After the block type keyword and any labels, the block _body_ is delimited
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by the `{` and `}` characters. Within the block body, further arguments
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and blocks may be nested, creating a hierarchy of blocks and their associated
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arguments.
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The Terraform language uses a limited number of _top-level block types,_ which
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are blocks that can appear outside of any other block in a configuration file.
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Most of Terraform's features (including resources, input variables, output
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values, data sources, etc.) are implemented as top-level blocks.
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## Identifiers
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Argument names, block type names, and the names of most Terraform-specific
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constructs like resources, input variables, etc. are all _identifiers_.
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Identifiers can contain letters, digits, underscores (`_`), and hyphens (`-`).
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The first character of an identifier must not be a digit, to avoid ambiguity
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with literal numbers.
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For complete identifier rules, Terraform implements
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[the Unicode identifier syntax](http://unicode.org/reports/tr31/), extended to
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include the ASCII hyphen character `-`.
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## Comments
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The Terraform language supports three different syntaxes for comments:
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* `#` begins a single-line comment, ending at the end of the line.
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* `//` also begins a single-line comment, as an alternative to `#`.
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* `/*` and `*/` are start and end delimiters for a comment that might span
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over multiple lines.
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The `#` single-line comment style is the default comment style and should be
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used in most cases. Automatic configuration formatting tools may automatically
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transform `//` comments into `#` comments, since the double-slash style is
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not idiomatic.
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## Character Encoding and Line Endings
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Terraform configuration files must always be UTF-8 encoded. While the
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delimiters of the language are all ASCII characters, Terraform accepts
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non-ASCII characters in identifiers, comments, and string values.
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Terraform accepts configuration files with either Unix-style line endings
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(LF only) or Windows-style line endings (CR then LF), but the idiomatic style
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is to use the Unix convention, and so automatic configuration formatting tools
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may automatically transform CRLF endings to LF.
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