129 lines
5.3 KiB
Plaintext
129 lines
5.3 KiB
Plaintext
---
|
|
page_title: Syntax - Configuration Language
|
|
description: >-
|
|
Key constructs of the native Terraform language syntax, including identifiers,
|
|
arguments, blocks, and comments.
|
|
---
|
|
|
|
# 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 relatively easy for humans to read and write.
|
|
The constructs in the Terraform language can also be expressed in
|
|
[JSON syntax](/language/syntax/json), 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](https://github.com/hashicorp/hcl/blob/main/hclsyntax/spec.md).
|
|
|
|
## Arguments and Blocks
|
|
|
|
The Terraform language syntax is built around two key syntax constructs:
|
|
arguments and blocks.
|
|
|
|
### Arguments
|
|
|
|
An _argument_ assigns a value to a particular name:
|
|
|
|
```hcl
|
|
image_id = "abc123"
|
|
```
|
|
|
|
The identifier before the equals sign is the _argument name_, and the expression
|
|
after the equals sign is the argument's value.
|
|
|
|
The context where the argument appears determines what value types are valid
|
|
(for example, each resource type has a schema that defines the types of its
|
|
arguments), but many arguments accept arbitrary
|
|
[expressions](/language/expressions), which allow the value to
|
|
either be specified literally or generated from other values programmatically.
|
|
|
|
-> **Note:** Terraform's configuration language is based on a more general
|
|
language called HCL, and HCL's documentation usually uses the word "attribute"
|
|
instead of "argument." These words are similar enough to be interchangeable in
|
|
this context, and experienced Terraform users might use either term in casual
|
|
conversation. But because Terraform also interacts with several _other_ things
|
|
called "attributes" (in particular, Terraform resources have attributes like
|
|
`id` that can be referenced from expressions but can't be assigned values in
|
|
configuration), we've chosen to use "argument" in the Terraform documentation
|
|
when referring to this syntax construct.
|
|
|
|
### Blocks
|
|
|
|
A _block_ is a container for other content:
|
|
|
|
```hcl
|
|
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.
|
|
|
|
The Terraform language uses a limited number of _top-level block types,_ which
|
|
are blocks that can appear outside of any other block in a configuration file.
|
|
Most of Terraform's features (including resources, input variables, output
|
|
values, data sources, etc.) are implemented as top-level blocks.
|
|
|
|
## 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](http://unicode.org/reports/tr31/), extended to
|
|
include the ASCII hyphen character `-`.
|
|
|
|
## 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.
|