---
page_title: HCL syntax reference
description: |-
  HCL syntax combines declarative structure with expressions into Packer templates. Learn how to use HCL syntax to define infrastructure as code. 
---

# HCL syntax reference

This topic provides reference information about the native HashiCorp configuration language (HCL) syntax you can use to create Packer templates. For information about HCL constructs in JSON, refer to the [JSON syntax reference](/packer/docs/templates/hcl_templates/syntax-json).

## Introduction

Learning the HCL syntax helps you understand the building blocks that Packer template constructs
are built from. HCL is a rich language designed to be easy for humans to read and write. 
Otherwhich 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 in order to use Packer, 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/hcl2/hclsyntax/spec.md).

## Arguments and Blocks

HCL 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 source type has a schema that defines the types of its
arguments), but many arguments accept arbitrary
[expressions](/packer/docs/templates/hcl_templates/expressions), which allow the value to
either be specified literally or generated from other values programmatically.

### Blocks

A _block_ is a container for other content:

```hcl
source "amazon-ebs" "example" {
  ami_name = "abc123"

  tags = {
    # ...
  }
}
```

A block has a _type_ (`source` in this example). Each block type defines
how many _labels_ must follow the type keyword. The `source` block type
expects two labels, which are `amazon-ebs` 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 `tags` 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.

HCL 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 Packer'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 Packer-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, Packer implements
[the Unicode identifier syntax](http://unicode.org/reports/tr31/), extended to
include the ASCII hyphen character `-`.

## Comments

HCL 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

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

Packer 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.
