---
layout: "docs"
page_title: "Configuration Language"
sidebar_current: "configuration"
description: |-
  Packer uses text files to describe infrastructure and to set variables.
  These text files are called Packer _configurations_ and are
  written in the HCL language.
---

# HCL Configuration Language

Packer uses the Hashicorp Configuration Language - HCL - designed to allow
concise descriptions of the required steps to get to a build file. This page
describes the features of HCL2 exhaustively, if you would like to give a quick
try to HCL2, you can also read the quicker [HCL2 getting started
guide](/guides/hcl).

## Builds

The main purpose of the HCL language is defining builds and sources. All other
language features exist only to make the definition of builds more flexible and
convenient.


`packer build` takes one argument. When a directory is passed, all files in the
folder with a name ending with ".pkr.hcl" or ".pkr.json" will be parsed using
the HCL2 format. When a file ending with ".pkr.hcl" or ".pkr.json" is passed it
will be parsed using the HCL2 schema. For every other case; the *JSON only* old
packer schema will be used.

## Arguments, Blocks, and Expressions

The syntax of the HCL language consists of only a few basic elements:

```hcl
source "amazon-ebs" "main" {
  ami_name = "main-ami"
}

<BLOCK TYPE> "<BLOCK LABEL>" "<BLOCK LABEL>" {
  # Block body
  <IDENTIFIER> = <EXPRESSION> # Argument
}
```

- _Blocks_ are containers for other content and usually represent the
  configuration of some kind of object, like a source. Blocks have a
  _block type,_ can have zero or more _labels,_ and have a _body_ that contains
  any number of arguments and nested blocks. Most of Packer's features are
  controlled by top-level blocks in a configuration file.
- _Arguments_ assign a value to a name. They appear within blocks.
- _Expressions_ represent a value, either literally or by referencing and
  combining other values. They appear as values for arguments, or within other
  expressions.

For full details about Packer's syntax, see:

- [Configuration Syntax](./syntax.html)
- [Expressions](./expressions.html)

## Code Organization

The HCL language uses configuration files that are named with the `.pkr.hcl`
file extension. There is also [a JSON-based variant of the
language](./syntax-json.html) that is named with the `.pkr.json` file
extension.

Configuration files must always use UTF-8 encoding, and by convention are
usually maintained with Unix-style line endings (LF) rather than Windows-style
line endings (CRLF), though both are accepted.

## Configuration Ordering

The ordering of root blocks is not significant. The order of `provisioner` or
`post-processor` blocks within a `build` is the only major feature where block
order matters.