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.

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:

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
• Expressions

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 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.