Introduction to Packer HCL2

Note: Packer version 1.5.0 introduced support for HCL2 templates as a beta feature. As of version 1.7.0, HCL2 support is no longer in beta and is the preferred way to write Packer configuration(s).

It is not necessary to know all of the details of the HCL syntax in order to use Packer, and so this page summarizes the most important details to get you started. If you are interested, you can find a full definition of HCL syntax in the HCL native syntax specification.

Arguments and Blocks

The HCL syntax is built around two key syntax constructs: arguments and blocks.

# block
source "amazon-ebs" "example" {

  # argument
  ami_name = "abc123"
}

Comments

The HCL 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.

Multi-line strings

A multi-line string value can be provided using heredoc syntax.

variable "long_key" {
  type = "string"
  default = <<EOF
This is a long key.
Running over several lines.
It could be super handy for a boot_command.
EOF
}

Building blocks can be split in files

Currently Packer offers the source and the build root blocks. These two building blocks can be defined in any order and a build can import one or more source. Usually a source defines what we currently call a builder and a build can apply multiple provisioning steps to a source. For example:

# folder/sources.pkr.hcl
source "amazon-ebs" "example-1" {
  ami_name = "example-1-ami"
}

source "virtualbox-iso" "example-2" {
  boot_command = <<EOF
<esc><esc><enter><wait>
/install/vmlinuz noapic
...
EOF
}

# folder/build.pkr.hcl
build {
  sources = [
    "source.amazon-ebs.example-1",
    "source.virtualbox-iso.example-2"
  ]

  provisioner "shell" {
    inline = [
      "echo 'it is alive !'"
    ]
  }
}