» Template Engine

All strings within templates are processed by a common Packer templating engine, where variables and functions can be used to modify the value of a configuration parameter at runtime.

The syntax of templates uses the following conventions:

  • Anything template related happens within double-braces: {{ }}.
  • Functions are specified directly within the braces, such as {{timestamp}}.
  • Template variables are prefixed with a period and capitalized, such as {{.Variable}}.

» Functions

Functions perform operations on and within strings, for example the {{timestamp}} function can be used in any string to generate the current timestamp. This is useful for configurations that require unique keys, such as AMI names. By setting the AMI name to something like My Packer AMI {{timestamp}}, the AMI name will be unique down to the second. If you need greater than one second granularity, you should use {{uuid}}, for example when you have multiple builders in the same template.

Here is a full list of the available functions for reference.

  • build_name - The name of the build being run.
  • build_type - The type of the builder being used currently.
  • isotime [FORMAT] - UTC time, which can be formatted. See more examples below in the isotime format reference.
  • lower - Lowercases the string.
  • pwd - The working directory while executing Packer.
  • template_dir - The directory to the template for the build.
  • timestamp - The current Unix timestamp in UTC.
  • uuid - Returns a random UUID.
  • upper - Uppercases the string.
  • user - Specifies a user variable.
  • packer_version - Returns Packer version.

» Specific to Amazon builders:

  • clean_ami_name - AMI names can only contain certain characters. This function will replace illegal characters with a '-" character. Example usage since ":" is not a legal AMI name is: {{isotime | clean_ami_name}}.

» Specific to Google Compute builders:

  • clean_image_name - GCE image names can only contain certain characters and the maximum length is 63. This function will convert upper cases to lower cases and replace illegal characters with a "-" character. Example:

    "mybuild-{{isotime | clean_image_name}}" will become mybuild-2017-10-18t02-06-30z.

    Note: Valid GCE image names must match the regex (?:[a-z](?:[-a-z0-9]{0,61}[a-z0-9])?)

    This engine does not guarantee that the final image name will match the regex; it will not truncate your name if it exceeds 63 characters, and it will not validate that the beginning and end of the engine's output are valid. For example, "image_name": {{isotime | clean_image_name}}" will cause your build to fail because the image name will start with a number, which is why in the above example we prepend the isotime with "mybuild".

» Template variables

Template variables are special variables automatically set by Packer at build time. Some builders, provisioners and other components have template variables that are available only for that component. Template variables are recognizable because they're prefixed by a period, such as {{ .Name }}. For example, when using the shell builder template variables are available to customize the execute_command parameter used to determine how Packer will run the shell command.

{
    "provisioners": [
        {
            "type": "shell",
            "execute_command": "{{.Vars}} sudo -E -S bash '{{.Path}}'",
            "scripts": [
                "scripts/bootstrap.sh"
            ]
        }
    ]
}

The {{ .Vars }} and {{ .Path }} template variables will be replaced with the list of the environment variables and the path to the script to be executed respectively.

» isotime Function Format Reference

Formatting for the function isotime uses the magic reference date Mon Jan 2 15:04:05 -0700 MST 2006, which breaks down to the following:

Day of Week Month Date Hour Minute Second Year Timezone
Numeric - 01 02 03 (15) 04 05 06 -0700
Textual Monday (Mon) January (Jan) - - - - - MST

The values in parentheses are the abbreviated, or 24-hour clock values

Note that "-0700" is always formatted into "+0000" because isotime is always UTC time.

Here are some example formatted time, using the above format options:

isotime = June 7, 7:22:43pm 2014

{{isotime "2006-01-02"}} = 2014-06-07
{{isotime "Mon 1504"}} = Sat 1922
{{isotime "02-Jan-06 03\_04\_05"}} = 07-Jun-2014 07\_22\_43
{{isotime "Hour15Year200603"}} = Hour19Year201407

Please note that double quote characters need escaping inside of templates (in this case, on the ami_name value):

{
  "builders": [
    {
      "type": "amazon-ebs",
      "access_key": "...",
      "secret_key": "...",
      "region": "us-east-1",
      "source_ami": "ami-fce3c696",
      "instance_type": "t2.micro",
      "ssh_username": "ubuntu",
      "ami_name": "packer {{isotime \"2006-01-02\"}}"
    }
  ]
}