» 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.
  • clean_resource_name - Image names can only contain certain characters and have a maximum length, eg 63 on GCE & 80 on Azure. clean_resource_name will convert upper cases to lower cases and replace illegal characters with a "-" character. Example:

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

Note: Valid Azure image names must match the regex
`^[^_\\W][\\w-._)]{0,79}$`

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 the maximum number of
allowed characters, and it will not validate that the beginning and end of
the engine's output are valid. For example, `"image_name": {{isotime |
clean_resource_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".
Exact behavior of `clean_resource_name` will depend on which builder it is
being applied to; refer to build-specific docs below for more detail on how
each function will behave.
  • env - Returns environment variables. See example in using home variable
  • isotime [FORMAT] - UTC time, which can be formatted. See more examples below in the isotime format reference. strftime FORMAT - UTC time, formated using the ISO C standard format FORMAT. See jehiah/go-strftime for a list of available format specifier.
  • lower - Lowercases the string.
  • packer_version - Returns Packer version.
  • pwd - The working directory while executing Packer.
  • replace - ( old, new string, n int, s ) Replace returns a copy of the string s with the first n non-overlapping instances of old replaced by new.
  • replace_all - ( old, new string, s ) ReplaceAll returns a copy of the string s with all non-overlapping instances of old replaced by new.
  • split - Split an input string using separator and return the requested substring.
  • 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.

» Specific to Amazon builders:

  • clean_resource_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_resource_name}}.

» Specific to Google Compute builders:

  • clean_resource_name - GCE image names can only contain certain characters and the maximum length is

    1. This function will convert upper cases to lower cases and replace illegal characters with a "-" character. Example:

    "mybuild-{{isotime | clean_resource_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".

» Specific to Azure builders:

  • clean_resource_name - Azure managed image names can only contain certain characters and the maximum length is 80. This function will replace illegal characters with a "-" character. Example:

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

    Note: Valid Azure image names must match the regex ^[^_\\W][\\w-._)]{0,79}$

    This engine does not guarantee that the final image name will match the regex; it will not truncate your name if it exceeds 80 characters, and it will not validate that the beginning and end of the engine's output are valid. It will truncate invalid characters from the end of the name when converting illegal characters. For example, "managed_image_name: "My-Name::" will be converted to "managed_image_name: "My-Name"

» 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

For those unfamiliar with GO date/time formatting, here is a link to the documentation: go date/time formatting

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\"}}"
    }
  ]
}

» split Function Format Reference

The function split takes an input string, a seperator string, and a numeric component value and returns the requested substring.

Please note that you cannot use the split function on user variables, because we can't nest the functions currently. This function is indended to be used on builder variables like build_name. If you need a split user variable, the best way to do it is to create a separate variable.

Here are some examples using the above options:

build_name = foo-bar-provider

{{split build_name "-" 0}} = foo
{{split "fixed-string" "-" 1}} = string

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

{
  "post-processors": [
    [
      {
        "type": "vagrant",
        "compression_level": 9,
        "keep_input_artifact": false,
        "vagrantfile_template": "tpl/{{split build_name \"-\" 1}}.rb",
        "output": "output/{{build_name}}.box",
        "only": [
            "org-name-provider"
        ]
      }
    ]
  ]
}

» replace Function Format Reference

Here are some examples using the replace options:

build_name = foo-bar-provider

{{ replace_all "-" "/" build_name }}  = foo/bar/provider
{{ build_name | replace "-" "/" 1 }} = foo/bar-provider