---
description: |
  The JSON template engine processes strings in JSON templates. Learn how to use JSON template engine functions and variables.
page_title: JSON template engine reference
---

# JSON template engine reference 

This topic describes the Packer engine that processes JSON templates. 

`@include 'from-1.5/legacy-json-warning.mdx'`

## Description

All strings within templates are processed by a common Packer templating
engine. The engine uses variables and functions 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](/packer/docs/templates/legacy_json_templates/user-variables#using-home-variable)
- `build` - This engine will allow you to access, from provisioners and post-processors, special variables that
  provide connection information and basic instance state information.
  Usage example:

  ```json
  {
    "type": "shell-local",
    "environment_vars": ["TESTVAR={{ build `PackerRunUUID`}}"],
    "inline": ["echo $TESTVAR"]
  }
  ```

  Valid variables to request are:

  - **ID**: Represents the VM being provisioned. For example, in Amazon it is the instance ID; in DigitalOcean,
    it is the Droplet ID; in VMware, it is the VM name.

  - **Host**, **Port**, **User** and **Password**: The host, port, user, and password that Packer uses to access the machine.
    Useful for using the shell local provisioner to run Ansible or Inspec against the provisioned instance.

  - **ConnType**: Type of communicator being used. For example, for SSH communicator this will be "ssh".

  - **PackerRunUUID**: Current build's unique ID. Can be used to specify build artifacts.
    An example of that, is when multiple builds runs at the same time producing the same artifact.
    It's possible to differentiate these artifacts by naming them with the builds' unique IDs.

  - **PackerHTTPIP**, **PackerHTTPPort**, and **PackerHTTPAddr**: HTTP IP, port, and address of the file server Packer creates to serve items in the "http" dir to the VM. The HTTP address is displayed in the format `IP:PORT`.

  - **SSHPublicKey** and **SSHPrivateKey**: The public and private key that Packer uses to connect to the instance.
    These are unique to the SSH communicator and are unset when using other communicators.
    **SSHPublicKey** and **SSHPrivateKey** can have escape sequences and special characters so their output should be single quoted to avoid surprises. For example:

    ```json
    { ... "provisioners": [{
      "type": "shell",
      "inline": [ "echo '{{ build `SSHPrivateKey`}}' > /tmp/packer-session.pem" ]
      }]
    }
    ```

  For backwards compatibility, `WinRMPassword` is also available through this
  engine, though it is no different than using the more general `Password`.

  This function is only for use within specific options inside of
  _provisioners_ -- these options will be listed as being template engines
  in the provisioner documentation.

  For builder-specific builder variables, please also refer to the builder docs:

  - Amazon EC2: [chroot](/packer/plugins/builders/amazon/chroot#build-shared-information-variables),
    [EBS Volume](/packer/plugins/builders/amazon/ebsvolume#build-shared-information-variables),
    [EBS](/packer/plugins/builders/amazon/ebs#build-shared-information-variables),
    [EBS Surrogate](/packer/plugins/builders/amazon/ebssurrogate#build-shared-information-variables),
    [Instance](/packer/plugins/builders/amazon/instance#build-shared-information-variables).

  This engine is in beta; please report any issues or requests on the Packer
  issue tracker on GitHub.

- `isotime [FORMAT]` - UTC time, which can be
  [formatted](https://pkg.go.dev/time#example_Time_Format). See more
  examples below in [the `isotime` format
  reference](/packer/docs/templates/legacy_json_templates/engine#isotime-function-format-reference).
  `strftime FORMAT` - UTC time, formated using the ISO C standard format
  `FORMAT`. See
  [jehiah/go-strftime](https://github.com/jehiah/go-strftime) for a list
  of available format specifiers.

  Please note that if you are using a large number of builders,
  provisioners or post-processors, using the isotime engine directly in the
  plugin configuration may cause the timestamp to be slightly diffferent for
  each plugin. This is because the timestamp is generated when each plugin is
  launched rather than in the initial Packer process. In order to avoid this
  and make sure the timestamp is consistent across all plugins, set it as a user
  variable and then access the user variable within your plugins.

- `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 Unix timestamp in UTC when the Packer process was
  launched. Please note that if you are using a large number of builders,
  provisioners or post-processors, the timestamp may be slightly
  different for each one because it is from when the plugin is
  launched not the initial Packer process. In order to avoid this and make
  the timestamp consistent across all plugins, set it as a user variable
  and then access the user variable within your plugins.
- `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

  63. 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_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".

#### 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`](/packer/plugins/builders/vmware/iso) builder template variables
are available to customize the
[`execute_command`](/packer/docs/provisioners/shell#execute_command) parameter
used to determine how Packer will run the shell command.

```json
{
  "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.

-> **Note:** In addition to template variables, you can specify your own
user variables. See the [user variable](/packer/docs/templates/legacy_json_templates/user-variables)
documentation for more information on user variables.

## `isotime` Function Format Reference

The `isotime` template engine uses Go to generate timestamps. If you're
unfamiliar with Go, then the way you format the timestamp is going to
feel a bit unusual compared to how you may be used to formatting
datetime strings.

Full docs and examples for the Go time formatting function can be found
[here](https://pkg.go.dev/time#example_Time_Format)

However, the formatting basics are worth describing here. From the [Go docs](https://pkg.go.dev/time#pkg-constants):

> These are predefined layouts for use in Time.Format and time.Parse. The
> reference time used in the layouts is the specific time:
>
> Mon Jan 2 15:04:05 MST 2006
>
> which is Unix time 1136239445. Since MST is GMT-0700, the reference time
> can be thought of as
>
> 01/02 03:04:05PM '06 -0700
>
> To define your own format, write down what the reference time would look like
> formatted your way; see the values of constants like ANSIC, StampMicro or
> Kitchen for examples. The model is to demonstrate what the reference time
> looks like so that the Format and Parse methods can apply the same
> transformation to a general time value.

So what does that look like in a Packer template function? Here's an example
of how you'd declare a variable using the isotime function.

```json
"variables": {
  "myvar": "packer-{{isotime `2006-01-02 03:04:05`}}"
}
```

You can try and modify the following examples in a packer template or in
`packer console` to get an idea of how to set different timestamps:

| Input                                            | Output                       |
| ------------------------------------------------ | ---------------------------- |
| `` "packer-{{isotime `2006-01-02`}}" ``          | "packer-2021-05-17"          |
| `` "packer-{{isotime `Jan-_2-15:04:05.000`}}" `` | "packer-May-17-23:40:16.786" |
| `` "packer-{{isotime `3:04PM`}}" ``              | "packer-11:40PM"             |
| `"{{ 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"           |

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:

<table class="table table-bordered table-condensed">
  <thead>
    <tr>
      <th></th>
      <th align="center">Day of Week</th>
      <th align="center">Month</th>
      <th align="center">Date</th>
      <th align="center">Hour</th>
      <th align="center">Minute</th>
      <th align="center">Second</th>
      <th align="center">Year</th>
      <th align="center">Timezone</th>
    </tr>
  </thead>
  <tr>
    <th>Numeric</th>
    <td align="center">-</td>
    <td align="center">01</td>
    <td align="center">02</td>
    <td align="center">03 (15)</td>
    <td align="center">04</td>
    <td align="center">05</td>
    <td align="center">06</td>
    <td align="center">-0700</td>
  </tr>
  <tr>
    <th>Textual</th>
    <td align="center">Monday (Mon)</td>
    <td align="center">January (Jan)</td>
    <td align="center">-</td>
    <td align="center">-</td>
    <td align="center">-</td>
    <td align="center">-</td>
    <td align="center">-</td>
    <td align="center">MST</td>
  </tr>
</table>

_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.

## `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 intended 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:

```liquid
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):

```json
{
  "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:

```liquid
build_name = foo-bar-provider

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