packer-cn/website/source/docs/templates/engine.html.md

---
description: |
    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.
layout: docs
page_title: 'Template Engine - Templates'
sidebar_current: 'docs-templates-engine'
---

# 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.
-   `env` - Returns environment variables. See example in [using home
    variable](/docs/templates/user-variables.html#using-home-variable)
-   `isotime [FORMAT]` - UTC time, which can be
    [formatted](https://golang.org/pkg/time/#example_Time_Format). See more
    examples below in [the `isotime` format
    reference](/docs/templates/engine.html#isotime-function-format-reference).
-   `lower` - Lowercases the string.
-   `pwd` - The working directory while executing Packer.
-   `sed` - Use [a golang implementation of sed](https://github.com/rwtodd/Go.Sed) to parse an input string.
-   `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.
-   `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".

#### Specific to Azure builders:

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

``` liquid
{
    "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](/docs/templates/user-variables.html)
documentation for more information on user variables.

# 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:

<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.

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

``` liquid
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):

``` json
{
  "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\"}}"
    }
  ]
}
```

-&gt; **Note:** See the [Amazon builder](/docs/builders/amazon.html)
documentation for more information on how to correctly configure the Amazon
builder in this example.

# 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:

``` 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"
        ]
      }
    ]
  ]
}
```

# sed Function Format Reference

See the library documentation https://github.com/rwtodd/Go.Sed for notes about
the difference between this golang implementation of sed and the regexes you may
be used to. A very simple example of this functionality:

```
    "provisioners": [
      {
          "type": "shell-local",
          "environment_vars": ["EXAMPLE={{ sed \"s/null/awesome/\" build_type}}"],
          "inline": ["echo build_type is $EXAMPLE\n"]
      }
    ]
```