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docs	User Variables in Templates

User Variables

User variables allow your templates to be further configured with variables from the command-line, environmental variables, or files. This lets you parameterize your templates so that you can keep secret tokens, environment-specific data, and other types of information out of your templates. This maximizes the portability and shareability of the template.

Using user variables expects you know how configuration templates work. If you don't know how configuration templates work yet, please read that page first.

Usage

User variables must first be defined in a variables section within your template. Even if you want a variable to default to an empty string, it must be defined. This explicitness makes it easy for newcomers to your template to understand what can be modified using variables in your template.

The variables section is a simple key/value mapping of the variable name to a default value. A default value can be the empty string. An example is shown below:

{
  "variables": {
    "aws_access_key": "",
    "aws_secret_key": ""
  },

  "builders": [{
    "type": "amazon-ebs",
    "access_key": "{{user `aws_access_key`}}",
    "secret_key": "{{user `aws_secret_key`}}",
    ...
  }]
}

In the above example, the template defines two variables: aws_access_key and aws_secret_key. They default to empty values. Later, the variables are used within the builder we defined in order to configure the actual keys for the Amazon builder.

If the default value is null, then the user variable will be required. This means that the user must specify a value for this variable or template validation will fail.

Using the variables is extremely easy. Variables are used by calling the user function in the form of {{user `variable`}}. This function can be used in any value within the template, in builders, provisioners, anything. The user variable is available globally within the template.

Environmental Variables

Environmental variables can be used within your template using user variables. The env function is available only within the default value of a user variable, allowing you to default a user variable to an environmental variable. An example is shown below:

{
  "variables": {
    "my_secret": "{{env `MY_SECRET`}}",
  },

  ...
}

This will default "my_secret" to be the value of the "MY_SECRET" environmental variable (or the empty string if it does not exist).

Why can't I use environmental variables elsewhere? User variables are the single source of configurable input to a template. We felt that having environmental variables used anywhere in a template would confuse the user about the possible inputs to a template. By allowing environmental variables only within default values for user variables, user variables remain as the single source of input to a template that a user can easily discover using packer inspect.

Setting Variables

Now that we covered how to define and use variables within a template, the next important point is how to actually set these variables. Packer exposes two methods for setting variables: from the command line or from a file.

From the Command Line

To set variables from the command line, the -var flag is used as a parameter to packer build (and some other commands). Continuing our example above, we could build our template using the command below. The command is split across multiple lines for readability, but can of course be a single line.

$ packer build \
    -var 'aws_access_key=foo' \
    -var 'aws_secret_key=bar' \
    template.json

As you can see, the -var flag can be specified multiple times in order to set multiple variables. Also, variables set later on the command-line override earlier set variables if it has already been set.

Finally, variables set from the command-line override all other methods of setting variables. So if you specify a variable in a file (the next method shown), you can override it using the command-line.

From a File

Variables can also be set from an external JSON file. The -var-file flag reads a file containing a basic key/value mapping of variables to values and sets those variables. The JSON file is simple:

{
  "aws_access_key": "foo",
  "aws_secret_key": "bar"
}

It is a single JSON object where the keys are variables and the values are the variable values. Assuming this file is in variables.json, we can build our template using the following command:

$ packer build -var-file=variables.json template.json

The -var-file flag can be specified multiple times and variables from multiple files will be read and applied. As you'd expect, variables read from files specified later override a variable set earlier if it has already been set.

And as mentioned above, no matter where a -var-file is specified, a -var flag on the command line will always override any variables from a file.