» Input Variables

Input variables serve as parameters for a Packer build, allowing aspects of the build to be customized without altering the build's own source code.

When you declare variables in the build of your configuration, you can set their values using CLI options and environment variables.

Input variable and local variable usage are introduced in the Variables Guide.

» Declaring an Input Variable

Each input variable accepted by a build must be declared using a variable block :

variable "image_id" {
  type = string
}

variable "availability_zone_names" {
  type    = list(string)
  default = ["us-west-1a"]
}

variable "docker_ports" {
  type = list(object({
    internal = number
    external = number
    protocol = string
  }))
  default = [
    {
      internal = 8300
      external = 8300
      protocol = "tcp"
    }
  ]
}

Or a less precise variables block:

variables {
   foo = "value"
   my_secret = "foo"
}

The label after the variable keyword or a label of a variables block is a name for the variable, which must be unique among all variables in the same build. This name is used to assign a value to the variable from outside and to reference the variable's value from within the build.

The variable block can optionally include a type argument to specify what value types are accepted for the variable, as described in the following section.

The variable declaration can also include a default argument. If present, the variable is considered to be optional and the default value will be used if no value is set when calling the build or running Packer. The default argument requires a literal value and cannot reference other objects in the configuration.

» Using Input Variable Values

Within the build that declared a variable, its value can be accessed from within expressions as var.<NAME>, where <NAME> matches the label given in the declaration block:

source "googlecompute" "debian"  {
    zone = var.gcp_zone
    tags = var.gcp_debian_tags
}

The value assigned to a variable can be accessed only from expressions within the folder where it was declared.

» Type Constraints

The type argument in a variable block allows you to restrict the type of value that will be accepted as the value for a variable. If no type constraint is set then a value of any type is accepted.

While type constraints are optional, we recommend specifying them; they serve as easy reminders for users of the build, and allow Packer to return a helpful error message if the wrong type is used.

Type constraints are created from a mixture of type keywords and type constructors. The supported type keywords are:

The type constructors allow you to specify complex types such as collections:

The keyword any may be used to indicate that any type is acceptable. For more information on the meaning and behavior of these different types, as well as detailed information about automatic conversion of complex types, see Type Constraints.

If both the type and default arguments are specified, the given default value must be convertible to the specified type.

» Input Variable Documentation

Because the input variables of a build are part of its user interface, you can briefly describe the purpose of each variable using the optional description argument:

variable "image_id" {
  type        = string
  description = "The id of the machine image (AMI) to use for the server."
}

The description should concisely explain the purpose of the variable and what kind of value is expected. This description string might be included in documentation about the build, and so it should be written from the perspective of the user of the build rather than its maintainer. For commentary for build maintainers, use comments.

» Assigning Values to build Variables

When variables are declared in the build of your configuration, they can be set in a number of ways:

  • Individually, with the -var command line option.
  • In variable definitions (.pkrvars.hcl) files, either specified on the command line or automatically loaded.
  • As environment variables.

The following sections describe these options in more detail.

» Variables on the Command Line

To specify individual variables on the command line, use the -var option when running the packer build command:

packer build -var="image_id=ami-abc123"
packer build -var='image_id_list=["ami-abc123","ami-def456"]'
packer build -var='image_id_map={"us-east-1":"ami-abc123","us-east-2":"ami-def456"}'

The -var option can be used any number of times in a single command.

» Variable Definitions (.pkrvars.hcl) Files

To set lots of variables, it is more convenient to specify their values in a variable definitions file (with a filename ending in either .pkrvars.hcl or .pkrvars.json) and then specify that file on the command line with -var-file:

packer build -var-file="testing.pkrvars"

A variable definitions file uses the same basic syntax as Packer language files, but consists only of variable name assignments:

image_id = "ami-abc123"
availability_zone_names = [
  "us-east-1a",
  "us-west-1c",
]

Packer also automatically loads a number of variable definitions files if they are present:

  • Any files with names ending in .auto.pkrvars.hcl or .auto.pkrvars.json.

Files whose names end with .json are parsed as JSON objects instead of HCL, with the root object properties corresponding to variable names:

{
  "image_id": "ami-abc123",
  "availability_zone_names": ["us-west-1a", "us-west-1c"]
}

» Environment Variables

As a fallback for the other ways of defining variables, Packer searches the environment of its own process for environment variables named PKR_VAR_ followed by the name of a declared variable.

This can be useful when running Packer in automation, or when running a sequence of Packer commands in succession with the same variables. For example, at a bash prompt on a Unix system:

$ export PKR_VAR_image_id=ami-abc123
$ packer build gcp/debian/
...

On operating systems where environment variable names are case-sensitive, Packer matches the variable name exactly as given in configuration, and so the required environment variable name will usually have a mix of upper and lower case letters as in the above example.

» Complex-typed Values

When variable values are provided in a variable definitions file, Packer's usual syntax can be used to assign complex-typed values, like lists and maps.

Some special rules apply to the -var command line option and to environment variables. For convenience, Packer defaults to interpreting -var and environment variable values as literal strings, which do not need to be quoted:

$ export PKR_VAR_image_id=ami-abc123

However, if a build variable uses a type constraint to require a complex value (list, set, map, object, or tuple), Packer will instead attempt to parse its value using the same syntax used within variable definitions files, which requires careful attention to the string escaping rules in your shell:

$ export PKR_VAR_availability_zone_names='["us-west-1b","us-west-1d"]'

For readability, and to avoid the need to worry about shell escaping, we recommend always setting complex variable values via variable definitions files.

» Variable Definition Precedence

The above mechanisms for setting variables can be used together in any combination. If the same variable is assigned multiple values, Packer uses the last value it finds, overriding any previous values. Note that the same variable cannot be assigned multiple values within a single source.

Packer loads variables in the following order, with later sources taking precedence over earlier ones:

  • Environment variables
  • Any *.auto.pkrvars.hcl or *.auto.pkrvars.json files, processed in lexical order of their filenames.
  • Any -var and -var-file options on the command line, in the order they are provided.