» Input Variables

Input variables serve as parameters for a Terraform module, allowing aspects of the module to be customized without altering the module's own source code, and allowing modules to be shared between different configurations.

When you declare variables in the root module of your configuration, you can set their values using CLI options and environment variables. When you declare them in child modules, the calling module should pass values in the module block.

Input variable usage is introduced in the Getting Started guide section Input Variables.

» Declaring an Input Variable

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

variable "image_id" {
  type = string
}

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

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

The name of a variable can be any valid identifier except the following:

These names are reserved for meta-arguments in module configuration blocks, and cannot be declared as variable names.

The variable declaration 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 module or running Terraform. The default argument requires a literal value and cannot reference other objects in the configuration.

» Using Input Variable Values

Within the module 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:

resource "aws_instance" "example" {
  instance_type = "t2.micro"
  ami           = var.image_id
}

The value assigned to a variable can be accessed only from expressions within the module 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 module, and allow Terraform 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 module 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 module, and so it should be written from the perspective of the user of the module rather than its maintainer. For commentary for module maintainers, use comments.

» Assigning Values to Root Module Variables

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

  • In a Terraform Cloud workspace.
  • Individually, with the -var command line option.
  • In variable definitions (.tfvars) files, either specified on the command line or automatically loaded.
  • As environment variables.

The following sections describe these options in more detail. This section does not apply to child modules, where values for input variables are instead assigned in the configuration of their parent module, as described in Modules.

» Variables on the Command Line

To specify individual modules on the command line, use the -var option when running the terraform plan and terraform apply commands:

terraform apply -var="image_id=ami-abc123"
terraform apply -var='image_id_list=["ami-abc123","ami-def456"]'
terraform apply -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 (.tfvars) 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 .tfvars or .tfvars.json) and then specify that file on the command line with -var-file:

terraform apply -var-file="testing.tfvars"

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

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

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

  • Files named exactly terraform.tfvars or terraform.tfvars.json.
  • Any files with names ending in .auto.tfvars or .auto.tfvars.json.

Files whose names end with .json are parsed instead as JSON objects, 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, Terraform searches the environment of its own process for environment variables named TF_VAR_ followed by the name of a declared variable.

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

$ export TF_VAR_image_id=ami-abc123
$ terraform plan
...

On operating systems where environment variable names are case-sensitive, Terraform 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, Terraform'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, Terraform defaults to interpreting -var and environment variable values as literal strings, which do not need to be quoted:

$ export TF_VAR_image_id=ami-abc123

However, if a root module variable uses a type constraint to require a complex value (list, set, map, object, or tuple), Terraform 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 TF_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, Terraform uses the last value it finds, overriding any previous values.

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

  • Environment variables
  • The terraform.tfvars file, if present.
  • The terraform.tfvars.json file, if present.
  • Any *.auto.tfvars or *.auto.tfvars.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. (This includes variables set by a Terraform Cloud workspace.)