Variable Configuration

Variables define the parameterization of Terraform configurations. Variables can be overridden via the CLI. Variable usage is covered in more detail in the getting started guide. This page covers configuration syntax for variables.

This page assumes you’re familiar with the configuration syntax already.

Example

A variable configuration looks like the following:

variable "key" {
  type = "string"
}

variable "images" {
  type = "map"

  default = {
    us-east-1 = "image-1234"
    us-west-2 = "image-4567"
  }
}

variable "zones" {
  default = ["us-east-1a", "us-east-1b"]
}

Description

The variable block configures a single input variable for a Terraform configuration. Multiple variables blocks can be used to add multiple variables.

The name given to the variable block is the name used to set the variable via the CLI as well as reference the variable throughout the Terraform configuration.

Within the block (the { }) is configuration for the variable. These are the parameters that can be set:

• type (optional) - If set this defines the type of the variable. Valid values are string, list, and map. If this field is omitted, the variable type will be inferred based on the default. If no default is provided, the type is assumed to be string.

• default (optional) - This sets a default value for the variable. If no default is provided, the variable is considered required and Terraform will error if it is not set. The default value can be any of the data types Terraform supports. This is covered in more detail below.

• description (optional) - A human-friendly description for the variable. This is primarily for documentation for users using your Terraform configuration. A future version of Terraform will expose these descriptions as part of some Terraform CLI command.

Note: Default values can be strings, lists, or maps. If a default is specified, it must match the declared type of the variable.

String values are simple and represent a basic key to value mapping where the key is the variable name. An example is:

variable "key" {
  type    = "string"
  default = "value"
}

A map allows a key to contain a lookup table. This is useful for some values that change depending on some external pivot. A common use case for this is mapping cloud images to regions. An example:

variable "images" {
  type = "map"
  default = {
    us-east-1 = "image-1234"
    us-west-2 = "image-4567"
  }
}

A list can also be useful to store certain variables. For example:

variable "users" {
  type    = "list"
  default = ["admin", "ubuntu"]
}

The usage of maps, lists, strings, etc. is documented fully in the interpolation syntax page.

Syntax

The full syntax is:

variable NAME {
  [type = TYPE]
  [default = DEFAULT]
  [description = DESCRIPTION]
}

where DEFAULT is:

VALUE

[
  VALUE,
  ...
]

{
  KEY = VALUE
  ...
}

Environment Variables

Environment variables can be used to set the value of a variable. The key of the environment variable must be TF_VAR_name and the value is the value of the variable.

For example, given the configuration below:

variable "image" {}

The variable can be set via an environment variable:

$ TF_VAR_image=foo terraform apply

Maps and lists can be specified using environment variables as well using HCL syntax in the value.

For a list variable like so:

variable "somelist" {
  type = "list"
}

The variable could be set like so:

$ TF_VAR_somelist='["ami-abc123", "ami-bcd234"]' terraform plan

Similarly, for a map declared like:

variable "somemap" {
  type = "map"
}

The value can be set like this:

$ TF_VAR_somemap='{foo = "bar", baz = "qux"}' terraform plan

Variable Files

Variables can be collected in files and passed all at once using the -var-file=foo.tfvars flag.

If a file named terraform.tfvars is present in the current directory, Terraform automatically loads it to populate variables. If the file is named something else, you can pass the path to the file using the -var-file flag.

Variables files use HCL or JSON to define variable values. Strings, lists or maps may be set in the same manner as the default value in a variable block in Terraform configuration. For example:

foo = "bar"
xyz = "abc"
somelist = [
  "one",
  "two",
]
somemap = {
  foo = "bar"
  bax = "qux"
}

The -var-file flag can be used multiple times per command invocation:

terraform apply -var-file=foo.tfvars -var-file=bar.tfvars

Note: Variable files are evaluated in the order in which they are specified on the command line. If a variable is defined in more than one variables file, the last value specified is effective.

Precedence example:

Both these files have the variable baz defined:

foo.tfvars

baz = "foo"

bar.tfvars

baz = "bar"

When they are passed in the following order:

terraform apply -var-file=foo.tfvars -var-file=bar.tfvars

The result will be that baz will contain the value bar because bar.tfvars has the last definition loaded.

See the source of this document at Terraform.io