» Input Variables

Input variables serve as parameters for a Terraform module.

When used in the root module of a configuration, variables can be set from CLI arguments and environment variables. For child modules, they allow values to pass from parent to child.

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

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

» Example

Input variables can be defined as follows:

variable "key" {
  type = "string"
}

variable "images" {
  type = "map"

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

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

» Description

The variable block configures a single input variable for a Terraform module. Each block declares a single variable.

The name given in the block header is used to assign a value to the variable via the CLI and to reference the variable elsewhere in the configuration.

Within the block body (between { }) is configuration for the variable, which accepts the following arguments:

  • 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 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, Terraform will raise an error if a value is not provided by the caller. The default value can be of any of the supported data types, as described below. If type is also set, the given value must be of the specified type.

  • description (Optional) - A human-friendly description for the variable. This is primarily for documentation for users using your Terraform configuration. When a module is published in Terraform Registry, the given description is shown as part of the documentation.

The name of a variable can be any valid identifier. However, due to the interpretation of module configuration blocks, the names source, version and providers are reserved for Terraform's own use and are thus not recommended for any module intended to be used as a child module.

The default value of an input variable must be a literal value, containing no interpolation expressions. To assign a name to an expression so that it may be re-used within a module, use Local Values instead.

» Strings

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 multi-line string value can be provided using heredoc syntax.

variable "long_key" {
  type = "string"
  default = <<EOF
This is a long key.
Running over several lines.
EOF
}

Terraform performs automatic conversion from string values to numeric and boolean values based on context, so in practice string variables may be used to set arguments of any primitive type. For boolean values in particular there are some caveats, described under Booleans below.

» Maps

A map value is a lookup table from string keys to string values. This is useful for selecting a value based on some other provided value.

A common use of maps is to create a table of machine images per region, as follows:

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

» Lists

A list value is an ordered sequence of strings indexed by integers starting with zero. For example:

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

» Booleans

Although Terraform can automatically convert between boolean and string values, there are some subtle implications of these conversions that should be completely understood when using boolean values with input variables.

It is recommended for now to specify boolean values for variables as the strings "true" and "false", to avoid some caveats in the conversion process. A future version of Terraform will properly support boolean values and so relying on the current behavior could result in backwards-incompatibilities at that time.

For a configuration such as the following:

variable "active" {
  default = false
}

The false is converted to a string "0" when running Terraform.

Then, depending on where you specify overrides, the behavior can differ:

  • Variables with boolean values in a tfvars file will likewise be converted to "0" and "1" values.

  • Variables specified via the -var command line flag will be literal strings "true" and "false", so care should be taken to explicitly use "0" or "1".

  • Variables specified with the TF_VAR_ environment variables will be literal string values, just like -var.

A future version of Terraform will fully support first-class boolean types which will make the behavior of booleans consistent as you would expect. This may break some of the above behavior.

When passing boolean-like variables as parameters to resource configurations that expect boolean values, they are converted consistently:

  • "1" and "true" become true
  • "0" and "false" become false

The behavior of conversion in this direction (string to boolean) will not change in future Terraform versions. Therefore, using these string values rather than literal booleans is recommended when using input variables.

» Environment Variables

Environment variables can be used to set the value of an input variable in the root module. The name of the environment variable must be TF_VAR_ followed by the variable 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

Values for the input variables of a root module can be gathered in variable definition files and passed together using the -var-file=FILE option.

For all files which match terraform.tfvars or *.auto.tfvars present in the current directory, Terraform automatically loads them to populate variables. If the file is located somewhere else, you can pass the path to the file using the -var-file flag. It is recommended to name such files with names ending in .tfvars.

Variables files use HCL or JSON syntax 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

» Variable Merging

When multiple values are provided for the same input variable, map values are merged while all other values are overridden by the last definition.

For example, if you define a variable twice on the command line:

$ terraform apply -var foo=bar -var foo=baz

Then the value of foo will be baz, since it was the last definition seen.

However, for maps, the values are merged:

$ terraform apply -var 'foo={quux="bar"}' -var 'foo={bar="baz"}'

The resulting value of foo will be:

{
  quux = "bar"
  bar = "baz"
}

There is no way currently to unset map values in Terraform. Whenever a map is modified either via variable input or being passed into a module, the values are always merged.

» Variable Precedence

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.

Definition files passed using the -var-file flag will always be evaluated after those in the working directory.

Values passed within definition files or with -var will take precedence over TF_VAR_ environment variables, as environment variables are considered defaults.