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Input Variables

You now have enough Terraform knowledge to create useful configurations, but we're still hardcoding access keys, AMIs, etc. To become truly shareable and commitable to version control, we need to parameterize the configurations. This page introduces input variables as a way to do this.

Defining Variables

Let's first extract our access key, secret key, and region into a few variables. Create another file variables.tf with the following contents. Note that the file can be named anything, since Terraform loads all files ending in .tf in a directory.

variable "access_key" {}
variable "secret_key" {}
variable "region" {
	default = "us-east-1"
}

This defines three variables within your Terraform configuration. The first two have empty blocks {}. The third sets a default. If a default value is set, the variable is optional. Otherwise, the variable is required. If you run terraform plan now, Terraform will error since the required variables are not set.

Using Variables in Configuration

Next, replace the AWS provider configuration with the following:

provider "aws" {
	access_key = "${var.access_key}"
	secret_key = "${var.secret_key}"
	region = "${var.region}"
}

This uses more interpolations, this time prefixed with var.. This tells Terraform that you're accessing variables. This configures the AWS provider with the given variables.

Assigning Variables

There are three ways to assign variables.

First, if you execute terraform plan or apply without doing anythiing, Terraform will ask you to input the variables interactively. These variables are not saved, but provides a nice user experience for getting started with Terraform.

For another option, you can set it directly on the command-line with the -var flag. Any command in Terraform that inspects the configuration accepts this flag, such as apply, plan, and refresh:

$ terraform plan \
  -var 'access_key=foo' \
  -var 'secret_key=bar'
...

Once again, setting variables this way will not save them, and they'll have to be input repeatedly as commands are executed.

The third way, and the way to persist variable values, is to create a file and assign variables within this file. Create a file named "terraform.tfvars" with the following contents:

access_key = "foo"
secret_key = "bar"

If a "terraform.tfvars" file is present in the current directory, Terraform automatically loads it to populate variables. If the file is named something else, you can use the -var-file flag directly to specify a file. These files are the same syntax as Terraform configuration files. And like Terraform configuration files, these files can also be JSON.

We recommend using the "terraform.tfvars" file, and ignoring it from version control.

Mappings

We've replaced our sensitive strings with variables, but we still are hardcoding AMIs. Unfortunately, AMIs are specific to the region that is in use. One option is to just ask the user to input the proper AMI for the region, but Terraform can do better than that with mappings.

Mappings are a way to create variables that are lookup tables. An example will show this best. Let's extract our AMIs into a mapping and add support for the "us-west-2" region as well:

variable "amis" {
	default = {
		us-east-1 = "ami-aa7ab6c2"
		us-west-2 = "ami-23f78e13"
	}
}

A variable becomes a mapping when it has a default value that is a map like above. There is no way to create a required map.

Then, replace the "aws_instance" with the following:

resource "aws_instance" "example" {
	ami = "${lookup(var.amis, var.region)}"
	instance_type = "t1.micro"
}

This introduces a new type of interpolation: a function call. The lookup function does a dynamic lookup in a map for a key. The key is var.region, which specifies that the value of the region variables is the key.

While we don't use it in our example, it is worth noting that you can also do a static lookup of a mapping directly with ${var.amis.us-east-1}.

We set defaults, but mappings can also be overridden using the -var and -var-file values. For example, if the user wanted to specify an alternate AMI for us-east-1:

$ terraform plan -var 'amis.us-east-1=foo'
...

Terraform provides variables for parameterizing your configurations. Mappings let you build lookup tables in cases where that makes sense. Setting and using variables is uniform throughout your configurations.

In the next section, we'll take a look at output variables as a mechanism to expose certain values more prominently to the Terraform operator.

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