Shashi Techlogues: Terraform different approaches for variable assignment.

In this post, we will discuss different approaches to variable assignment in Terraform.

Here are the different approaches:

Variable Defaults
Command-line flags
From a file
Environment variables

We will go into detail one by one and see examples of using them.

1. Variable Defaults

In this approach, we create a variables.tf file and define a variable and assign a default value to a variable.

variable "instance_type" {
    default = "t2.micro"
}

This is referred to as a variable default. If no value is mentioned for the variable, then the default value will be assigned.

Here comes the question: what will happen if we don’t provide a default value?

Let’s try it out. We removed the default value and now our variable looks like this.

variable "instance_type" {}

And do a terraform plan:

terraform plan

You will get an output like this:

var.instance_type
  Enter a value:

So, if you have not defined a default value, Terraform will ask you for the value from the command line.

2. Command-Line Flags

In the similar example above, if we have a variable and we have not provided a default value, or we want to override the value, we can do so by providing the value in the command line.

terraform plan --var="instance_type=t2.small"

3. From a File

Another way you can provide a value is using a file. Create a terraform.tfvars file and provide a value for the variables.

# terraform.tfvars
instance_type = "t2.micro"

Now when you do terraform plan, the instance type value will be taken from the terraform.tfvars file.

Note: File naming is important here. Terraform by default only looks for the terraform.tfvars file.

Let’s say for some reason you want to use a custom file name. Create a new file custom.tfvars and delete the terraform.tfvars file.

Now, in order to use custom.tfvars, you can provide the file name in the CLI like below.

terraform plan -var-file="custom.tfvars"

4. Environment Variables

Finally, let’s explore the environment-based approach.

Let’s set an environment variable.

Windows

setx TF_VAR_instance_type t2.micro

The TF_VAR_<VARIABLE> <VALUE> format is Terraform-specific.

Linux / macOS

export TF_VAR_instance_type=t2.micro

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Terraform different approaches for variable assignment.