Using Variables in Terraform

Overview

Questions

• How can we make Terraform configurations flexible instead of hard-coded?
• How do we define and use variables in Terraform?

Objectives

• Define input variables in a variables.tf file.
• Use variables inside Terraform resources.
• Override variables from the command line or with terraform.tfvars.

Why Variables Matter

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In the previous episodes, we hard-coded details like the AMI ID, instance type, and allowed SSH CIDR. This is fine for experimentation, but not practical when:

• You deploy to different environments.
• You need to give learners flexibility.
• You plan to reuse your Terraform modules.

Terraform variables make your configuration more reusable and easier to maintain.

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Creating a variables.tf File

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Create a new file:

BASH

touch variables.tf

Add the following:

variable "aws_region" {
  description = "AWS region for all resources"
  type        = string
  default     = "us-west-2"
}

variable "instance_type" {
  description = "EC2 instance type"
  type        = string
  default     = "t3.micro"
}

variable "instance_name" {
  description = "Name tag for the EC2 instance"
  type        = string
  default     = "terraform-demo"
}

Callout

By convention, variables are collected in their own file. Terraform loads all .tf files automatically, so file names don’t affect execution order.

Using Variables in main.tf

Update your EC2 resource to reference variables:

provider "aws" {
  region = var.aws_region
}

resource "aws_instance" "demo" {
  ami           = "ami-0adebe3adcec1fa34"
  instance_type = var.instance_type

  tags = {
    Name = var.instance_name
  }
}

Overriding Variables

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1. Override at apply time terraform apply -var=“instance_type=t3.small”
2. Use a .tfvars file

Create a terraform.tfvars file:

BASH

instance_type = "t3.large"
instance_name = "demo-from-tfvars"

Now apply normally:

BASH

terraform apply

Terraform automatically loads terraform.tfvars.

Challenge

Challenge

Challenge: Create Your Own Variable

Add a new variable called ssh_cidr that lets the learner specify which IP is allowed to SSH into the instance.

Then update your Security Group to use it.

Show me the solution

Add to variables.tf:

BASH

variable "ssh_cidr" {
  description = "CIDR allowed for SSH"
  type        = string
  default     = "0.0.0.0/0"
}

Update the security group:

BASH

ingress {
  description = "SSH"
  from_port   = 22
  to_port     = 22
  protocol    = "tcp"
  cidr_blocks = [var.ssh_cidr]
}

Key Points

• Variables make Terraform code reusable and easier to maintain.
• Variables are defined once and used many times across configurations.
• You can override variables with -var or terraform.tfvars.