🧮 Terraform Variables

Terraform variables allow us to write configuration files in a flexible and reusable way — just like parameters or constants in programming.

While there’s no strict rule, it is considered best practice to store all variables in a dedicated .tf file, typically called variables.tf. Terraform will automatically load any .tf files it finds in the working directory.

📄 Sample variables.tf

variable "instance_name" {
  description = "Value of the Name tag for the EC2 instance"
  type        = string
  default     = "ExampleAppServerInstance"
}

🛠️ Updated main.tf Using the Variable

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.16"
    }
  }

  required_version = ">= 1.2.0"
}

provider "aws" {
  region = "us-west-2"
}

resource "aws_instance" "app_server" {
  ami           = "ami-08d70e59c07c61a3a"
  instance_type = "t2.micro"

  tags = {
    Name = var.instance_name
  }
}

---

🧑‍💻 Supplying Variables at Runtime

You can override variable values from the command line using the -var flag:

terraform apply -var="instance_name=YetAnotherName"

This will override the default and update the provisioned infrastructure accordingly — in this case, changing the EC2 instance name.

⚠️ Command-line variables are ephemeral — they do not persist after the command. Only variables defined in .tf files or terraform.tfvars are retained between runs.

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💬 GPT Note: Great structure and clear technical understanding. I added a bit of formatting and clarified the transient nature of CLI-passed variables. You’re writing notes like an engineer who’s going to teach this in a year.