Terraform Tutorial — Declaring Variables

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The variable block declares an input variable. A well-declared variable includes a type, description, and optionally a default value.

The variable Block

variable "region" {
  type        = string
  default     = "us-east-1"
  description = "AWS region where all resources will be created"
}
Attribute Required? Purpose
type Recommended The data type (string, number, list, etc.)
default Optional Value used if no other value is provided
description Recommended Human-readable explanation (shown in docs and prompts)
sensitive Optional Hides the value in plan and output
nullable Optional Whether null is an acceptable value
validation Optional Custom validation rules

Variables Without Defaults

If a variable has no default, Terraform requires you to provide a value:

variable "environment" {
  type        = string
  description = "Deployment environment (dev, staging, prod)"
}

If you run terraform plan without providing this value:

var.environment
  Deployment environment (dev, staging, prod)

  Enter a value:

Terraform prompts interactively. In CI/CD, this causes the pipeline to hang — always provide values via .tfvars or environment variables.

Using Variables in Resources

Reference variables with the var. prefix:

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

variable "environment" {
  type    = string
  default = "dev"
}

resource "aws_instance" "web" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = var.instance_type

  tags = {
    Environment = var.environment
  }
}

When to Use Defaults

Scenario Use Default?
Value is the same 90% of the time ✅ Yes — set the common value as default
Value must be explicitly chosen (environment name, project name) ❌ No — force the user to provide it
Sensitive value (password, API key) ❌ No — never put secrets in defaults

Variable File Organization

By convention, all variable declarations go in variables.tf:

# variables.tf

variable "region" {
  type        = string
  default     = "us-east-1"
  description = "AWS region"
}

variable "environment" {
  type        = string
  description = "Deployment environment"
}

variable "project_name" {
  type        = string
  default     = "myapp"
  description = "Project name used in resource naming"
}

This makes it easy for teammates to see all inputs at a glance.

What’s Next?

You’ve declared variables. Now let’s learn the multiple ways to provide values — .tfvars files, CLI flags, and environment variables.

← Variable Types tfvars and Environment Variables →

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