Terraform Variables and Outputs

Terraform becomes reusable and environment-friendly when you separate what changes from what stays the same.

• Variables = inputs (things you set)
• Outputs = values you export (things you read)

Learning outcomes

By the end you’ll know:

• how to define variables (type, default, validation)
• how to pass variables into root modules / modules
• how to create outputs for inspection and wiring modules together

1) Input variables

Simple variable

variable "aws_region" {
  type    = string
  default = "us-east-1"
}

Read it like:

provider "aws" {
  region = var.aws_region
}

Required variable (no default)

variable "project_name" {
  type = string
}

Provide it via CLI:

terraform apply -var "project_name=my-app"

Or with a variable file:

terraform apply -var-file=dev.tfvars

Example dev.tfvars:

project_name = "my-app"
aws_region    = "us-east-1"

2) Variable types (practical)

Terraform types help validation and editor hints.

List

variable "allowed_cidrs" {
  type    = list(string)
  default = ["203.0.113.0/24", "198.51.100.0/24"]
}

Map

variable "tags" {
  type = map(string)
  default = {
    owner = "platform"
    env   = "dev"
  }
}

Object

variable "vpc" {
  type = object({
    cidr_block = string
    enable_dns = bool
  })

  default = {
    cidr_block = "10.0.0.0/16"
    enable_dns = true
  }
}

3) Variable validation

Example:

variable "environment" {
  type = string

  validation {
    condition     = contains(["dev", "staging", "prod"], var.environment)
    error_message = "environment must be dev, staging, or prod"
  }
}

4) Outputs

Outputs expose values from your configuration.

Simple output

output "bucket_name" {
  value = aws_s3_bucket.example.bucket
}

Output with description and sensitive value (concept)

output "db_password" {
  description = "Database password (sensitive)"
  value       = aws_db_instance.example.password
  sensitive   = true
}

Inspect outputs

After apply:

terraform output

5) Module inputs and outputs

If you have a module, you pass variables into it.

Example root calling a module:

module "vpc" {
  source = "./modules/vpc"

  cidr_block = "10.0.0.0/16"
  tags = {
    owner = "platform"
  }
}

Module outputs can be consumed by the caller:

output "vpc_id" {
  value = module.vpc.vpc_id
}

Rule of thumb: a module should “export” the few values that other stacks/modules need.

6) Cleanup reminder

terraform destroy