Hard-coding values everywhere makes configs impossible to reuse. Terraform gives you variables for inputs, outputs for results, and references to wire resources together — so one clean config can build many environments.

🎯 Learning Objectives

By the end of this lesson you will:

• Define and use input variables
• Return useful values with outputs
• Reference one resource’s attributes from another
• Understand terraform.tfvars and variable precedence

Input Variables

Variables turn hard-coded values into parameters:

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

variable "instance_count" {
type    = number
default = 2
}

variable "environment" {
type = string
# no default → Terraform will prompt or require a value
}

Use them with var.<name>:

provider "aws" {
region = var.region
}

Supplying Variable Values

Terraform reads variables from several sources, in order of precedence (later wins):

A typical terraform.tfvars:

region         = "eu-west-1"
instance_count = 3
environment    = "production"

Outputs

Outputs surface useful values after an apply — like a server’s IP or a bucket’s name:

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

output "bucket_arn" {
value = aws_s3_bucket.data.arn
}

bucket_arn  = "arn:aws:s3:::my-devops-course-bucket"
bucket_name = "my-devops-course-bucket"

Outputs are also how one Terraform configuration hands values to another.

Referencing Resources

The real power: use one resource’s attributes when defining another. Terraform automatically works out the correct creation order from these references.

resource "aws_vpc" "main" {
cidr_block = "10.0.0.0/16"
}

resource "aws_subnet" "public" {
vpc_id     = aws_vpc.main.id    # reference the VPC
cidr_block = "10.0.1.0/24"
}

Because the subnet references aws_vpc.main.id, Terraform knows to create the VPC first. This dependency graph is built automatically — you never sequence things by hand.

🧪 Hands-on Lab

variable "filename" {
type    = string
default = "note.txt"
}

resource "local_file" "note" {
filename = var.filename
content  = "Environment: production"
}

output "content" {
value = local_file.note.content
}

local_file.note: Creation complete after 0s
Apply complete! Resources: 1 added.
content = "Environment: production"

🧠 Knowledge Check

💼 Interview Preparation

Summary

You can now parameterise configs with variables, return values with outputs, and wire resources together with references that build Terraform’s dependency graph automatically. Next, we tackle the most important operational topic: state.