# terraform / variables / configuration

Write Once, Deploy Everywhere: Input Variables

Input variables make your Terraform code dynamic, reusable, and configurable. Instead of hardcoding an instance type into main.tf and editing it for every environment, you declare a placeholder once and feed it different values from outside the configuration.

The Three Files →Variable Types

$ terraform apply✓

devt2.micro

testingt2.medium

productionm5.large

main.tf

Environments

Code Edits

# the problem

Without Variables, You Keep Editing Code

Say your company has three environments — Development, Testing, Production — and each needs a different EC2 instance type. Hardcode the value and main.tf has to change every single time you deploy somewhere new.

Environment	EC2 Instance
Development	`t2.micro`
Testing	`t2.medium`
Production	`m5.large`

main.tfHARDCODED — NOT RECOMMENDED

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resource "aws_instance" "web" {
  ami           = "ami-0123456789abcdef"
  instance_type = "t2.micro"
}

// Testing: change to "t2.medium"
// Production: change to "m5.large"
// You keep editing this file every time.

# the fix

The Three-File Model

Instead of changing main.tf, keep it the same and only change the values. A typical project splits into three files, each with exactly one job.

declares

variables.tf

Tells Terraform "my project needs these values." It's an empty placeholder — no value lives here, just a name and a type. Someone will provide the value later.

provides

terraform.tfvars

Supplies the actual value for each declared variable. This is the file that changes between Development, Testing, and Production.

uses

main.tf

References the variable with var.instance_type instead of a literal string. This file never has to change again.

Reading Order

variables.tf → declares instance_type

↓

terraform.tfvars → instance_type = "t2.medium"

↓

main.tf → instance_type = var.instance_type

↓ Terraform substitutes the value

instance_type = "t2.medium"

variables.tfDECLARATION

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variable "instance_type" {
  type = string
}

terraform.tfvarsVALUE

instance_type = "t2.medium"

main.tfUSAGE

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resource "aws_instance" "web" {
  ami           = "ami-0123456789abcdef"
  instance_type = var.instance_type
}

terminal$ terraform apply

Terraform will perform the following actions:

+ aws_instance.web

instance_type = "t2.medium"

Plan: 1 to add.

Apply complete!

# six kinds

Variable Types

String — instance_type

Stores text. The most common variable type — instance types, regions, key pairs, AMI IDs are all strings.

variables.tf

variable "instance_type" {
  type = string
}

terraform.tfvars

instance_type = "t2.micro"

main.tf

instance_type = var.instance_type

Number — disk_size

Stores numeric values — EBS volume sizes, auto-scaling desired capacity, port numbers.

variables.tf

variable "disk_size" {
  type = number
}

terraform.tfvars

disk_size = 100

main.tf

size = var.disk_size
// → 100 GB EBS volume

Bool — monitoring

Stores true or false — feature toggles like detailed monitoring, encryption, or public access.

variables.tf

variable "monitoring" {
  type = bool
}

terraform.tfvars

monitoring = true

main.tf

monitoring = var.monitoring
// → Detailed Monitoring: Enabled

List — subnets

Stores an ordered collection of values of the same type, accessed by index.

variables.tf

variable "subnets" {
  type = list(string)
}

terraform.tfvars

subnets = [
  "subnet-111",
  "subnet-222",
  "subnet-333"
]

main.tf

subnet_id = var.subnets[0]
// → launched in subnet-111

Map — tags

Stores key-value pairs, all values sharing the same type — the classic shape for resource tags.

variables.tf

variable "tags" {
  type = map(string)
}

terraform.tfvars

tags = {
  Name = "WebServer"
  Environment = "Development"
  Owner = "DevOps"
}

main.tf

tags = var.tags

Object — server

Bundles several related values of different types into a single structured variable.

variables.tf

variable "server" {
  type = object({
    instance_type = string
    disk = number
    monitoring = bool
  })
}

terraform.tfvars

server = {
  instance_type = "t3.large"
  disk = 200
  monitoring = true
}

main.tf

instance_type = var.server.instance_type
monitoring = var.server.monitoring

# convenience & safety

Default Values & Validation

defaults

Skip terraform.tfvars Entirely

When almost every server uses the same region, give the variable a default right in variables.tf. There's no need to repeat it in terraform.tfvars — Terraform falls back to the default automatically.

variables.tfDEFAULT VALUE

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variable "region" {
  default = "ap-south-1"
}

validation

Enforcing Company Policy

Say policy only allows t2.micro or t2.medium. A validation block rejects anything else with a clear error message before Terraform ever calls the provider.

variables.tfVALIDATION BLOCK

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variable "instance_type" {
  validation {
    condition = contains(
      ["t2.micro","t2.medium"],
      var.instance_type
    )
    error_message = "Only t2.micro or t2.medium allowed."
  }
}

CORRECT

instance_type = "t2.micro" → Terraform Apply Successful.

WRONG

instance_type = "m5.large" → Error: Only t2.micro or t2.medium allowed.

# where values come from

Loading Values

terraform.tfvars

Terraform loads this file automatically on every apply — no flag needed.

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region = "ap-south-1"
instance_type = "t2.micro"
disk_size = 100

*.auto.tfvars

Any file ending in .auto.tfvars — like dev.auto.tfvars, qa.auto.tfvars, prod.auto.tfvars — loads automatically too, no -var-file flag required.

dev.auto.tfvars

instance_type = "t2.micro"

TF_VAR_*

CI/CD pipelines (Jenkins, GitHub Actions, GitLab, Azure DevOps) inject values as environment variables instead of files.

$ export TF_VAR_instance_type="t3.large"

$ terraform apply

# putting it together

Complete Flow

variables.tf → declares instance_type

↓

terraform.tfvars → instance_type = "t2.medium"

↓

main.tf → instance_type = var.instance_type

↓ Terraform replaces

instance_type = "t2.medium"

↓

AWS API

↓

Creates EC2 · Instance Type = t2.medium

Easy Way to Remember

File	Why	Example
`variables.tf`	Declares what the project needs	`instance_type`, `region`
`terraform.tfvars`	Provides the actual values	`instance_type = "t2.medium"`
`main.tf`	Uses values to create infrastructure	`instance_type = var.instance_type`
`TF_VAR_*`	Passes values from CI/CD or terminal	`export TF_VAR_instance_type="m5.large"`

The code in main.tf is written once. Dev, QA, and Production all run the same code — only the input values change, through terraform.tfvars, .auto.tfvars, or TF_VAR_ environment variables.