Using Input Variables

Replacing hardcoded values with input variables in OpenTofu (a drop-in replacement for Terraform) makes your infrastructure code more modular, configurable, and reusable. In this guide, you’ll learn how to:

Define variables in a dedicated file
Reference them in resource blocks
Override defaults at runtime
Understand OpenTofu’s variable precedence

This approach applies to any provider—from local files to AWS EC2 instances—so you can eliminate literal strings and numbers scattered across your configuration.

Defining Variables
Referencing Variables
Example: Local File & Random Pet
Example: AWS EC2 Instance
Overriding Variable Values
Variable Precedence
Links and References

Defining Variables

Best practice is to keep all variable definitions in variables.tf (you can also place them alongside resources in main.tf, but separation improves readability).

// variables.tf
variable "filename" {
  description = "Path to the output file"
  default     = "/root/pets.txt"
}

variable "content" {
  description = "Text content written into the file"
  default     = "We love pets!"
}

variable "prefix" {
  description = "Prefix for random pet names"
  default     = "Mrs"
}

variable "separator" {
  description = "Separator between prefix and pet name"
  default     = "."
}

variable "length" {
  description = "Length of the random suffix"
  default     = "1"
}

Each block names the variable and can include:

description (optional): clarifies its purpose
default: used when no other value is supplied
type (optional): for stricter validation (e.g., string, number, list(string))

Do not store sensitive credentials (like passwords or API keys) in default. Use environment variables, a secure vault provider, or encrypted files instead.

Referencing Variables

In your main.tf, replace literal values with var.<name> references:

resource "local_file" "pet" {
  filename = var.filename
  content  = var.content
}

resource "random_pet" "my-pet" {
  prefix    = var.prefix
  separator = var.separator
  length    = var.length
}

When referencing variables, do not wrap var.name in quotes.
Correct: filename = var.filename
Incorrect: filename = "var.filename"

Apply these changes:

$ tofu apply

If you need different defaults, edit variables.tf (for example, set length = "2") and re-run tofu apply.

Example: AWS EC2 Instance

Use variables to configure cloud resources just as easily:

// main.tf
resource "aws_instance" "webserver" {
  ami           = var.ami
  instance_type = var.instance_type
}

// variables.tf
variable "ami" {
  description = "AMI ID for the EC2 instance"
  default     = "ami-0edab43b6fa892279"
}

variable "instance_type" {
  description = "EC2 instance type"
  default     = "t2.micro"
}

You can still override these at apply time without changing the file.

Overriding Variable Values

OpenTofu supports four primary ways to supply or override variable values:

Method	Description	Example
Prompted Input	Leave out `default` and answer interactive prompt	`hcl\nvariable "ami" {}\nvariable "instance_type" {}\n` `$ tofu apply`
CLI Flags	Pass `-var` flags on the command line	`bash\n$ tofu apply \\\n -var "ami=ami-0edab43b6fa892279" \\\n -var "instance_type=t2.micro"\n`
Environment Variables	Set `TF_VAR_<name>` in your shell	`bash\nexport TF_VAR_ami="ami-0edab43b6fa892279"\nexport TF_VAR_instance_type="t2.micro"\n$ tofu apply\n`
tfvars Files	Use `.tfvars` or `.tfvars.json` files	variables.tfvars: `hcl\nami = "ami-0edab43b6fa892279"\ninstance_type = "t2.micro"\n` `$ tofu apply -var-file=variables.tfvars`

Variable Precedence

When the same variable is defined multiple times, OpenTofu applies them in this order (lowest → highest):

Environment variables (TF_VAR_name)
terraform.tfvars
.auto.tfvars / .auto.tfvars.json (alphabetical)
-var-file
-var flags

Example scenario:

export TF_VAR_type="t2.nano"          # (1)
# terraform.tfvars: type = "t3.micro"  # (2)
# vars.auto.tfvars: type = "t3.small"  # (3)
$ tofu apply -var "type=t2.medium"     # (5)

The final instance_type will be t2.medium, since -var overrides all others.

Course Introduction

Introduction to Infrastructure as Code

OpenTofu Basics

OpenTofu State

Working with OpenTofu

Remote State

OpenTofu Provisioners

OpenTofu Import, Tainting Resources and Deubugging

OpenTofu Modules

OpenTofu Functions and Conditional Expressions

OpenTofu: Beyond Basics

Course Conclusion

Getting Started with OpenTofu

Using Input Variables

Table of Contents

Defining Variables

Referencing Variables

Example: AWS EC2 Instance

Overriding Variable Values

Variable Precedence

Links and References

Watch Video

​Table of Contents

​Defining Variables

​Referencing Variables

​Example: AWS EC2 Instance

​Overriding Variable Values

​Variable Precedence

​Links and References

Watch Video

Table of Contents

Defining Variables

Referencing Variables

Example: AWS EC2 Instance

Overriding Variable Values

Variable Precedence

Links and References