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.

Table of Contents

• 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))

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
}

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:

Variable Precedence

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

1. Environment variables (TF_VAR_name)
2. terraform.tfvars
3. .auto.tfvars / .auto.tfvars.json (alphabetical)
4. -var-file
5. -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.

Links and References

• OpenTofu GitHub
• Terraform Input Variables
• AWS EC2 Documentation