Demo Using Variables in OpenTofu

In this guide, we’ll cover multiple methods to pass input variables into your OpenTofu configurations. You’ll learn how to:

• Inject variables via environment variables
• Understand OpenTofu’s variable precedence
• Supply custom variable files
• Declare and override variables in your project

By the end, you’ll be confident working with TF_VAR_ exports, .tfvars files, and command-line flags.

1. Injecting Variables with Environment Variables

The simplest approach is to set an environment variable prefixed with TF_VAR_. OpenTofu picks these up automatically:

export TF_VAR_filename="./data/baseball.txt"

2. Variable Definition Precedence

OpenTofu resolves variables based on a fixed priority list. The highest precedence is given to values passed with -var or -var-file. Below is the full order:

For more details, see OpenTofu Input Variables{:target="_blank"}.

3. Supplying a Custom Variable File

You can group variables in a file (e.g., variables.tfvars) and pass it with -var-file:

tofu apply -var-file=variables.tfvars

OpenTofu will load all key-value pairs from that file.

Hands-On: Working with Variables in a Sample Project

Navigate to /root/OpenTofu/projects/variables. The directory contains:

• basket.auto.tfvars
• main.tf
• terraform.tfvars
• throw.auto.tfvars

4. Running tofu plan Without Declared Variables

If main.tf references var.filename but you haven’t declared the variable, you’ll see:

$ cd /root/OpenTofu/projects/variables
$ tofu plan
Warning: Value for undeclared variable
Error: Reference to undeclared input variable

  on main.tf line 2, in resource "local_file" "games":
   2:   filename = var.filename

An input variable with the name "filename" has not been declared. This variable can be declared with a variable "filename" {} block.

5. Declaring the filename Variable

Create a variables.tf file next to main.tf:

variable "filename" {
  description = "Path to the output file"
  type        = string
}

Re-run tofu plan—the error should disappear.

6. Overriding Variables on the Command Line

To set or override filename at runtime:

tofu apply -var='filename=/root/tennis.txt'

Since -var has the highest precedence, it will override any environment or .tfvars settings.

Links and References

• OpenTofu Documentation
• Terraform Variables
• Managing Terraform Variables{:target="_blank"}