> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Using Input Variables

> This article explains how to use input variables in OpenTofu to create flexible and reusable infrastructure code.

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](#defining-variables)
* [Referencing Variables](#referencing-variables)
* [Example: Local File & Random Pet](#example-local-file--random-pet)
* [Example: AWS EC2 Instance](#example-aws-ec2-instance)
* [Overriding Variable Values](#overriding-variable-values)
* [Variable Precedence](#variable-precedence)
* [Links and References](#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).

```hcl theme={null}
// 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)`)

<Callout icon="triangle-alert" color="#FF6B6B">
  Do **not** store sensitive credentials (like passwords or API keys) in `default`. Use environment variables, a secure vault provider, or encrypted files instead.
</Callout>

***

## Referencing Variables

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

```hcl theme={null}
resource "local_file" "pet" {
  filename = var.filename
  content  = var.content
}

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

<Callout icon="lightbulb" color="#1CB2FE">
  When referencing variables, do **not** wrap `var.name` in quotes.\
  Correct: `filename = var.filename`\
  Incorrect: `filename = "var.filename"`
</Callout>

Apply these changes:

```bash theme={null}
$ 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:

```hcl theme={null}
// 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`<br />`$ 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**:<br />`hcl\nami           = "ami-0edab43b6fa892279"\ninstance_type = "t2.micro"\n`<br />`$ tofu apply -var-file=variables.tfvars` |

***

## 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:

```bash theme={null}
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](https://github.com/opentofu/opentofu)
* [Terraform Input Variables](https://developer.hashicorp.com/terraform/language/values/variables)
* [AWS EC2 Documentation](https://docs.aws.amazon.com/ec2/index.html)

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