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In this lesson you’ll learn how to pass input variables into OpenTofu configurations using environment variables, variable files, and command-line flags. You’ll also see how OpenTofu determines which value to use when multiple sources provide the same variable.

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1) Using environment variables

To provide an input variable from the environment, export it using the TF_VAR_ prefix followed by the variable name. For example, to set a variable named filename: 
export TF_VAR_filename="/root/baseball.txt"
OpenTofu recognizes environment variables that match the TF_VAR_<name> pattern and treats them as input values for the root module.
Environment variables set with TF_VAR_<name> are convenient for CI or temporary local overrides. They are lower priority than command-line flags, but generally take precedence over variable definition files such as *.tfvars.
Use cases for environment variables:
  • CI/CD secrets or temporary overrides that you do not want checked into source control.
  • Local overrides during development without changing .tfvars files.
  • Quick testing of alternate inputs.

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2) Variable definition precedence

When multiple sources supply a value for the same variable, OpenTofu follows a deterministic precedence order. The general precedence (highest to lowest) is:
Precedence (highest → lowest)Source
1. Last-specified CLI flags-var and -var-file flags provided on the command line
2. Environment variablesTF_VAR_<name>
3. Variable definition files*.tfvars, terraform.tfvars, and *.auto.tfvars (auto-loaded)
4. Declared defaults in configurationvariable "<name>" { default = ... } in .tf files
Notes:
  • If you pass multiple -var or -var-file flags, the flag specified last on the command line takes priority among them.
  • Auto-loaded *.auto.tfvars files are combined; explicit -var-file will override values from those files.

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3) Using a custom variable file with apply

To supply a custom variables file at apply time, use the -var-file flag: 
tofu apply -var-file=variables.tfvars
Because -var-file is a CLI flag, its values take precedence over both environment variables and auto-loaded *.tfvars files. This is useful when you need an environment-specific or sensitive values file that should override defaults.

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4) Example project and troubleshooting

Assume the following files exist under /root/opentofu-projects/variables:
  • main.tf
  • variables.tf (may be missing initially)
  • throw.auto.tfvars
  • basket.auto.tfvars
  • terraform.tfvars
Contents of main.tf (a resource that expects a variable filename): 
resource "local_file" "games" {
  filename = var.filename
  content  = "football"
}
An auto var file throw.auto.tfvars might contain: 
filename = "/root/baseball.txt"
Troubleshooting: undeclared variable error If main.tf references var.filename but the configuration does not declare a variable "filename" {} block, running tofu plan will warn that a value was found in an auto-loaded file and then error because the input variable is undeclared. Example console output: 
$ tofu plan
Warning: Value for undeclared variable

The root module does not declare a variable named "filename" but a value was found in file "throw.auto.tfvars". If you meant to use this value, add a "variable" block to the configuration.

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.
Fix: declare the variable in the root module (for example, in variables.tf): 
variable "filename" {}
You do not need to provide a default; declaring the variable allows OpenTofu to accept values from .tfvars files, environment variables, or CLI flags. After adding the variable block, tofu plan will proceed and use the appropriate value according to the precedence rules above.

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5) Example: command-line -var overrides

Passing a value via the CLI -var flag overrides values from .tfvars files and environment variables: 
tofu apply -var 'filename="/root/tennis.txt"'
Given these sources:
  • throw.auto.tfvars contains filename = "/root/baseball.txt"
  • Environment: TF_VAR_filename="/root/baseball.txt"
  • Command line: -var 'filename="/root/tennis.txt"'
OpenTofu will choose /root/tennis.txt because -var on the command line has the highest priority.
If you have multiple .tfvars files and *.auto.tfvars files, remember that auto-loaded files are combined; however explicit -var-file or -var on the CLI will override those values.

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Summary

  • Export variables for quick, ephemeral overrides: export TF_VAR_<name>=<value>.
  • Always declare variables in the root module with variable "<name>" {} to avoid undeclared variable errors.
  • Use tofu apply -var-file=FILE or tofu apply -var 'name=value' to provide or override values at runtime; CLI flags take highest precedence.
  • Auto-loaded *.auto.tfvars files are convenient for defaults but can be overridden by environment variables and the CLI.

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