Why Environment variables (TF_VAR_) in Terraform? - Purpose & Use Cases

The Big Idea

What if you could change your cloud setup without touching a single line of code?

The Scenario

Imagine you have to configure your cloud infrastructure by typing every setting directly into your Terraform files or commands each time you deploy.

For example, you want to change the server size or region for different projects, so you edit the files manually every time.

The Problem

This manual way is slow and risky.

You might forget to change a value, causing errors or deploying wrong resources.

It's also hard to share configurations safely without exposing sensitive data.

The Solution

Using environment variables with the TF_VAR_ prefix lets you set values outside your code.

This means you can easily switch settings by changing environment variables, without touching your Terraform files.

It keeps your configurations clean, safe, and flexible.

Before vs After

✗ Before

terraform apply -var 'region=us-east-1' -var 'size=small'

✓ After

export TF_VAR_region=us-east-1
export TF_VAR_size=small
terraform apply

What It Enables

You can quickly and safely customize your infrastructure settings across different environments without changing your code.

Real Life Example

A developer working on multiple projects can set environment variables for each project's cloud region and server size, then run the same Terraform commands without editing files.

Key Takeaways

Manual configuration is slow and error-prone.

TF_VAR_ environment variables let you separate settings from code.

This makes infrastructure deployment safer, faster, and more flexible.

Practice

1. What is the purpose of using environment variables with the TF_VAR_ prefix in Terraform?

easy

A. To set Terraform input variables without hardcoding them in configuration files

B. To define backend storage for Terraform state files

C. To specify the Terraform provider version

D. To enable debugging output during Terraform runs

Solution

1. Step 1: Understand Terraform variable usage

   Terraform variables allow customization of configurations without changing code files.

2. Step 2: Role of TF_VAR_ prefix

   Environment variables prefixed with TF_VAR_ automatically set Terraform input variables, keeping code clean and secure.

3. Final Answer:

   To set Terraform input variables without hardcoding them in configuration files -> Option A

4. Quick Check:

   Environment variables with TF_VAR_ prefix set variables [OK]

Hint: TF_VAR_ prefix sets Terraform variables via environment [OK]

Common Mistakes:

• Confusing TF_VAR_ with backend configuration
• Using TF_VAR_ to set provider versions
• Expecting TF_VAR_ to enable debug logs

2. Which of the following is the correct way to set a Terraform variable named region using an environment variable?

easy

A. TF_VARregion=us-west-2

B. set TF_VAR-region=us-west-2

C. export TF_VAR_region=us-west-2

D. terraform var region=us-west-2

Solution

1. Step 1: Identify correct environment variable syntax

   Terraform expects environment variables for variables to be prefixed with TF_VAR_ followed by the variable name.

2. Step 2: Correct shell export command

   In Unix-like shells, export TF_VAR_region=us-west-2 correctly sets the variable.

3. Final Answer:

   export TF_VAR_region=us-west-2 -> Option C

4. Quick Check:

   Use export TF_VAR_variable=value [OK]

Hint: Use export TF_VAR_variable=value to set variables [OK]

Common Mistakes:

• Using dash instead of underscore in TF_VAR_ prefix
• Missing export keyword in Unix shell
• Trying to set variables with terraform command directly

3. Given the Terraform variable declaration:

variable "instance_type" { default = "t2.micro" }

and the environment variable set as export TF_VAR_instance_type=t3.medium, what will be the value of var.instance_type during Terraform apply?

medium

A. "t2.micro" (default value)

B. "t3.medium" (from environment variable)

C. null (no value set)

D. Error: variable instance_type not set

Solution

1. Step 1: Understand variable precedence

   Terraform uses environment variables with TF_VAR_ prefix to override default variable values.

2. Step 2: Apply environment variable value

   Since TF_VAR_instance_type is set to "t3.medium", this value overrides the default "t2.micro".

3. Final Answer:

   "t3.medium" (from environment variable) -> Option B

4. Quick Check:

   Environment variable overrides default value [OK]

Hint: Environment variable overrides default Terraform variable [OK]

Common Mistakes:

• Assuming default always applies ignoring environment variable
• Expecting error if environment variable is set
• Confusing null with default value

4. You set the environment variable TF_VAR_count=3 but Terraform still uses the default value count = 1 from the variable declaration. What is the most likely cause?

medium

A. The environment variable name is case-sensitive and should be TF_VAR_Count

B. The environment variable was set after running terraform init

C. The variable count is declared as a string, but environment variable is numeric

D. The shell session where Terraform runs does not have the environment variable exported

Solution

1. Step 1: Check environment variable visibility

   Terraform reads environment variables from the shell session it runs in; if not exported, Terraform won't see it.

2. Step 2: Confirm export of variable

   Setting TF_VAR_count=3 without export means it's not passed to child processes like Terraform.

3. Final Answer:

   The shell session where Terraform runs does not have the environment variable exported -> Option D

4. Quick Check:

   Environment variables must be exported to be visible [OK]

Hint: Always export environment variables before running Terraform [OK]

Common Mistakes:

• Assuming variable names are case-insensitive
• Thinking terraform init caches variable values
• Confusing variable type mismatch as cause

5. You want to securely provide a sensitive variable db_password to Terraform without storing it in code or plain text files. Which approach using environment variables is best practice?

hard

A. Set export TF_VAR_db_password=your_password in your shell before running Terraform

B. Hardcode the password in terraform.tfvars file

C. Use terraform apply -var 'db_password=your_password' every time

D. Store the password in a public GitHub repository and reference it

Solution

1. Step 1: Avoid storing sensitive data in code files

   Hardcoding passwords in files or public repos risks exposure.

2. Step 2: Use environment variables for sensitive data

   Setting TF_VAR_db_password in the shell keeps secrets out of code and version control.

3. Final Answer:

   Set export TF_VAR_db_password=your_password in your shell before running Terraform -> Option A

4. Quick Check:

   Environment variables keep secrets out of code [OK]

Hint: Use TF_VAR_ environment variables to keep secrets out of code [OK]

Common Mistakes:

• Storing secrets in terraform.tfvars files
• Passing secrets on command line risking history leaks
• Publishing secrets in public repositories