Input variable precedence order in Terraform - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Variable Precedence in Terraform

In Terraform, multiple sources can provide values for input variables. Which source has the highest precedence when determining the final value of a variable?

A A value passed directly via the command line using -var

B A default value defined in the variable block

C A value set in environment variables with the TF_VAR_ prefix

D A value set in a terraform.tfvars file

💡 Hint

Think about which method directly overrides others when running Terraform commands.

Values passed directly via the command line using -var have the highest precedence and override values from tfvars files, environment variables, and defaults.

🧠 Conceptual

intermediate

Order of Variable Sources in Terraform

Arrange the following Terraform variable sources from highest to lowest precedence:

• Default value in variable block
• Environment variable with TF_VAR_ prefix
• Command line -var option
• terraform.tfvars file

A 3,4,2,1

B 3,2,4,1

C 4,3,2,1

D 2,3,4,1

💡 Hint

Remember that command line options override environment variables, which override tfvars files, which override defaults.

The correct order from highest to lowest precedence is: command line -var option, environment variables with TF_VAR_ prefix, terraform.tfvars file, then default values.

❓ service_behavior

advanced

Effect of Multiple Variable Sources on Terraform Plan

You have a variable region with a default value of us-east-1. You set an environment variable TF_VAR_region=us-west-2 and also pass -var='region=eu-central-1' on the command line. What region will Terraform use during the plan?

A us-east-1

B us-west-2

C Terraform will throw an error due to conflicting values

D eu-central-1

💡 Hint

Which input source has the highest priority?

The command line -var option overrides environment variables and default values, so Terraform uses eu-central-1.

❓ security

advanced

Security Implications of Variable Precedence

Which Terraform variable input method is the safest to store sensitive data like passwords, considering variable precedence and exposure risk?

A Using environment variables with TF_VAR_ prefix

B Storing sensitive data in terraform.tfvars file checked into version control

C Passing sensitive data via command line -var option

D Defining sensitive data as default values in variable blocks

💡 Hint

Consider which method avoids exposing secrets in command history or version control.

Using environment variables with TF_VAR_ prefix is safer because it avoids storing secrets in files or command history, reducing exposure risk.

❓ Architecture

expert

Resolving Variable Conflicts in Terraform Modules

You have a root module and a child module. The root module passes a variable instance_type with value t3.micro via -var. The child module has a variable instance_type with a default t2.small. The root module also sets instance_type in a terraform.tfvars file as t3a.small. Which instance_type value will the child module receive during apply?

A t3a.small

B Terraform will fail due to conflicting variable values

C t3.micro

D t2.small

💡 Hint

Remember how variable values are passed from root to child modules and the precedence of input sources.

The root module's command line -var option overrides the terraform.tfvars file and default values. The child module receives the value passed from the root module, which is t3.micro.

Practice

1. In Terraform, which source has the highest priority when setting the value of an input variable?

easy

A. Terraform variable definition files (.tfvars)

B. Environment variables

C. Command-line flags (CLI flags)

D. Default values in the variable block

Solution

1. Step 1: Understand Terraform variable precedence

   Terraform uses a specific order to decide variable values: CLI flags first, then environment variables, then .tfvars files, and lastly default values.

2. Step 2: Identify the highest priority source

   Since CLI flags are checked first, they override all other sources.

3. Final Answer:

   Command-line flags (CLI flags) -> Option C

4. Quick Check:

   CLI flags > env vars > tfvars > defaults [OK]

Hint: Remember: CLI flags always override others [OK]

Common Mistakes:

• Thinking environment variables override CLI flags
• Assuming .tfvars files have highest priority
• Confusing default values as highest priority

2. Which of the following is the correct way to pass a variable value via CLI flag when running Terraform?

easy

A. terraform apply -var-file=region=us-west-1

B. terraform apply --set region=us-west-1

C. terraform apply -env region=us-west-1

D. terraform apply -var 'region=us-west-1'

Solution

1. Step 1: Recall CLI flag syntax for variables

   The correct syntax to pass a variable via CLI is using -var followed by 'key=value'.

2. Step 2: Match the correct option

   terraform apply -var 'region=us-west-1' uses -var 'region=us-west-1', which is the correct syntax.

3. Final Answer:

   terraform apply -var 'region=us-west-1' -> Option D

4. Quick Check:

   Use -var 'key=value' for CLI variable input [OK]

Hint: Use -var 'key=value' to pass variables via CLI [OK]

Common Mistakes:

• Using --set instead of -var
• Confusing environment variable syntax with CLI flags
• Using -var-file incorrectly for single variables

3. Given the following Terraform variable declaration and usage:
<pre>variable "env" { default = "dev" } # Command run: # terraform apply -var 'env=prod' # Environment variable: # TF_VAR_env=staging # tfvars file content: # env = "qa" What value will Terraform use for the variable env during this apply?

medium

A. "prod" (from CLI flag)

B. "dev" (default value)

C. "staging" (from environment variable)

D. "qa" (from tfvars file)

Solution

1. Step 1: List variable sources and their precedence

   Terraform checks CLI flags first, then environment variables, then tfvars files, then defaults.

2. Step 2: Identify the highest priority value provided

   CLI flag sets env=prod, which overrides environment variable (staging), tfvars (qa), and default (dev).

3. Final Answer:

   "prod" (from CLI flag) -> Option A

4. Quick Check:

   CLI flag value "prod" is used [OK]

Hint: CLI flags beat env vars and tfvars files [OK]

Common Mistakes:

• Choosing environment variable over CLI flag
• Picking tfvars value over environment variable
• Assuming default value is used when others exist

4. You run Terraform with the environment variable TF_VAR_region=us-east-1 set, but your configuration uses a region variable with a default value of us-west-2. You also have a terraform.tfvars file setting region = "eu-central-1". However, Terraform still uses us-west-2. What is the most likely cause?

medium

A. You passed a CLI flag overriding all other values

B. The environment variable name is incorrect; it should be TF_REGION

C. The variable is marked as sensitive and ignores external values

D. The terraform.tfvars file is not loaded automatically

Solution

1. Step 1: Check environment variable naming

   TF_VAR_region is correct naming for environment variable to set variable 'region'.

2. Step 2: Recall precedence order

   CLI flags > env vars > tfvars > defaults. Env var us-east-1 should override tfvars and default.

3. Step 3: Identify cause of default value

   To override the env var and use default us-west-2, a CLI flag like -var 'region=us-west-2' must have been passed.

4. Final Answer:

   You passed a CLI flag overriding all other values -> Option A

5. Quick Check:

   CLI flags > env vars > tfvars > defaults [OK]

Hint: CLI flags override env vars and tfvars [OK]

Common Mistakes:

• Assuming environment variable name is wrong
• Thinking sensitive variables block external values
• Ignoring possibility of CLI flag override

5. You want to ensure a Terraform variable instance_type always uses the value from a .tfvars file, ignoring any CLI flags or environment variables. Which approach correctly enforces this behavior?

hard

A. Remove default value and only set instance_type in the .tfvars file; avoid passing CLI flags or env vars

B. Terraform cannot enforce this; CLI flags always override .tfvars files

C. Use a local variable to override instance_type with the .tfvars value inside the configuration

D. Use a validation rule in the variable block to reject values not matching the .tfvars file

Solution

1. Step 1: Understand variable precedence limits

   Terraform's precedence is fixed: CLI flags > env vars > .tfvars > defaults. Cannot natively prioritize .tfvars over CLI/env.

2. Step 2: Confirm no enforcement mechanism

   There is no way to access or prioritize the .tfvars value separately from the resolved input variable.

3. Step 3: Why other options fail

   A relies on user discipline; B validation cannot reference .tfvars; C locals use the already-resolved var value from highest precedence.

4. Final Answer:

   Terraform cannot enforce this; CLI flags always override .tfvars files -> Option B

5. Quick Check:

   Precedence fixed, cannot override CLI/env with tfvars [OK]

Hint: Terraform precedence cannot be changed natively [OK]

Common Mistakes:

• Assuming Terraform can block CLI flag precedence natively
• Relying on validation rules to enforce external values
• Thinking removing defaults affects precedence order