TF-004 Read, generate and modify configuration Practice Question
A module requires an input variable named 'vpc_id'. How should the calling configuration pass the VPC ID from another module's output?
⚠ Common exam trap
The Terraform exam often tests the misconception that module outputs require an explicit `.outputs.` attribute, leading candidates to choose Option A, when in fact Terraform flattens outputs into direct module attributes for simplicity.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
vpc_id = module.vpc.vpc_id
In Terraform, when you need to reference an output from a child module, the syntax is `module.<module_name>.<output_name>`. The `.outputs.` prefix is not used; Terraform automatically exposes module outputs as direct attributes of the module resource. Therefore, `module.vpc.vpc_id` correctly retrieves the `vpc_id` output from the module named `vpc`.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
vpc_id = module.vpc.outputs.vpc_id
Why it's wrong here
The syntax `module.vpc.outputs.vpc_id` is incorrect for referencing module outputs in Terraform. While modules do define outputs, they are directly exposed as attributes of the module block itself, not nested under an explicit `outputs` keyword. The correct approach is to access the output directly using `module.<MODULE_NAME>.<OUTPUT_NAME>` without the intermediate `.outputs` attribute.
- ✗
vpc_id = var.vpc_id
Why it's wrong here
Using `var.vpc_id` would attempt to reference an input variable named `vpc_id` defined within the *current* Terraform configuration or module where this assignment is being made. This is not suitable when the intent is to retrieve an output value from a *separate, already defined module* named `vpc`. The `var.` prefix is exclusively for accessing input variables, not outputs from other modules.
- ✗
vpc_id = vpc.module.vpc_id
Why it's wrong here
The syntax `vpc.module.vpc_id` is structurally incorrect and will result in a parsing error in Terraform. When referencing an output from a called module, the `module` keyword must always precede the local name assigned to that module instance. The correct order is `module.<LOCAL_MODULE_NAME>.<OUTPUT_NAME>`, making `vpc.module` an invalid construct for this purpose.
- ✓
vpc_id = module.vpc.vpc_id
Why this is correct
This syntax, `vpc_id = module.vpc.vpc_id`, is the correct and standard method for referencing an output value from a module in Terraform. The `module` keyword signifies that a module instance is being referenced, `vpc` is the local name given to that specific module instance within the configuration, and `vpc_id` is the name of the output variable defined within the `vpc` module itself. This allows the output of one module to be used as an input for another resource or module.
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Written by Johnson Ajibi, MSc IT Security
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