Cloud Build Substitution Variables: Must Start with Underscore
This PCD practice question tests your understanding of building and testing applications. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Refer to the exhibit. The Cloud Build fails with an error that the image name is invalid. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The answer is that the substitution variable $MY_PROJECT is invalid because Cloud Build substitution variable naming rules require all user-defined variables to start with an underscore. This is a strict syntax enforced by Cloud Build: any variable like $_MY_PROJECT is valid, but $MY_PROJECT is interpreted as a literal string or an environment variable that does not exist, causing the image name to be malformed. On the Google Professional Cloud Developer exam, this rule frequently appears in troubleshooting scenarios where a build fails due to an invalid image tag or parameter, testing your understanding of how Cloud Build processes substitutions versus shell environment variables. A common trap is assuming that any variable name works, but the underscore prefix is mandatory for user-defined substitutions—built-in variables like $PROJECT_ID are exceptions. To remember this, think of the underscore as the “user flag”: if you define it, underscore it.
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
✓
The substitution variable $MY_PROJECT is not a valid substitution because it is not prefixed with underscore.
Option B is correct because in Cloud Build, user-defined substitution variables must be prefixed with an underscore. The variable `$MY_PROJECT` lacks the required underscore prefix, making it invalid. This causes the image name to be malformed, leading to the 'invalid image name' error.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The push step should be after the deploy step.
Why it's wrong here
Push step is properly after the build step.
✓
The substitution variable $MY_PROJECT is not a valid substitution because it is not prefixed with underscore.
Why this is correct
User-defined substitutions must start with underscore; $MY_PROJECT is not valid.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The Docker build step fails because the Dockerfile is missing.
Why it's wrong here
The error is about the image name, not the build step.
✗
The image name contains uppercase letters that are not allowed.
Why it's wrong here
Uppercase letters are allowed in image names.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud Build often tests the subtle distinction between default substitutions (like `$PROJECT_ID`) and user-defined substitutions (which require an underscore prefix), leading candidates to assume all variables follow the same syntax.
Detailed technical explanation
How to think about this question
Cloud Build substitution variables are defined in the build configuration file or passed at runtime, and they must start with an underscore (e.g., `$_MY_VAR`). The `$PROJECT_ID` variable is a default substitution that does not require an underscore, but user-defined variables do. This strict naming convention prevents conflicts with environment variables and ensures predictable substitution behavior.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Building and testing applications — This question tests Building and testing applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The substitution variable $MY_PROJECT is not a valid substitution because it is not prefixed with underscore. — Option B is correct because in Cloud Build, user-defined substitution variables must be prefixed with an underscore. The variable `$MY_PROJECT` lacks the required underscore prefix, making it invalid. This causes the image name to be malformed, leading to the 'invalid image name' error.
What should I do if I get this PCD question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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