The correct answer is that the Dockerfile violates the best practice of combining `apt-get update` and `apt-get install` in a single RUN command. When these commands are separated, Docker caches the layer from `apt-get update`, meaning subsequent builds reuse a stale package index, which can lead to installing outdated or vulnerable packages. Combining them into one RUN command ensures the update and install happen atomically, guaranteeing a fresh package index and reducing overall image size by eliminating unnecessary intermediate layers. On the CompTIA Linux+ XK0-005 exam, this concept tests your understanding of Dockerfile optimization and layer caching, often appearing as a scenario where a candidate must identify why a build produces unexpectedly large images or uses old packages. A common trap is thinking separate commands improve readability, but the exam emphasizes efficiency and security. Remember the memory tip: “Update and install, one line to fill—cache won’t kill.”
XK0-005 Scripting, Containers and Automation Practice Question
This XK0-005 practice question tests your understanding of scripting, containers and automation. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Exhibit
Refer to the exhibit.
FROM ubuntu:20.04
RUN apt-get update
RUN apt-get install -y python3
COPY app.py /app/
CMD ["python3", "/app/app.py"]
Given the Dockerfile in the exhibit, which best practice is being violated?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Not combining apt-get update and install in one RUN command
The Dockerfile violates the best practice of combining `apt-get update` and `apt-get install` in a single RUN command. When these are separated, Docker caches the layer from `apt-get update`, so subsequent builds may use a stale package index, potentially installing outdated or vulnerable packages. Combining them ensures that the update and install happen atomically, reducing image size and guaranteeing a fresh package index.
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.
✓
Not combining apt-get update and install in one RUN command
Why this is correct
Should be combined: RUN apt-get update && apt-get install -y python3
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Using a non-LTS base image
Why it's wrong here
20.04 is LTS.
✗
Not using a .dockerignore file
Why it's wrong here
Missing .dockerignore is not a best practice violation for this simple case.
✗
Running apt-get update without cache cleanup
Why it's wrong here
rm -rf /var/lib/apt/lists/* is recommended, but not the primary violation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the nuance that separating `apt-get update` and `apt-get install` is a caching and security violation, not just a style issue, and candidates may mistakenly focus on cache cleanup or .dockerignore as the primary problem.
Detailed technical explanation
How to think about this question
Docker layers are cached based on the exact command string; separating `apt-get update` and `apt-get install` into two RUN commands means the update layer is cached and reused even if the package index changes, leading to potential security gaps. Combining them with `&&` ensures a single layer, and adding `--no-install-recommends` and cleanup of `/var/lib/apt/lists/*` further minimizes image size. In CI/CD pipelines, this prevents stale builds and reduces attack surface.
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 practitioner preparing for the XK0-005 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this XK0-005 question in full detail.
Scripting, Containers and Automation — This question tests Scripting, Containers and Automation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Not combining apt-get update and install in one RUN command — The Dockerfile violates the best practice of combining `apt-get update` and `apt-get install` in a single RUN command. When these are separated, Docker caches the layer from `apt-get update`, so subsequent builds may use a stale package index, potentially installing outdated or vulnerable packages. Combining them ensures that the update and install happen atomically, reducing image size and guaranteeing a fresh package index.
What should I do if I get this XK0-005 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: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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