Quick Answer
The answer is `usermod -aG docker bob`. This command is correct because the `-a` (append) flag, when used with `-G` (supplementary groups), adds user 'bob' to the 'docker' group without removing his existing supplementary group memberships. Without the `-a` flag, the `-G` option alone would replace the entire supplementary group list, effectively removing bob from any groups not specified in the command. On the Linux Foundation Certified System Administrator LFCS exam, this is a classic trap: many candidates forget the `-a` flag and inadvertently strip a user of their existing groups, which can break access to critical resources. The exam tests your understanding that `usermod -G` is destructive by default, while `-aG` is additive. A simple memory tip is to think of the `-a` as “append” or “add” — if you want to add a user to a supplementary group without removing existing groups, always append the `-a` flag.
This LFCS practice question tests your understanding of user and group management. 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.
An administrator wants to add user 'bob' to the supplementary group 'docker' without removing bob from any existing groups. Which command accomplishes this?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
usermod -g docker bob
Why wrong: -g changes the primary group, not supplementary.
usermod -aG docker bob
-aG appends docker to the list of supplementary groups.
usermod -G docker bob
Why wrong: -G without -a replaces all supplementary groups with only docker, removing bob from any other groups.
usermod -G docker bob
Why wrong: Same as B; no -a flag.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
usermod -aG docker bob
Option B is correct because the `-a` (append) flag combined with `-G` (supplementary groups) ensures user 'bob' is added to the 'docker' group without removing him from any existing supplementary groups. Without `-a`, the `-G` flag replaces the user's current supplementary group list with the specified groups.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
usermod -g docker bob
Why it's wrong here
-g changes the primary group, not supplementary.
usermod -aG docker bob
Why this is correct
-aG appends docker to the list of supplementary groups.
Clue confirmation
The clue word "which command" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
usermod -G docker bob
Why it's wrong here
-G without -a replaces all supplementary groups with only docker, removing bob from any other groups.
usermod -G docker bob
Why it's wrong here
Same as B; no -a flag.
Common exam traps
The trap here is that candidates often confuse `-g` (primary group) with `-G` (supplementary groups) and overlook the necessity of the `-a` flag to append rather than replace supplementary group memberships.
Detailed technical explanation
The `usermod` command modifies `/etc/group` and `/etc/gshadow` to update supplementary group memberships. The `-a` flag is only meaningful with `-G`; it appends to the current list of supplementary groups rather than overwriting it. In real-world scenarios, forgetting `-a` can accidentally remove a user from critical groups like `sudo` or `wheel`, locking them out of administrative privileges.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the LFCS 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.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
User and Group Management — study guide chapter
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FAQ
User and Group Management — This question tests User and Group Management — Read the scenario before looking for a memorised answer..
The correct answer is: usermod -aG docker bob — Option B is correct because the `-a` (append) flag combined with `-G` (supplementary groups) ensures user 'bob' is added to the 'docker' group without removing him from any existing supplementary groups. Without `-a`, the `-G` flag replaces the user's current supplementary group list with the specified groups.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "which command". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This LFCS practice question is part of Courseiva's free Linux Foundation certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LFCS exam.
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