Quick Answer
The correct command is grep -rwin 'error' /var/log. This works because the -w flag enforces whole-word matching using word boundaries, so it only matches 'error' as a standalone word, not substrings like 'error404' or 'error-prone', while -r enables recursive directory traversal, -i ignores case, and -n prints line numbers. On the Linux Foundation Certified System Administrator LFCS exam, this tests your ability to combine grep flags precisely to filter logs without false positives—a common trap is forgetting -w and relying on -i alone, which still matches partial words. A strong memory tip is to think of the mnemonic "RINse with WIN" (Recursive, Ignore case, Numbers, Whole Word) to recall the flag order, or simply remember that -w is your "word boundary" shield against substring matches.
This LFCS practice question tests your understanding of essential commands. 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.
A system administrator runs 'grep -r 'error' /var/log' and gets many false positives. They want to search only for the exact word 'error' as a whole word, case-insensitively, and display line numbers. Which command should they use?
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.
grep -rwi 'error' /var/log
Why wrong: Missing -n for line numbers.
grep -rin 'error' /var/log
Why wrong: No whole word matching.
grep -rwn 'error' /var/log
Why wrong: Missing -i for case-insensitivity.
grep -rwin 'error' /var/log
Correct: recursive, whole word, case-insensitive, line numbers.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
grep -rwin 'error' /var/log
Option D is correct because it combines all required flags: `-r` for recursive search, `-w` for whole-word matching (using word boundaries), `-i` for case-insensitive search, and `-n` for displaying line numbers. The `-w` flag ensures that only the exact word 'error' is matched, not substrings like 'error404' or 'error-prone', which eliminates false positives.
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.
grep -rwi 'error' /var/log
Why it's wrong here
Missing -n for line numbers.
grep -rin 'error' /var/log
Why it's wrong here
No whole word matching.
grep -rwn 'error' /var/log
Why it's wrong here
Missing -i for case-insensitivity.
grep -rwin 'error' /var/log
Why this is correct
Correct: recursive, whole word, case-insensitive, line numbers.
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.
Common exam traps
The trap here is that candidates often forget the `-w` flag is needed for whole-word matching, assuming `-i` alone is sufficient, or they confuse the order of flags and omit one of the required options.
Detailed technical explanation
The `-w` flag in GNU grep enforces whole-word matching by anchoring the pattern with `\b` word boundary assertions (equivalent to `\berror\b` in extended regex). This is critical in log files where 'error' may appear as part of a larger token (e.g., 'error_handler'). Without `-w`, grep's default substring matching would cause false positives, which is a common pitfall in log analysis.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Essential Commands — This question tests Essential Commands — Read the scenario before looking for a memorised answer..
The correct answer is: grep -rwin 'error' /var/log — Option D is correct because it combines all required flags: `-r` for recursive search, `-w` for whole-word matching (using word boundaries), `-i` for case-insensitive search, and `-n` for displaying line numbers. The `-w` flag ensures that only the exact word 'error' is matched, not substrings like 'error404' or 'error-prone', which eliminates false positives.
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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