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 user wants to continuously monitor the last 20 lines of a log file that is being written to by a running service. Which command achieves 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.
head -20 /var/log/syslog
Why wrong: head shows the beginning of the file, not the end, and does not follow changes.
less /var/log/syslog
Why wrong: less shows file content page by page but does not automatically follow updates.
tail -f /var/log/syslog
tail -f displays the last lines and updates in real time.
cat /var/log/syslog
Why wrong: cat outputs the entire file at once and does not follow.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
tail -f /var/log/syslog
Option C is correct because the `tail -f` command displays the last 10 lines of a file by default and then continues to output new lines as they are appended, making it ideal for real-time monitoring of a growing log file. The `-20` option is not specified here, but `tail -f` without a line count still shows the last 10 lines and follows updates; if exactly 20 lines were required, the command would be `tail -20 -f /var/log/syslog`. The `-f` flag (follow) keeps the file open and polls for changes, typically using inotify on Linux, to output new data immediately.
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.
head -20 /var/log/syslog
Why it's wrong here
head shows the beginning of the file, not the end, and does not follow changes.
less /var/log/syslog
Why it's wrong here
less shows file content page by page but does not automatically follow updates.
tail -f /var/log/syslog
Why this is correct
tail -f displays the last lines and updates in real time.
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.
cat /var/log/syslog
Why it's wrong here
cat outputs the entire file at once and does not follow.
Common exam traps
The trap here is that candidates often confuse `tail -f` with `tail -n 20` (which shows the last 20 lines but does not follow) or mistakenly think `head` can monitor the end of a file, leading them to choose option A or B without recognizing the need for the `-f` flag to achieve continuous monitoring.
Trap categories for this question
Command / output trap
head shows the beginning of the file, not the end, and does not follow changes.
Detailed technical explanation
The `tail -f` command uses the inotify subsystem on Linux to monitor file changes, which is more efficient than polling because it receives events from the kernel when the file is modified. In real-world scenarios, such as monitoring Apache access logs or system logs during troubleshooting, `tail -f` allows administrators to see entries in near real-time without reloading the file manually. A subtle behavior is that if the log file is rotated (e.g., by logrotate), `tail -f` continues to follow the original inode unless the `--follow=name` option is used, which tracks the file by name across rotations.
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.
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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: tail -f /var/log/syslog — Option C is correct because the `tail -f` command displays the last 10 lines of a file by default and then continues to output new lines as they are appended, making it ideal for real-time monitoring of a growing log file. The `-20` option is not specified here, but `tail -f` without a line count still shows the last 10 lines and follows updates; if exactly 20 lines were required, the command would be `tail -20 -f /var/log/syslog`. The `-f` flag (follow) keeps the file open and polls for changes, typically using inotify on Linux, to output new data immediately.
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 25, 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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