LFCS Essential Commands Practice Question
An administrator needs to combine two sorted text files, /tmp/a.txt and /tmp/b.txt, into a single sorted stream on standard output while also removing duplicate lines that appear in both files. Which command should be used?
⚠ Common exam trap
The trap here is assuming uniq can accept two files and merge them, when its second argument is an output file.
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
✓
sort -u /tmp/a.txt /tmp/b.txt
Sorting both files together with duplicate suppression yields the union of their lines in order, which matches the request. Merge mode preserves duplicates, uniq cannot read two inputs and would clobber the second path, and comm -12 returns only the shared lines, so none of those alternatives produce the required combined, deduplicated stream.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
sort -m /tmp/a.txt /tmp/b.txt
Why it's wrong here
The -m option merges already-sorted inputs efficiently but does not remove duplicate lines; any line present in both files appears twice in the output. Since the requirement explicitly includes eliminating duplicates, merge-only behavior leaves the task incomplete even though the ordering is correct.
- ✗
comm -12 /tmp/a.txt /tmp/b.txt
Why it's wrong here
comm -12 prints only lines common to both files, that is the intersection, and it discards lines unique to either file. The administrator wants a union with duplicates removed, not an intersection, so this command silently drops most of the desired content.
- ✓
sort -u /tmp/a.txt /tmp/b.txt
Why this is correct
Passing both files as arguments to sort merges them into one input stream, and -u suppresses duplicate lines after sorting. Because the input files are already sorted, the result is a single ordered stream with duplicates from either file removed, which is exactly the stated goal.
- ✗
uniq /tmp/a.txt /tmp/b.txt
Why it's wrong here
uniq accepts a single input file and an optional output file, so the second argument is treated as the output destination rather than a second input. It would read only the first file, collapse adjacent duplicates there, and overwrite the second file, which is both incorrect and destructive.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Linux Foundation exam blueprint
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.