Select the TWO true statements about the uniq command.
LPIC-1 Exam 101-500, objective 103. GNU and Unix commands medium
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The options
Choose 2.
Not correct uniq sorts its input before removing duplicates.
Wrong. uniq never reorders anything; it compares each line only with the previous one. Duplicates that are far apart pass through untouched.
Not correct uniq -u prints one copy of every line, including the duplicated ones.
Wrong. -u prints only the lines that appear exactly once in a run; lines that were duplicated are omitted entirely.
Correct uniq only collapses duplicate lines that are adjacent, so its input is normally sorted first.
Correct. This is the single most important fact about uniq, and the reason `sort file | uniq` is such a common pipeline.
Correct uniq -c prefixes each output line with the number of times it occurred.
Correct. `sort file | uniq -c | sort -nr` is the standard way to rank lines by frequency, for example in log analysis.
Not correct uniq -d prints all lines, marking the duplicated ones.
Wrong. -d prints only the duplicated lines, one copy per group, and nothing else. Nothing is marked.
Why
uniq is a streaming filter with a one-line memory: it compares the current line with the previous one only. Sort first, or the result is meaningless. -c counts occurrences, -d shows only repeated lines, -u shows only lines that never repeated, and -i ignores case. `sort -u` does the sort and the deduplication in one step, but it cannot count.
Where this comes from
- Cited
- LPI exam objective 103.2
- What it says
- Remove or count duplicate lines with uniq, which requires sorted input.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 103 and space the ones you get wrong.