uniq command: collapse repeated lines and count them
uniq collapses adjacent identical lines. Learn why it is always paired with `sort`, how `-c` counts occurrences, and what `-d` and `-u` are for, by running the commands in a real terminal in your browser.
Updated: 2026-09-06
Syntax
uniq [options] fileIt collapses adjacent identical lines into one.
Try it first
A log where the same error repeats.
$ cat logs/errors.log
ERROR connection refused
ERROR connection refused
ERROR connection refused
WARN slow query
ERROR timeout
ERROR timeoutThrough uniq, each run of identical lines becomes a single line.
$ uniq logs/errors.log
ERROR connection refused
WARN slow query
ERROR timeout-c says how many times each run repeated.
$ uniq -c logs/errors.log
3 ERROR connection refused
1 WARN slow query
2 ERROR timeoutThe same error hit three times in a row.
Scattered duplicates survive
This is where uniq surprises people.
$ cat emails.txt
ada@example.com
linus@example.com
ada@example.com
grace@example.com
linus@example.com
ada@example.com
$ uniq emails.txt
ada@example.com
linus@example.com
ada@example.com
grace@example.com
linus@example.com
ada@example.comNot one line was removed, because the duplicates are not next to each other. Sort first.
$ sort emails.txt | uniq
ada@example.com
grace@example.com
linus@example.comIn practice, uniq almost always follows a sort.
Count, then order by the count
$ sort emails.txt | uniq -c
3 ada@example.com
1 grace@example.com
2 linus@example.com
$ sort emails.txt | uniq -c | sort -nr
3 ada@example.com
2 linus@example.com
1 grace@example.comThose three steps, group and count and order, are the standard way to read a log.
$ cut -d' ' -f1 logs/access.log | sort | uniq -c | sort -nr
3 200
2 404
1 500Only the repeats, only the one-offs
-d keeps lines that appeared more than once.
$ sort emails.txt | uniq -d
ada@example.com
linus@example.com-u keeps the lines that appeared exactly once.
$ sort emails.txt | uniq -u
grace@example.comUse -d to hunt for duplicates and -u to find what only happened once.
When you actually reach for it
uniq is what you type to find out how many times the same thing happened.
| Situation | What to type |
|---|---|
| Tally, most frequent first | sort access.log | uniq -c | sort -nr |
| Find duplicate entries | sort emails.txt | uniq -d |
| Find the one-offs | sort emails.txt | uniq -u |
| Count a repeated error in place | uniq -c errors.log |
| Just remove duplicates | sort -u emails.txt |
Things that trip people up
Without sort in front, nothing happens.
uniq compares neighbours only.
If duplicates refuse to disappear, that is the reason.
For removal alone, sort -u is enough.
$ sort -u emails.txt
ada@example.com
grace@example.com
linus@example.comWhat uniq adds is -c and -d.
Directories are not accepted.
$ uniq logs
uniq: logs: Is a directoryA missing file says so.
$ uniq nothing.txt
uniq: nothing.txt: No such file or directoryPractise it hands-on
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Frequently asked questions
- The duplicates are still there.
- uniq only compares each line with the one before it. Sort the input first so identical lines end up next to each other.
- How is this different from sort -u?
- If you only want the duplicates gone, `sort -u` is enough. uniq earns its place through `-c` (counts) and `-d` (duplicates only).
- How do I get the most common first?
- `sort | uniq -c | sort -nr`. Group, count, then order by that count.
- What is the difference between -d and -u?
- `-d` prints only lines that appeared more than once; `-u` prints only lines that appeared exactly once. Use -d to find repeats and -u to find one-offs.
- Why are the counts padded?
- `-c` right-aligns them, which keeps the columns straight and lets you pipe the result into `sort -nr` unchanged.