Linux commands: uniq

Oct 4, 2020

Learn how the Linux uniq command finds and removes duplicate lines in text, why you pair it with sort, and how -d, -u, and -c change what it reports.

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uniq is a command useful to sort lines of text.

You can get those lines from a file, or using pipes from the output of another command:

uniq dogs.txt

ls | uniq

You need to consider this key thing: uniq will only detect adjacent duplicate lines.

This implies that you will most likely use it along with sort:

sort dogs.txt | uniq

The sort command has its own way to remove duplicates with the -u (unique) option. But uniq has more power.

By default it removes duplicate lines:

Terminal showing cat dogs.txt with duplicate names, then sort dogs.txt | uniq removing duplicates to show each name once

You can tell it to only display duplicate lines, for example, with the -d option:

sort dogs.txt | uniq -d

Terminal showing cat dogs.txt with duplicate names, then uniq -d displaying only the duplicate lines Roger and Syd

You can use the -u option to only display non-duplicate lines:

Terminal showing cat dogs.txt with duplicate names, then uniq -u displaying only non-duplicate lines Ivica Luna Tina Vanille

You can count the occurrences of each line with the -c option:

Terminal showing cat dogs.txt with duplicates, then uniq -c displaying count numbers before each name 1 Ivica 1 Luna 2 Roger 2 Syd

Use the special combination:

sort dogs.txt | uniq -c | sort -nr

to then sort those lines by most frequent:

Terminal showing sort uniq -c sort -nr output with names sorted by frequency 2 Syd 2 Roger 1 Vanille 1 Tina 1 Luna 1 Ivica

The uniq command works on Linux, macOS, WSL, and anywhere you have a UNIX environment

For a quick paste-and-clean pass over a block of text, I built a free line sorter that sorts lines and removes duplicates.

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