Deduplicate Lines in a File on the Command Line

September 2025

This is what I use to remove duplicate lines from a text file on the command line:

sort -u input.txt > output.txt

Notes

Lots of examples on the web show using sort along with uniq like this:
```
sort input.txt | uniq > output.txt
```
That does the same thing as as sort -u.
The input file has to be sorted first because uniq only compares lines that are on top of each other.
Lots of examples on the web show using the -u flag. It removes all instances of the duplicate lines. It's probably not what you want most of the time

For example, given this already sorted input.txt file:
```
alfa
alfa
bravo
charlie
charlie
delta
```
You get these outputs from the uniq command:
```
uniq input.txt
```
Output
```
alfa
bravo
charlie
delta
```
verses with the -u flag:
```
uniq -u input.txt
```
Output
```
bravo
delta
```

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