How to catch duplicate entries in text file in linux [duplicate]

Question

Text file:

 1 1 2 2 3 3 1 1 

I want to catch 1 1 as duplicated

Answer 1

Your question is not quite clear, but you can filter out duplicate lines with uniq:

sort file.txt | uniq 

or simply

sort -u file.txt 

(thanks RobEarl)

You can also print only repeating lines with

sort file.txt | uniq -d 

Answer 2

One way using GNU awk:

awk 'array[$0]++' file.txt  

Results:

1 1