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How to print only the duplicate values from a text file?

Suppose there is a column of numeric values like following:

File1:

1 2
3
3
3
4
4
4
5
6

I want the output:

3  4

Ans::

You can use uniq(1) for this:

uniq -d file1.txt

This will print out the duplicates only.

sankar@new-host shell script]$ uniq -d file1.txt

+ uniq -d uniq1.txt

2

4

6

7

2. [sankar@new-host shell script]$ cat uniq1.txt  | uniq -c | awk '$1 > 1 { print $2 }'

+ uniq -c

+ awk '$1 > 1 { print $2 }'

+ cat uniq1.txt

2

4

6

7