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

You can use uniq(1) for this if the file is sorted:

uniq -d file.txt

If the file is not sorted, run it through sort(1) first:

sort file.txt | uniq -d

This will print out the duplicates only.

Technically the input does not need to be in sorted order, but the duplicates in the file need to be consecutive. The usual way to achieve that is to sort the file.

uniq requires your list to be ordered, sort defaults to alphabetical

sort path/to/your/filename | uniq -d

or

cat fileName | sort | uniq -d

Execute this: perl -ne 'print if $a{$_}++' filename.txt