how to show lines in common (reverse diff)?

Question

I have a series of text files for which I\'d like to know the lines in common rather than the lines which are different between them. Command line unix or windows is fine.

Answer 1

Easiest way to do is :

awk 'NR==FNR{a[$1]++;next} a[$1] ' file1 file2

Files are not necessary to be sorted.