Extracting Similarity Between Two (Text) Files

When you need to compare difference between two files, you have diff.
When you need to find similarity between such files, as far as I know, you need to devise your own script.
This is my oneliner script for such need (assuming files to be compared named file-01 & file-02):

for ((i=1;i<=$(wc -l file-01|awk '{print $1}');i+=1)); do grep $(awk -v a=$i 'NR==a' file-01) file-02 ; done

Ugly, but working...