You can use a combination of `comm` and `uniq` to implement set intersection and union. https://ss64.com/bash/comm.html

> Return the unique lines in the file words.txt that don't exist in countries.txt

    comm -23 <(sort words.txt | uniq) <(sort countries.txt | uniq)

>Return the lines that are in both words.txt and countries.txt:

    comm -12 <(sort words.txt | uniq) <(sort countries.txt | uniq)