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)

I wrote these years ago. They're damn handy. It's true that they're not implemented in Bash (that would be nuts), but having them on hand lets me do much more on the command line than would otherwise be possible.

  ~/bin/union
  ===========
  #! /usr/bin/awk -f

  !acc[$0]++


  ~/bin/intersection
  ==================
  #! /usr/bin/awk -f

  !buf[$0]++ {acc[$0] += 1}

  ENDFILE {
    delete buf;
    files++
  }

  END {
    for (k in acc) if (acc[k] == files) print k
  }

  ~/bin/set-diff
  ==============
  #! /usr/bin/awk -f

  ! filenum { acc[$0] = 1    }
  filenum   { delete acc[$0] }

  ENDFILE { filenum++ }

  END {
    for (k in acc) print k
  }

In bash? How would you even implement a set in bash without just doing linear greps? Or did someone add sets to bash 20 years ago and I never got the memo?