Comment by sciurus

This is a neat hack! For two input files, the intersection and relative complement can be done more straightforwardly with comm.

https://en.wikipedia.org/wiki/Comm

  # show only items in both a and b
  comm -1 -2 a_list b_list

  # show only items unique to a
  comm -2 -3 a_list b_list

  # show only items unique to b
  comm -1 -3 a_list b_list