Comment by buddydvd

I tried out your command and got different line count compared to the output of the commands mentioned in the post. The man page for comm says it assumes inputs to be pre-sorted. To do that, you need to sort the input files before passing them to comm.

  # show only items in both a and b
  comm -1 -2 <(sort -u a_list) <(sort -u b_list)

  # show only items unique to a
  comm -2 -3 <(sort -u a_list) <(sort -u b_list)

  # show only items unique to b
  comm -1 -3 <(sort -u a_list) <(sort -u b_list)