select_lines

#!/bin/bash

help() {
   echo 'usage: select_lines (only-left|only-right|in-both) left_file right_file'
   echo '       select_lines --help'
   echo
   echo '   Depending on first argument will emit a list of'
   echo '   lines that are either only in the left, the right'
   echo '   or in both files.'
   echo
   echo '   (The script will create sorted temporary copies'
   echo '    of both files...)'
   echo
   echo '    Thanks to mr.spuratic in https://stackoverflow.com/a/18205289'
   echo
   exit 1
}

[ "$1" == "--help" ] && help
[ "$3" == "" ] && echo "ERROR: missing parameters" && echo && help

left_sorted=$( mktemp )
right_sorted=$( mktemp )

sort "$2" > "$left_sorted"
sort "$3" > "$right_sorted"

case "$1" in
   only-left)
     diff --old-group-format="%<"       \
          --new-group-format=""         \
          --unchanged-group-format=""   \
        "$left_sorted" \
        "$right_sorted"
     ;;
   only-right)
     diff --old-group-format=""         \
          --new-group-format="%>"       \
          --unchanged-group-format=""   \
        "$left_sorted" \
        "$right_sorted"
     ;;
   in-both)
     diff --old-group-format=""         \
          --new-group-format=""         \
          --unchanged-group-format="%=" \
        "$left_sorted" \
        "$right_sorted"
     ;;
   *)
     echo "ERROR: unkown first parameters" && echo && help
     ;;
esac

rm "$left_sorted"
rm "$right_sorted"