#! /bin/bash
IFS=$'\n'
XYZ=($(<a1))
while read -r line
do
  for row in "${XYZ[@]}"
  do
    if [[ "$row" == "$line" ]]; then
      continue 2 
    fi
  done
  printf "%s\n" "$line"
done < a2

#! /bin/bash4
declare -A XYZ

while read -r line
do
  XYZ[$line]=x
done < a1

while read -r line
do
  if [[ ${XYZ[$line]} != x ]]; then
    printf "%s\n" "$line"
  fi
done < a2