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Bash: Array difference

Loop approach

A=("alpha" "beta" "gamma" "delta" "epsilon")
B=("alpha" "beta" "gamma" "delta")
NOT_IN_A=()

for A_ITEM in "${A[@]}"; do
  for B_ITEM in "${B[@]}"; do
    [[ $A_ITEM == "$B_ITEM" ]] && continue 2
  done
  NOT_IN_A+=( "$A_ITEM" )
done

echo "${NOT_IN_A[@]}" # epsilon

With `comm` command

A=("alpha" "beta" "gamma" "delta" "epsilon")
B=("alpha" "beta" "gamma" "delta")
NOT_IN_A=($(comm -23 <( printf '%s\n' "${A[@]}" | LC_ALL=C sort ) <( printf '%s\n' "${B[@]}" | LC_ALL=C sort )))

echo "${NOT_IN_A[@]}" # epsilon