join files with mktemp

#!/bin/bash
base='hoge_base.txt'
files=('hoge2.txt' 'hoge3.txt')
temp1=$(mktemp)
temp2=$(mktemp)
trap 'rm -f "${temp1}" "${temp2}"' ERR EXIT
cp "${base}" "${temp1}"
for ((i = 0; i < ${#files[@]}; ++i)); do
  if ((i % 2 == 0)); then
    join -o auto -a1 -e '---' "${temp1}" "${files[$i]}" >"${temp2}"
  else
    join -o auto -a1 -e '---' "${temp2}" "${files[$i]}" >"${temp1}"
  fi
done
if ((${#files[@]} % 2 == 1)); then
  cat "${temp2}"
else
  cat "${temp1}"
fi