Get Last Character of Variable

main() {
  HI="12"
  last_char="${HI: -1}"

  echo "last_char='${last_char}'"
}

main "${@}" || exit 1

main() {
  last_char="${HI_I_DONT_EXIST: -1}"

  echo "last_char='${last_char}'"
}

main "${@}" || exit 1