function permission2digit ({
  read,
  write,
  exec
}) {
  const arr = [
    read ? '1' : '0',
    write ? '1' : '0',
    exec ? '1' : '0'
  ]
  return Number('0b' + arr.join('')).toString()
}
permission to digit
@param {object}  {read: Boolean, write: Boolean, exec: Boolean}
@return {string}