check ipv4 validity

bash_check_ipv4_validity.sh

# the argument must in a dot decimal ipv4 format

function is_valid_ipv4() {
  if [ -z "$1" ]; then
    return 0;
  fi

  IFS="."
  local isvalid=1
  local bytectr=0

  for byte in $1; do
    bytectr=$(( $bytectr + 1 ))

    # check if the 1st byte is greater than 0
    if [ ${bytectr} -eq 1 ] && [ ${byte} -le 0 ]; then
      isvalid=0
      break
    fi

    if [ ${byte} -lt 0 ] || [ ${byte} -gt 255 ]; then
      isvalid=0
      break
    fi
  done

  if [ ${bytectr} -ne 4 ]; then
    isvalid=0
  fi

  unset IFS

  return ${isvalid}
}