Dear Victor
Yes, -r works fine but actually what I like to want is "20sec delayed" wake up.
Finally I found hwclock command solved this problem as following.
WRONG:
# shutdown -h now
...
# shutdown -h now
CORRECT:
# shutdown -h now
...
# hwclock -w
# echo 0 > /sys/class/rtc/rtc0/wakealarm
# echo +20 > /sys/class/rtc/rtc0/wakealarm
# shutdown -h now
Just in case, could you explain why such command is need?
Thank you for your kindness.
BR