I am working on a home automation project which uses a mini-pc to perform certain function on scheduled time. I don’t want the PC to run all time to save electicity cost and be efficient in usage. So I started looking for option to power ON the PC just before doing the job and power OFF when completing the job.


  • Beelink GK Mini Mini PC which supports Auto Power On, Wake On LAN, PXE Boot, RTC Wake
  • Ubuntu 20.04.3 LTS

Automatic Power ON

The PC I have supports RTC wake on S5(powered off) state. To configure, we have to enter into BIOS/boot setting by pressing F7 during PC startup.

Once entered in to blue screen go to Boot –> S5 RTC Wake Settings Configure the time to wake up.

This worked like charm with the windows operating system which comes with the PC. But did not work with Ubuntu. Also this BIOS level wake is not going to help me if I want to wake up multiple times in a day.

Time to look at RTC wake up alarm at the operating system level. Also it is allowing one wakeup alarm at a time. Even worse, the alarm gets removed if the PC turned on manually before the wake up time. This is a real challenge.

Thinking of…

  • intercept shutdown to call a script so we can place the alarm every time.
  • write the script to calculate the time to set alarm based on the list of wake up times configured in a file. Then finally call perform the shutdown.

What if someone unplug without proper shutdown? Change BIOS setting to automatic OS startup as soon as power connected. Configure to suspend the OS when idle for 2 hour. (What idle means? No user interaction or no CPU intensive programs). I dont want to suspend when my automation running. Maybe create script which justs sets the wakeup alarm. Then run it in background every 1 hour. Maybe the simple answer is - ask family not to touch this automation PC. Schedule to poweroff and poweron by planning the automation well.


#List the alarm times in the orderly fashion and spearated with space
echo $alarms
for alarm in ${alarms[@]}; do

  DESIRED=$((`date +%s -d "$alarm"`))
  NOW=$((`date +%s`))
  if [ $DESIRED -gt $NOW ]; then
    echo $alarm - $ALARM_TIME
if [ -z $ALARM_TIME ]; then
	echo "No more alarm for today"

echo "Next wakeup" $ALARM_TIME

# Argument check
if [ $# -lt 1 ]; then
    echo "Usage: set_next_wakeup HH:MM <HH:MM>"

# Check whether specified time today or tomorrow
TODAY=`date '+%F'`
ALARM_TIME_MS=`date -d "${TODAY} ${ALARM_TIME}" +%s`
NOW=$((`date +%s`))
echo "NOW:" $NOW
if [ ${ALARM_TIME_MS} -lt $NOW ]; then
    echo "Alarm time ${ALARM_TIME} - ${ALARM_TIME_MS} for today alread passed. Choosing tomorrow"
    ALARM_TIME_MS=$((${ALARM_TIME_MS} + 24*60*60))
#sudo echo 0 > /sys/class/rtc/rtc0/wakealarm
cat /sys/class/rtc/rtc0/wakealarm
#tail /var/log/kern.log
sleep 2
#tail /var/log/kern.log
#sudo echo ${ALARM_TIME_MS} > /sys/class/rtc/rtc0/wakealarm
sudo rtcwake --mode no --time ${ALARM_TIME_MS}
cat /sys/class/rtc/rtc0/wakealarm
#tail /var/log/kern.log
sleep 2
#tail /var/log/kern.log

Example run/test

automation@automation-mini:~$ sudo ./set_next_wakeup 14:15 15:30 18:20
14:15 15:30 18:20
15:30 - 15:30
Next wakeup 15:30
ALARM_DATE_TIME: Sat 15 Jan 2022 03:30:00 PM EST
ALARM_TIME_MS: 1642278600
NOW: 1642277255
rtcwake: assuming RTC uses UTC ...
rtcwake: wakeup using /dev/rtc0 at Sat Jan 15 20:30:00 2022
automation@automation-mini:~$ date
Sat 15 Jan 2022 03:07:46 PM EST

To suspend…

sudo systemctl suspend

To configure suspend in crontab use sudo crontab -e

29 5 * * * systemctl suspend

Automatic Power OFF

You may shedule your automation task as regular user using crontab -e command. But schedule the shutdown as root user by configuring at sudo crontab -e. Find the exact location of shutdown using which shutdown and use it as below.

43 5 * * * /usr/sbin/shutdown -h now

Shutting down or stopping power supply could lose the wakeup alarm. So we have to suspend instead of poweroff.

Automatic suspend

You may shedule your automation task as regular user using crontab -e command. But schedule the suspend as root user by configuring at sudo crontab -e.

43 5 * * * sudo systemctl suspend

Challenges and Learning

To power off, I tried to use shutdown and poweroff commands in crontab.

43 5 * * * shutdown -h now

It did not work as I hoped.

How I started troubleshooting?

I can confirm the cron ran as scheduled by greping syslog

$ grep CRON /var/log/syslog
Jan  9 05:43:01 automation-mini CRON[2482]: (root) CMD (/sbin/shutdown -h now)
Jan  9 05:43:41 automation-mini cron[597]: (CRON) INFO (pidfile fd = 3)
Jan  9 05:43:41 automation-mini cron[597]: (CRON) INFO (Running @reboot jobs)

Just writing the stdout and stderr logs to file.

43 5 * * * shutdown -h now > /home/automation/shutdown.log 2>&1

I saw logs like…

/bin/sh: 1: shutdown: not found

To overcome this, I found the exact location of shutdown command which shutdown and used it.

43 5 * * * /usr/sbin/shutdown -h now > /home/automation/shutdown.log 2>&1

Which takes me to next challenge.

Also I created a sh file and place the shutdown as part of the job.

echo "Automation begins" >> /home/automation/automation.log 2>&1
echo "Automation ends" >> /home/automation/automation.log 2>&1
/usr/sbin/shutdown -h now >> /home/automation/automation.log 2>&1

From the log file…

Failed to set wall message, ignoring: Interactive authentication required.
Failed to power off system via logind: Interactive authentication required.
Failed to open initctl fifo: Permission denied
Failed to talk to init daemon.

It did not work, because I used crontab -e to configure it. So the command end up asking for password interactively in the background. You know I will not know.

Kernel settings

If we set rtcwake but not really suspend, we will see below annoying message in /var/log/kern.log

Jan 15 14:05:32 automation-mini kernel: [ 6944.268665] hpet: Lost 7161 RTC interrupts
Jan 15 14:05:33 automation-mini kernel: [ 6945.448545] hpet: Lost 7161 RTC interrupts
Jan 15 14:05:35 automation-mini kernel: [ 6946.631170] hpet: Lost 7160 RTC interrupts

Also CPU gets stuck for unknown reasons

Message from syslogd@automation-mini at Jan 14 22:50:24 ...
 kernel:[ 6108.100772] watchdog: BUG: soft lockup - CPU#1 stuck for 22s! [echo:4468]

To resolve, add hpet=disable to GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub

GRUB_CMDLINE_LINUX_DEFAULT="quiet hpet=disable"

Once updated execute sudo update-grub

Other options not tried or used or gained confidence to share

  • specifying username in crontab instead of using sudo crontab -e -
      43 5 * * * root /usr/sbin/shutdown -h now > /home/automation/shutdown.log 2>&1

    Maybe this require giving more permission to the automation user (wheel/sudoers)