Can I run a script if the RTC is updated from NTP?

Magic Matt · October 6, 2020 at 6:30 PM

I would like to update an Arduino I have connected via I2C with the correct date and time once I'm sure the Pi has it. Since the Pi is getting the time via NTP, it seems the most obvious would be to have a script run when NTP is successful.

Is there a mechanism for this? I'm thinking systemd might be involved but not sure what target?

**Da Flex** · October 6, 2020 at 6:56 PM Official Post

You can activate "Wait for Network" at Settings -> LibreELEC -> Network and put your Arduino script into autostart.sh.

Magic Matt · October 6, 2020 at 8:19 PM

Wait for network completes before the NTP gets executed, and the NTP may update at any time, not only when you reboot.

**HiassofT** · October 6, 2020 at 9:15 PM

Create a oneshot systemd service and order it after time-sync.target.

so long,

Hias

**Da Flex** · October 6, 2020 at 11:56 PM Official Post

Or find the NTP update frequency and set an appropriate delay as first autostart.sh statement. The NTP update and the sync-to-Arduino doesn't have fire at the same time. You can also program a loop with such async timing.

Magic Matt · October 7, 2020 at 12:14 AM

The reason I want to grab the NTP is because I don't want to update the Arduino if the Date/Time is not valid (ie wrong because there was no Internet).

I've created this systemd entry and it seems to work, although I don't know if it still fires if the NTP gets updated after boot.

[Unit]

Description=Media Centre Control Panel NTP Updater

After=time-sync.target

[Service]

ExecStart=/storage/MediaCentre/NTPUpdate.sh

Type=Simple

[Install]

WantedBy=multi-user.target

**HiassofT** · October 7, 2020 at 9:46 AM

The service file looks fine, you should change the Type to "oneshot" though (simple is for programs continuously running in the background, eg if you want to do regular updates in a loop in your script).

For scripts that should be only run once after startup you should also set "RemainAfterExit=yes" in addition to "Type=oneshot".

so long,

Hias

Magic Matt · October 7, 2020 at 4:08 PM

The service seems to work, but systems is now adding a service that waits for time sync, which of there's no internet causes boot to hang indefinitely.

Time to rethink this.

**Da Flex** · October 7, 2020 at 5:08 PM Official Post

Time to try my approach. You're fixed on NTP too much. If you just send the RPi time to Arduino as a loop, it will be more stable in case of network / NTP errors.

**HiassofT** · October 7, 2020 at 8:12 PM

The hang is caused by multi-user.target getting an After= dependency on your service, in addition to the Wants=.

You can avoid that by setting DefaultDependencies=no and manually specifying Before, After and Conflicts. eg:

Code

DefaultDependencies=no
After=time-sync.target
Before=shutdown.target
Conflicts=shutdown.target

Then multi-user.target will only get the Wants= which pulls in your service but not the After= which delays multi-user.target.

so long,

Hias

Can I run a script if the RTC is updated from NTP?

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