Run systemd timer periodically but not on boot

Question

I would like to run a script every 30 minutes on my RbPi, but not immediately after boot.
Example: If I boot my RbPi at 15:15, the script should run at 15:30 not before.
Setup: My RbPi has no internet connection to sync its time, it relies on an external RTC. To sync the time I followed this post.

What I did so far:
I have the following two files, generated using
sudo systemctl edit --force --full test.service and
sudo systemctl edit --force --full test.timer

# test.service
[Unit]
Description=test

[Service]
Type=simple
ExecStart=echo "TEST"

# test.timer
[Unit]
Description=test

[Timer]
OnCalendar=*:0/30
Persistent=false

[Install]
WantedBy=default.target

Unfortunately this dummy script runs immediately during the boot sequence as shown by journalctl -r:

...

Oct 21 14:11:43 rbpi0 kernel: Bluetooth: BNEP (Ethernet Emulation) ver 1.3
Oct 21 14:11:43 rbpi0 systemd[1]: Finished Online ext4 Metadata Check for All Filesystems.
Oct 21 14:11:43 rbpi0 systemd[1]: e2scrub_all.service: Succeeded.
Oct 21 14:11:43 rbpi0 systemd[1]: Starting Daily man-db regeneration...
Oct 21 14:11:43 rbpi0 systemd-udevd[128]: Using default interface naming scheme 'v247'.
Oct 21 14:11:43 rbpi0 bluetoothd[443]: Starting SDP server
Oct 21 14:11:43 rbpi0 systemd[1]: Starting Rotate log files...
Oct 21 14:11:43 rbpi0 bluetoothd[443]: Bluetooth daemon 5.55
Oct 21 14:11:43 rbpi0 echo[455]: TEST
Oct 21 14:11:43 rbpi0 systemd[1]: Starting test...
Oct 21 14:11:42 rbpi0 systemd[1]: Starting Online ext4 Metadata Check for All Filesystems...
Oct 21 14:11:42 rbpi0 kernel: rtc-rv3028 1-0052: setting system clock to 2023-10-21T14:11:42 UTC (1697897502)

How can I avoid my script to run during the boot sequence but run only at *:30?

Answer 1

Systemd scripts are run asynchronously as root before boot and is not suitable.

Linux has a tool cron to run tasks at a given time.

It can run after boot, yearly, monthly, weekly, daily, hourly etc. and can run as root or as a user.

See man 5 crontab

I use to turn the LED on my Pi on/off overnight and it is used by the OS to perform tasks e.g. monthly log rotation.

Answer 2

I'd probably modify the script to check how long since boot. Something like this (Bash syntax):

up_sec=$(sed 's/\..*//' /proc/uptime)
if ! (( up_sec > 15 * 60 )); then
    echo "Not up long enough yet ($up_sec), skipping"
    exit 0
fi

Answer 3

The Linux at command allows you to run something at a particular time. The time can be specified in different ways.

For the example in the question, run this command at boot-up:

at -f /path/scriptname now + 15 minutes

See the man page for further details.

Answer 4

Eureka!

The issue was that i dindn't initialized the time correctly. Yes I followed this post but I used a script that wrapped the /sbin/hwclock --hctosys --utc --noadjfile command, delaying it in time. This let the boot sequence to start already creating a mess with the OnCalendar= entry (as described in the man page).

I was enough to add a service exactly as this, no need to change anything else.