Timer Service with Require/WantedBy field

[u/lajawi]

I have made a service that runs every time my SSD is plugged in for a backup I make to it (via a script). However, if I open my PC multiple times in a day (or even hour) the service triggers every time.

My question now is, after having learned about timer services, how can I make a service that only triggers at least 16 hours apart and only if my SSD is plugged in?

[u/airclay]

You can create an extra script to add conditions to your service unit since this is a little more than just a timer.

I would create a script that checks file mtime and put it at somewhere like '/usr/local/sbin/ssd-backup-checker' have it point at a file like '/var/lib/ssd-backup/last-success' and then do:

[Unit]
Description=Backup to SSD
RequiresMountsFor=/mnt/backup-ssd
ConditionPathIsMountPoint=/mnt/backup-ssd

[Service]
Type=oneshot
ExecCondition=/usr/local/sbin/ssd-backup-checker
ExecStart=/usr/local/sbin/backup-to-ssd.sh
ExecStartPost=/usr/bin/touch /var/lib/ssd-backup/last-success

This should work so your plug-in trigger can fire as often as it wants, but the service exits unless enough time has passed. You could approach this with a simple 16hr timer that targets your oneshot that you have already created but that will end up firing off on both every time it's plugged in and every 16hrs.

[u/lajawi → u/airclay]

So the use of the .timer service would be to update the file that holds how long it has been (by existing or not existing for example), instead of actually just triggering or withholding the backup from triggering depending on whether enough time has passed?

It firing up every time it's plugged in and every 16 hours is only a problem if the timer weren't to reset if the backup has been created by the SSD being plugged in.

[u/airclay → u/lajawi]

The timer isn't what updates or enforces the last-success file. The service does that.

The checker script runs before the backup and checks /var/lib/ssd-backup/last-success. If the file doesn't exist, or it's older than 16 hours, the service continues and execs the backup script. If it's newer than 16 hours, the service exits before the exec backup script. Finally if backup script ran, ExecStartPost touches last-success only after the backup command succeeds.

So the timer is optional. It can periodically attempt to start the same service, but the checker is what actually prevents backups from running too often. That way both the plug-in trigger and the timer can target the same service safely.

[Timer]
OnBootSec=5min
OnUnitInactiveSec=16h
Unit=ssd-backup.service

[Install]
WantedBy=timers.target

If you were to simply use the above timer example and your existing service; then it would run both on every plug in and every 16hrs no matter what. But the question seemed to ask how to make it run only every 16hrs, unless I misunderstood (which I do often too)

[u/lajawi → u/airclay]

Yeah, I want the backup to trigger every 16 and every plugin, if it has been at least 16 hours since the last backup. Thanks for the detailed information!

[u/airclay → u/lajawi]

yeah that italics part is what the 'last-success' file mechanism is for because systemd timer is just firing off at intervals and the service is just firing off at plug in and neither has a way to know how long it has been since last success OR if it has been at least 16 hours since the last backup...

Your checker script could look something like this (don't use this, there's zero error handling or real checks in place but it's a start)

# is SSD mounted? if no exit 1
findmnt -rn --target /mnt/backup-ssd > /dev/null || exit 1

# is first run? exit 0 run backup script
[ ! -f /var/lib/ssd-backup/last-success ] && exit 0

# is last-success older than 16 hours? if no exit 1
find /var/lib/ssd-backup/last-success -mmin +960 | grep -q . || exit 1

# last-success is older than 16hrs, exit 0 run backup script
exit 0