Linux: cron and systemd timers

This guide assumes you’ve already completed the prerequisites: a configured store with a non-interactive passphrase, and the plakar command you want to schedule.

Open your crontab:

$ crontab -e

Add a line to run the backup every day at 02:00, sending all output to a log file:

0 2 * * * /usr/local/bin/plakar at "@mybackups" backup -check /var/www >> /var/log/plakar-backup.log 2>&1

A plain cron job does not run a backup that was missed while the machine was off. If that matters, which is often the case, use a systemd timer instead, which catches up on missed runs.

systemd timers are the recommended approach on modern Linux. They keep a log in the journal, can catch up on runs missed while the machine was off, and are managed with the same tooling as the rest of the system.

Create a service unit that performs the backup, /etc/systemd/system/plakar-backup.service:

[Unit]
Description=Plakar daily backup
Wants=network-online.target
After=network-online.target

[Service]
Type=oneshot
ExecStart=/usr/local/bin/plakar at @mybackups backup -check /var/www
# If you supply the passphrase via the environment instead of the store config:
# Environment=PLAKAR_PASSPHRASE=mysuperpassphrase

Then a timer unit with the same base name, /etc/systemd/system/plakar-backup.timer:

[Unit]
Description=Run the Plakar daily backup

[Timer]
OnCalendar=*-*-* 02:00:00
Persistent=true

[Install]
WantedBy=timers.target

Persistent=true runs the job as soon as the machine comes back up if a scheduled time was missed while it was off. Adjust OnCalendar to taste (daily, hourly, *-*-* 02,14:00:00, and so on).

Enable and start the timer:

$ sudo systemctl daemon-reload
$ sudo systemctl enable --now plakar-backup.timer

Inspect and verify it:

$ systemctl list-timers plakar-backup.timer   # see the next run time
$ journalctl -u plakar-backup.service         # read the backup logs
$ sudo systemctl start plakar-backup.service  # trigger a run immediately to test

The example above runs as root, which reads configuration from /root/.config/plakar. To run as a specific user instead, add User= and Group= to the [Service] section and make sure that user has its own store configuration. Alternatively, install the units under ~/.config/systemd/user/ and manage them with systemctl --user; user timers run only while that user has an active session unless lingering is enabled (loginctl enable-linger <user>).

On a laptop you usually don’t want a backup to fire while you’re on battery. With systemd, add ConditionACPower=true to the [Unit] section of plakar-backup.service. When you’re on battery at the scheduled time, that run is skipped.

A plain cron job has no equivalent built-in condition, so use a systemd timer instead if this matters for your setup.