الدرس 16 · Networking, scripting and internals

Scheduling jobs: cron and timers

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A scheduler runs a command for you at set times, so a backup or a clean-up happens every night without anyone typing it. Follow one job, /opt/backup.sh at 02:30, through both Linux schedulers: a crontab line whose five time fields the cron daemon checks against the clock every minute, and a systemd timer that starts a matching service, logs to the journal and can catch up a run the machine missed while it was off.

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01When does the crontab line 30 2 * * * /opt/backup.sh run?

The fields are minute, hour, day of month, month, day of week. Minute 30 and hour 2, with * (any value) for the other three, means 02:30 every day.

02It is 02:29. Why does the cron daemon not run the job yet?

The cron daemon wakes once a minute and runs a line only when every time field matches the current time. At 02:29 the minute does not match; one minute later all five do.

03A cron job prints "backup done". Where does that text go by default?

cron collects a job's output and mails it to the owner of the crontab, or to the address in MAILTO. On a machine without mail set up, redirect it to a file instead, for example >> /home/ana/backup.log 2>&1.

04backup.timer has OnCalendar=--* 02:30:00 and no Unit= line. Which unit does it start?

A timer starts the unit with the same name and a different ending unless Unit= says otherwise, so backup.timer starts backup.service. Its output goes to the journal: journalctl -u backup.service.

05The machine is switched off from midnight to 08:15. What happens to the 02:30 run?

cron assumes the machine is always on, so a missed minute is simply missed (anacron exists for that). With Persistent=true, systemd stores the last run time on disk and starts the missed run as soon as the timer is active again.

المصطلحات الأساسية

المسرد

Scheduler: A program that runs commands for you at set times, such as cron or systemd timers.
Daemon: A program that keeps running in the background with no terminal, waiting for work, such as the cron daemon.
Crontab: Your table of cron jobs: one line per job, five time fields and a command. Edit it with crontab -e.
Time field: One of the five numbers in a crontab line: minute, hour, day of month, month, day of week. * means any value.
systemd timer: A .timer unit file that tells systemd when to start another unit, usually the .service with the same name.
OnCalendar=: The timer setting that names a calendar time, such as *-*-* 02:30:00 for 02:30 every day.
Journal: systemd's log. A service's output is stored there; read it with journalctl -u <unit>.
Persistent=true: A timer setting that saves the last run time on disk, so a run missed while the machine was off happens after boot.

تفاصيل إضافية

الملاحظات والمصادر

On Debian and Ubuntu the cron daemon is called cron; on Fedora and RHEL (cronie) it is crond. Both read crontab lines the same way.
If you restrict both day of month and day of week (neither is *), cron runs the job when either one matches, not only when both do.
In a crontab line a % sign means a new line, so write \% inside a command such as date +\%F.
Both schedulers run /opt/backup.sh directly, so the file needs its execute bit: sudo chmod +x /opt/backup.sh. Without it cron mails "/bin/sh: /opt/backup.sh: Permission denied" and systemd logs status=203/EXEC.
A system service runs as root unless its [Service] section has a User= line. The cron job runs as the owner of the crontab.

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