blog/source/_posts/2013-06-09-use-systemd-as-a-cron-replacement.markdown

layout	title	date	comments	categories	description
post	Use systemd as a cron replacement	2013-06-09 18:22	true	systemd linux cron	HOWTO: Replace cron with systemd

Since systemd 197 timer units support calendar time events, which makes systemd a full cron replacement. Why one would replace the good old cron? Well, because systemd is good at executing stuff and monitor its state!

• with the help of journalctl you get last status and logging output, which is a great thing to debug failing jobs:

$ systemctl status reflector-update.service
reflector-update.service - "Update pacman's mirrorlist using reflector"
   Loaded: loaded
(/etc/systemd/system/timer-weekly.target.wants/reflector-update.service)
   Active: inactive (dead)

Jun 09 17:58:30 higgsboson reflector[30109]: rating http://www.gtlib.gatech.edu/pub/archlinux/
Jun 09 17:58:30 higgsboson reflector[30109]: rating rsync://rsync.gtlib.gatech.edu/archlinux/
Jun 09 17:58:30 higgsboson reflector[30109]: rating http://lug.mtu.edu/archlinux/
Jun 09 17:58:30 higgsboson reflector[30109]: Server Rate       Time
...

• there are a lot of useful systemd unit options like IOSchedulingPriority, Nice or JobTimeoutSec
• it is possible to let depend units on other services, like mounting the nfs host before starting the mysql-backup.service or depending on the network.target.

So let's get it started. The first thing you might want to do, is to replace the default scripts located in the runparts directories /etc/cron.{daily,hourly,monthly,weekly}.

On my distribution (archlinux) these are logrotate, man-db, shadow and updatedb: For convenience I created a structure like /etc/cron.*:

mkdir /etc/systemd/system/timer-{hourly,daily,weekly}.target.wants

and added the following timer.

cd /etc/systemd/system
wget https://blog.higgsboson.tk/downloads/timers.tar
tar -xvf timers.tar && rm timers.tar

{% include_code /etc/systemd/system/timer-hourly.timer lang:ini cron-replacement/timer-hourly.timer %} {% include_code /etc/systemd/system/timer-hourly.target lang:ini cron-replacement/timer-hourly.target %} {% include_code /etc/systemd/system/timer-daily.timer lang:ini cron-replacement/timer-daily.timer %} {% include_code /etc/systemd/system/timer-daily.target lang:ini cron-replacement/timer-daily.target %} {% include_code /etc/systemd/system/timer-weekly.timer lang:ini cron-replacement/timer-weekly.timer %} {% include_code /etc/systemd/system/timer-weekly.target lang:ini cron-replacement/timer-weekly.target %}

... and enable them:

systemctl enable timer-hourly.timer
systemctl enable timer-daily.timer
systemctl enable timer-weekly.timer

These directories work like their cron equivalents, each service file located in such a directory will be executed at the given time.

Now move on to the service files. If you're not running Arch, the paths might be different on your system.

cd /etc/systemd/system
wget https://blog.higgsboson.tk/downloads/services.tar
tar -xvf services.tar && rm services.tar

{% include_code /etc/systemd/system/timer-daily.target.wants/logrotate.service lang:ini cron-replacement/logrotate.service %} {% include_code /etc/systemd/system/timer-daily.target.wants/man-db-update.service lang:ini cron-replacement/man-db-update.service %} {% include_code /etc/systemd/system/timer-daily.target.wants/mlocate-update.service lang:ini cron-replacement/mlocate-update.service %} {% include_code /etc/systemd/system/timer-daily.target.wants/verify-shadow.service lang:ini cron-replacement/verify-shadow.service %}

At last but not least you can disable cron:

systemctl stop cronie && systemctl disable cronie

If you want to execute at a special calendar events for example "every first day in a month" use the "OnCalendar=" option in the timer file. example:

[Unit]
Description=Daily Timer

[Timer]
OnCalendar=*-*-1 0:0:O
Unit=send-bill.target

[Install]
WantedBy=basic.target

That's all for the moment. Have a good time using the power of systemd!

Below some service files, I use:

{% include_code /etc/systemd/system/timer-weekly.target.wants/reflector-update.service lang:ini cron-replacement/reflector-update.service %} {% include_code /etc/systemd/system/timer-weekly.target.wants/pkgstats.service lang:ini cron-replacement/pkgstats.service %}

See this link for details about my shell-based pacman notifier {% include_code /etc/systemd/system/timer-daily.target.wants/pacman-update.service lang:ini cron-replacement/pacman-update.service %}