Use Systemd as a Cron Replacement

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:
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$ 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
/etc/systemd/system/timer-hourly.timer (timer-hourly.timer) download
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[Unit]
Description=Hourly Timer

[Timer]
OnBootSec=5min
OnUnitActiveSec=1h
Unit=timer-hourly.target

[Install]
WantedBy=basic.target
/etc/systemd/system/timer-hourly.target (timer-hourly.target) download
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[Unit]
Description=Hourly Timer Target
StopWhenUnneeded=yes
/etc/systemd/system/timer-daily.timer (timer-daily.timer) download
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[Unit]
Description=Daily Timer

[Timer]
OnBootSec=10min
OnUnitActiveSec=1d
Unit=timer-daily.target

[Install]
WantedBy=basic.target
/etc/systemd/system/timer-daily.target (timer-daily.target) download
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[Unit]
Description=Daily Timer Target
StopWhenUnneeded=yes
/etc/systemd/system/timer-weekly.timer (timer-weekly.timer) download
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[Unit]
Description=Weekly Timer

[Timer]
OnBootSec=15min
OnUnitActiveSec=1w
Unit=timer-weekly.target

[Install]
WantedBy=basic.target
/etc/systemd/system/timer-weekly.target (timer-weekly.target) download
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[Unit]
Description=Weekly Timer Target
StopWhenUnneeded=yes

… 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
/etc/systemd/system/timer-daily.target.wants/logrotate.service (logrotate.service) download
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[Unit]
Description=Update man-db

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/logrotate /etc/logrotate.conf
/etc/systemd/system/timer-daily.target.wants/man-db-update.service (man-db-update.service) download
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[Unit]
Description=Update man-db

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/mandb --quiet
/etc/systemd/system/timer-daily.target.wants/mlocate-update.service (mlocate-update.service) download
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[Unit]
Description=Update mlocate database

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/updatedb
/etc/systemd/system/timer-daily.target.wants/verify-shadow.service (verify-shadow.service) download
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[Unit]
Description=Verify integrity of password and group files

[Service]
Type=oneshot
ExecStart=/usr/sbin/pwck -r
ExecStart=/usr/sbin/grpck -r

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:

send-bill.timer
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[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:

/etc/systemd/system/timer-weekly.target.wants/reflector-update.service (reflector-update.service) download
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[Unit]
Description="Update pacman's mirrorlist using reflector"

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
Type=oneshot
ExecStart=/usr/bin/reflector --verbose -l 5 --sort rate --save /etc/pacman.d/mirrorlist
/etc/systemd/system/timer-weekly.target.wants/pkgstats.service (pkgstats.service) download
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[Unit]
Description=Run pkgstats

[Service]
User=nobody
ExecStart=/usr/bin/pkgstats

See this link for details about my shell-based pacman notifier

/etc/systemd/system/timer-daily.target.wants/pacman-update.service (pacman-update.service) download
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[Unit]
Description=Update pacman's package cache

[Service]
Nice=19
Type=oneshot
IOSchedulingClass=2
IOSchedulingPriority=7
Environment=CHECKUPDATE_DB=/var/lib/pacman/checkupdate
ExecStartPre=/bin/sh -c "/usr/bin/checkupdates > /var/log/pacman-updates.log"
ExecStart=/usr/bin/pacman --sync --upgrades --downloadonly --noconfirm --dbpath=/var/lib/pacman/checkupdate
cron, linux, systemd, timer

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