09 Jun 2013

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:

$ 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.thalheim.io/downloads/timers.tar
$ tar -xvf timers.tar && rm timers.tar

[Unit]
Description=Hourly Timer

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

[Install]
WantedBy=basic.target

[Unit]
Description=Hourly Timer Target
StopWhenUnneeded=yes

[Unit]
Description=Daily Timer

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

[Install]
WantedBy=basic.target

[Unit]
Description=Daily Timer Target
StopWhenUnneeded=yes

[Unit]
Description=Weekly Timer

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

[Install]
WantedBy=basic.target

[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

[Unit]
Description=Update man-db

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/logrotate /etc/logrotate.conf

[Unit]
Description=Update man-db

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/mandb --quiet

[Unit]
Description=Update mlocate database

[Service]
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/usr/bin/updatedb

[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:

[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:

[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

[Unit]
Description=Run pkgstats

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

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

[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 &gt; /var/log/pacman-updates.log"
ExecStart=/usr/bin/pacman --sync --upgrades --downloadonly --noconfirm --dbpath=/var/lib/pacman/checkupdate