---
layout: post
title: "Use systemd as a cron replacement"
date: 2013-06-09 18:22
comments: true
categories: 
- systemd
- timer
- linux
- cron
description: "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](http://www.freedesktop.org/software/systemd/man/systemd.exec.html) 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](http://superuser.com/questions/402781/what-is-run-parts-in-etc-crontab-and-how-do-i-use-it)
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](http://www.freedesktop.org/software/systemd/man/systemd.time.html) in the timer file.
example:

``` ini send-bill.timer
[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](https://bbs.archlinux.org/viewtopic.php?id=162989) 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 %}