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<!DOCTYPE html>
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<head>
<title>Cronjobs</title>
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<header><h1>Using Cronjobs to run scheduled tasks</h1></header>
<nav></nav>
<main>
<p>
Cron is a service that lets you run scheduled tasks. These tasks are called <strong> cronjobs. </strong> If you have already followed the initial course you will have already used cron when you set up certbot.
</p>
<h2> What tasks would I want to schedule? </h2>
<p>
You can schedule anything! Some examples of what you might have done already include:
<ul>
<li> <code> updatedb </code> to update your <code> locate </code> database </li>
<li> <code> certbot </code> to update renewing of your https certs </li>
</ul>
Some tasks that you might <em>want</em> to schedule may include:
<ul>
<li> Package updates - if you really just want to leave your server alone you can automated updating packages on your server </li>
<li> Backups - you may want to backup certain files every day and some every week, this is possible with cron </li>
</ul>
<p>
And many more, anything you can do can be turned into a cronjob.
</p>
<h2> How do I schedule things? </h2>
<p>
You can schedule tasks in the following ways:
<ol>
<li> Your users crontab </li>
<li> System wide cron directories </li>
</ol>
<h3> Your user crontab </h3>
<p>
This the preferred method for personal tasks and scripts, it's also the easiest to get started with. Run the command <code> crontab -e </code> to access your users crontab
</p>
<aside>
<p>
These crontabs are located at <code> /var/spool/cron/crontabs/ </code> If you wanted to you can edit these files instead of running <code> crontab -e </code>
</p>
</aside>
<p>
Once you have figured out the command you want to run you need to figure out how often you want to run it and when. I am going to schedule my system updates once a week on at 3:30 AM on a Monday.
</p>
<p>
We now have to convert this time (Every Monday at 3:30 AM) into a cron time. Cron uses a simple but effective way of scheduling when to run things.
</p>
<p>
Crontab expressions look like this <code> * * * * * command-to-run </code>
<pre>
.---------------- minute (0 - 59)
| .------------- hour (0 - 23)
| | .---------- day of month (1 - 31)
| | | .------- month (1 - 12
| | | | .---- day of week (0 - 6)
| | | | |
* * * * *
</pre>
<br>
<aside>
<p> Run the command <code> cat /etc/crontab </code> to see something similar to the above </p>
</aside>
<p>
So for our Monday at 3:30AM job we would do the following:
<pre>
<code> 30 3 * * 1 apt-get -y update && apt-get -y dist-upgrade </code>
</pre>
<br>
<aside class=callout>
<p>
For day of the week field (the last field), the week starts on a Sunday, so Monday is <code> 1 </code>
</p>
</aside>
<br>
<aside>
<p>
The website <a href="crontab.guru">https://crontab.guru</a> is insanely helpful for figuring out specific cron timings
</p>
</aside>
<p>
Lets add another Job, our backup job (for the purposes of this our backup command is just called <code> backup </code> We want to run <code> backup </code> Every evening at 11PM, once we work out the timings for this we can add the to the same file as the above by running <code> crontab -e </code> This would mean our full crontab would look like this:
<pre>
<code> 30 3 * * 1 apt-get -y update && apt-get -y dist-upgrade </code>
<code> 0 23 * * * backup </code>
</pre>
<h3> System wide cron directories </h3>
<p>
Run the command <code> ls /etc/cron* </code> you should see a list of directories and there contents. The directories should be something like the below
<ul>
<li> /etc/cron.d <em> This is a crontab like the ones that you create with </em> <code> crontab -e </code> </li>
<li> /etc/cron.hourly </li>
<li> /etc/cron.daily </li>
<li> /etc/cron.weekly </li>
<li> /etc/cron.monthly </li>
</ul>
<p>
The directories cron.{hourly,daily,weekly,monthly} are where you can put <strong> scripts </strong> to run at those times. You don't put normal cron entries here. I prefer to use these directories for system wide jobs that don't relate to an individual user. For example If we wanted to put our backup job in <code> /etc/cron.daily </code> we would do the following:
<ol>
<li> Create and edit the file <code> /etc/cron.daily/backup/ </code>:
<ul>
<li> <code> vim /etc/cron.daily/backup </code> </li>
</ul>
<li> Add the backup command in there, we can also add some logging because it's a full script. The full thing will look like this: </li>
<p>
<pre>
#!/bin/sh
# the logger command generates system logs
logger "starting backup daily job"
# backup is hypothetical backup command
backup
logger "finished backing up"
</pre>
</p>
</li>
<li> Save that file </li>
<li> Make that executable: </li>
<ul>
<li> <code> chmod +x /etc/cron.daily/backup </code> </li>
</ul>
</ol>
<p>
It's that easy, no messing around with cron timers! A downside of this is that you can't choose when these run, most of the time they will run at ~6:30am every hour,day,week,month.
</p>
<p>
That's it! There is not much more to know about cron. Hopefully you now know the appropriate cron method and have the ability to correctly schedule your tasks.
</p>
<h2> Contribution </h2>
<p>
This article was written by Mark McNally
</p>
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<li>
I am a (internet) LandChad, my website is <a href="https://mark.mcnally.je">here</a>
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<li>
Contact me about this article or just because <a href="mailto:mark@mcnally.je">here</a>
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I have a <a href="https://www.youtube.com/channel/UCMiInY8BhSUtCarO6uu6i_g">youtube channel</a> where I may post some LandChad related content as well as other content that you will hopefully find interesting
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