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@@ -1,7 +1,7 @@ <!DOCTYPE html> <html lang=en> <head> - <title>Cronjobs</title> + <title>Cronjobs – LandChad.net</title> <meta charset="utf-8"/> <link rel="shortcut icon" href="favicon.ico" type="image/x-icon" /> <link rel='stylesheet' type='text/css' href='style.css'> @@ -13,7 +13,7 @@ <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. + 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> @@ -31,73 +31,119 @@ And many more, anything you can do can be turned into a cronjob. </p> - <h2> How do I schedule things? </h2> + <h2>Basic Cronjobs</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> + 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> + Crontab expressions look like this <code> * * * * * command-to-run </code> + The five elements before the command tell when the command is supposed to be run automatically. <p> So for our Monday at 3:30AM job we would do the following: <p> - <pre> - .---------------- minute (0 - 59) + <pre><code> .---------------- minute (0 - 59) | .------------- hour (0 - 23) | | .---------- day of month (1 - 31) | | | .------- month (1 - 12 - | | | | .---- day of week (0 - 6) + | | | | .---- day of week (0 - 6) | | | | | * * * * * -30 3 * * 1 apt-get -y update && apt-get -y dist-upgrade - </pre> - <br> - - <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> Run the command <code> cat /etc/crontab </code> to see a basic expalnation on how to write crontab timings </p> - </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> - 30 3 * * 1 apt-get -y update && apt-get -y dist-upgrade - - 0 23 * * * backup - </pre> - - <h3> System wide cron directories </h3> +30 3 * * 1 apt -y update && apt -y dist-upgrade</code></pre> + + <h3>Some notes</h3> + <ul> + <li>On the day of the week option, Sunday is 0 and counting up from there, Saturday will be 6.</li> + <li><code>*</code> designates "everything". Our command above has a <code>*</code> in the day of month and month columns. This means it will run regardless of the day of the month or month.</li> + <li>The hour option uses 24 hour time. 3 = 3AM, while use 15 for 3PM.</li> + </ul> + + <h3>More examples</h3> + <p> + Let's 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>0 23 * * * backup</code></pre> + + <h3>Consecutive times</h3> + + <p> + Suppose we want a command to run every weekday. + We know we can put <code>1</code> (Monday), but we can also use <code>1-5</code> + to signify from day 1 (Monday) to day 5 (Friday). + </p> + + <pre><code>0 6 * * 1-5 echo "Wakey, wakey, wagie!" >> /home/wagie/alarm</code></pre> + + <p>The above <code>echo</code> command runs every Monday through Friday at 6:00AM.</p> + + <h3>Non-consecutive times</h3> + + <p> + We can also randomly specify non-consecutive arguments with a comma. + Suppose you have a script you want to run at the midday of the 1st, 15th, and 20th day of every month. + You can specify that my putting <code>1,15,20</code> for the day of the month argument: + </p> + + <pre><code>0 12 1,15,20 * * /usr/bin/pay_bills_script</code></pre> + + <h3>"Every X minutes/days/months"</h3> + + <p>We can also easily run a command very several minutes or months, without specifying the specific times: + </p> + + <pre><code>*/15 * * * * updatedb</code></pre> + + <p> + This cronjob will run the <code>updatedb</code> command every 15 minutes. + </p> + + <h3>Beware of this Rookie Mistake Though...</h3> + + <p> + Suppose you want to run a script once every other month. + You might be <em>tempted</em> write this: + </p> + + <pre><code>* * * */2 *</code></pre> + + <p> + That might <em>feel right</em>, but this script <em>will be running once every minute during that every other month</em>. + You should specify the first two arguments, because with <code>*</code> it will be running every minute and hour! + </p> + + <pre><code>0 0 1 */2 *</code></pre> + + <p>This makes the command run <em>only</em> at 0:00 (12:00AM) on the first day of every two months, which is what we really want.</p> + + <p> + Consult the website <a href="https://crontab.guru">crontab.guru</a> for an intuitive and interactive tester of cronjobs. + </p> + + <h2>User vs. Root Cronjobs</h2> + + <p> + It is important to note that user accounts all have different cronjobs. + If you have a user account <code>chad</code> and edit his crontab with <code>crontab -e</code>, + the commands you add will be run as the <code>chad</code> user, not <code>root</code> or anyone else. + </p> + + <p> + Bear in mind that if you need root access to run a particular command, + you will usually want to add it as root. + </p> + + <h2>System-wide cron directories</h2> + + <p> + <code>crontab -e</code> is the typical interface for adding cronjobs, but it's important to at least know that system-wide jobs are often stored in the file directory. + Some programs which need cronjobs will automatically install them in the following way. + </p> <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 @@ -110,60 +156,19 @@ </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. + 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. </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> - <h2> Contribution </h2> + <ul> + <li>Mark McNally -- <a href="https://mark.mcnally.je">website</a>, <a href="https://www.youtube.com/channel/UCMiInY8BhSUtCarO6uu6i_g">Youtube</a></li> + <li>Edits and examples by Luke</li> + </ul> - <p> - - Mark McNally -- <a href="https://mark.mcnally.je">website</a>, <a href="https://www.youtube.com/channel/UCMiInY8BhSUtCarO6uu6i_g">Youtube</a> - </p> - </main> <footer><a href="https://landchad.net">LandChad.net</a></br>Because Everyone should be an Internet LandChad.</br><li><a href="index.html"><img src="pix/chad.gif" alt="chad"></a></li><li><a href="rss.xml"><img src="pix/rss.svg" alt="RSS"></a></li><li><a href="pix/btc.png"><img src="pix/btc.svg" alt="BTC"></a></li><li><a href="pix/xmr.png"><img src="pix/xmr.svg" alt="XMR"></a></li><li><a href="https://github.com/lukesmithxyz/landchad"><img src="pix/git.svg" alt="Github"></a></footer> </body> </html> - - - - - - - - |