{"id":6401,"date":"2024-07-15T03:00:00","date_gmt":"2024-07-15T07:00:00","guid":{"rendered":"https:\/\/www.both.org\/?p=6401"},"modified":"2024-07-10T20:55:50","modified_gmt":"2024-07-11T00:55:50","slug":"automate-tasks-with-crontab","status":"publish","type":"post","link":"https:\/\/www.both.org\/?p=6401","title":{"rendered":"Automate Raspberry Pi tasks with crontab"},"content":{"rendered":"
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\r\n \r\n <\/i>\r\n <\/a>\r\n <\/span>\r\n<\/div><\/div>\n

I have a Raspberry Pi at home that I use to automate several things. For example, my printer is old enough that it doesn\u2019t support networking, so it\u2019s connected to the Raspberry Pi, which shares the printer to my home network using CUPS. I also use the Raspberry Pi as an at-home web server to work on websites, and as a \u201cfile server\u201d of sorts by using SFTP from my file manager.<\/p>\n\n\n\n

This became somewhat less useful last year when my Internet provider \u201cupgraded\u201d the home WiFi router. The new router has an added security feature where it disconnects any device from the network after about two or three days. Without a network connection, I can\u2019t print, backup files, or work on web pages.<\/p>\n\n\n\n

Restarting the Raspberry Pi would put it back on the network. So I needed to find a way to restart the Raspberry Pi every other day. And I needed it to happen automatically, and gracefully, when I wasn\u2019t likely to use it. That\u2019s the perfect use case for cron<\/strong> and crontab<\/strong>.<\/p>\n\n\n\n

Using \u2018cron\u2019 and \u2018crontab\u2019<\/h2>\n\n\n\n

On Unix-like systems, cron<\/code> is the system that executes tasks at specific times. The jobs that run, and when<\/em> they run, are controlled using a special file called the crontab<\/strong>. Every user can create their own crontab<\/strong> to run tasks according to their own schedule.<\/p>\n\n\n\n

The crontab<\/strong> file is arranged like a plain text \u201ctable\u201d with fields that define the time, day, month, and day of the week to run the job. You can find detailed information about the crontab<\/strong> file in the crontab<\/strong>(1) online manual page, with this command:<\/p>\n\n\n\n

$ man 5 crontab<\/code><\/pre>\n\n\n\n
The classic fields of the crontab<\/strong> file are:<\/p>\n\n\n\n
Field<\/th>Range of values<\/th><\/tr><\/thead>
1. Minute<\/td>0 to 59<\/td><\/tr>
2. Hour<\/td>0 to 23<\/td><\/tr>
3. Day of month<\/td>1 to 31<\/td><\/tr>
4. Month<\/td>1 to 12<\/td><\/tr>
5. Day of week<\/td>0 to 6<\/td><\/tr>
6. Command to run<\/td>(enter the full command here)<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
Modern crontab<\/strong> implementations, like the one on Linux, also support extensions to these fields. For example, the Vixie cron<\/code> implementation (by Paul Vixie) on most Linux distributions also accepts 0 to 7 as days of the week, where 0 and 7 are the same (Sunday). You can also use names for the months and days of the week, such as sun<\/code> for Sunday or jan<\/code> for January. But I like using numbers, so I\u2019ll use that here.<\/p>\n\n\n\n
For example, let\u2019s say you needed to run a tar<\/code> backup of a website, every Sunday at 6:00 AM. You can create this crontab<\/strong> entry:<\/p>\n\n\n\n
0 6 * * 0 \/bin\/tar czf $HOME\/website.tar.gz \/var\/www\/html<\/code><\/pre>\n\n\n\n
The first two fields indicate 0<\/code> for the minutes and 6<\/code> for the hour. The fifth field says 0<\/code> for the day of the week, which means \u201cSunday.\u201d The third and fourth fields are *<\/code> to mean \u201cany day of the month\u201d and \u201cany month.\u201d Together, these five fields will execute the tar<\/code> backup every Sunday at 6:00 AM.<\/p>\n\n\n\n
To enter the crontab<\/strong> file into the cron<\/code> system, you need to use the crontab<\/code> command. If your crontab<\/strong> file is called jobs<\/code>, then you would type this:<\/p>\n\n\n\n
$ crontab jobs<\/code><\/pre>\n\n\n\n
You can verify the contents of the crontab<\/strong> file with crontab -l<\/code> to list<\/em> (-l<\/code>) the contents of the file. You can also erase your crontab<\/strong> from the system with crontab -r<\/code> to remove<\/em> (-r<\/code>) the file.<\/p>\n\n\n\n
Scheduled reboot<\/h2>\n\n\n\n
In my case, I needed to reboot the Raspberry Pi before the router took it off the network. One way to do this is with the telinit<\/code> command, to change the \u201cSystem V\u201d run level. On Linux systems that run systemd, this executes the appropriate systemctl<\/code> command. But I learned on original Unix systems, so I prefer telinit<\/code>.<\/p>\n\n\n\n
To reboot every day at 7:00 AM, I started by creating this crontab<\/strong> file for the root<\/strong> user:<\/p>\n\n\n\n
0 7 * * * \/sbin\/telinit 6<\/code><\/pre>\n\n\n\n
The first two fields are 0<\/code> for the minutes and 7<\/code> for the hour, with *<\/code> for all of the other fields; this will run at 7:00 AM every day. I saved this file as \/root\/jobs<\/code> and I saved it into the system with this command as root<\/strong>:<\/p>\n\n\n\n
# crontab \/root\/jobs<\/code><\/pre>\n\n\n\n
But I didn\u2019t need to reboot every day; I only needed to reboot every other<\/em> day. And Vixie cron<\/code> has a neat syntax where you can specify \/<\/code> and then a number to create an alternate schedule. For example, 0-30\/2<\/code> for \u201cminutes\u201d would run a job every other minute: \u201c:00,\u201d \u201c:02,\u201d \u201c:04,\u201d \u2026 and so on until \u201c:26,\u201d \u201c:28,\u201d and \u201c:30.\u201d This runs on even minutes because \u201cminute\u201d can have values from 0 to 59, and zero is an even number.<\/p>\n\n\n\n
To reboot my Raspberry Pi every other<\/em> day, I updated the crontab<\/strong> entry to use *\/2<\/code> for \u201cday of the month.\u201d This runs the job every other day: 1, 3, 5, \u2026 until 27, 29, and 31. These are odd days because \u201cday of the month\u201d is a value between 1 and 31, and 1 is an odd number.<\/p>\n\n\n\n
0 7 *\/2 * * \/sbin\/telinit 6<\/code><\/pre>\n\n\n\n
With this entry, my Raspberry Pi rebooted itself at 7:00 AM every other<\/em> day of the month: June 1, June 3, June 5, \u2026 and so on for the rest of the month.<\/p>\n\n\n\n
Combining jobs<\/h2>\n\n\n\n
I figured as long as the Raspberry Pi would reboot every other day, I might as well apply updates at the same time. For that, I wanted to run the dnf<\/code> command to install any updates, then use telinit<\/code> to reboot the system.<\/p>\n\n\n\n
When you need to combine multiple tasks into a single crontab<\/strong> job, I recommend collecting the commands into a script. In my case, I created this Bash script called \/root\/bin\/update_reboot<\/code> to run dnf<\/code> and telinit<\/code>, plus keep a log of what it did:<\/p>\n\n\n\n
#!\/bin\/bash\n# apply updates, reboot anyway\n\nlogdir=\/root\/log\n[ -d $logdir ] || mkdir -p $logdir\n\ntoday=$(date '+%d')\n\ndnf -y update > $logdir\/dnf_update.$today.log\ntelinit 6<\/code><\/pre>\n\n\n\n
In this script, I wanted to keep a private log in \/root\/log<\/code> to capture the output from the dnf<\/code> command. The two lines that set the logdir<\/code> variable and run mkdir<\/code> ensure that my \/root\/log<\/code> directory is present, so I have a place to store logs.<\/p>\n\n\n\n
I use the $()<\/code> shell expansion to run date<\/code> to print today\u2019s day of the month as a zero-padded number, so I can use the date as part of the log file\u2019s name when I run the dnf<\/code> command to apply all updates. The last line reboots the system for me.<\/p>\n\n\n\n
Automating tasks made easy<\/h2>\n\n\n\n
While job automation might sound like something that only system administrators do, it\u2019s a feature that anyone<\/em> can use. If you need to do a task on a regular schedule, explore how you can use crontab<\/strong> to do the repetitive work for you.<\/p>\n","protected":false},"excerpt":{"rendered":"
If you need to do a task on a regular schedule, explore how you can use crontab to do the repetitive work for you.<\/p>\n","protected":false},"author":33,"featured_media":3868,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_lmt_disableupdate":"","_lmt_disable":"","footnotes":""},"categories":[5,487,89],"tags":[104,91,97],"class_list":["post-6401","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-linux","category-raspberry-pi","category-system-administration","tag-command-line","tag-linux","tag-sysadmin"],"modified_by":"David Both","_links":{"self":[{"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/6401","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/users\/33"}],"replies":[{"embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=6401"}],"version-history":[{"count":3,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/6401\/revisions"}],"predecessor-version":[{"id":6405,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/6401\/revisions\/6405"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/media\/3868"}],"wp:attachment":[{"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6401"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6401"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6401"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}