Setting Up a Basic Cron Job in Linux

Tania/
cronlinuxdevops

Recently, I had to set up a scheduled task on a server, which is a little intimidating and scary the first time around, so here's a quick article about the steps for setting up a basic cron job. In this article, we'll create a simple PHP script to send out an email once per day.

Prerequisites

  • Basic command line familiarity.
  • Ability to SSH into a Linux server.

Goals

  • Learn how to set a basic scheduled task (cron job) in a Linux server environment.
  • Set a cronjob to send out an email every day at a specified time.

Sending an Email from PHP

First, we can make a simple PHP script to test. I'm going to make a file that sends a simple email to myself.

<?php
$to = 'me@example.com';
$message = 'This is the message.';
$subject = 'Insert Subject Here';
$headers = 'From: noreply@example.com' . "\r\n" .
           'Reply-To: me@example.com';

@mail($to, $subject, $message, $headers);
?>

If you're not familiar with the mail() function in PHP, here is the official documentation. I'm setting variables for who the e-mail should send to, where it should come from, the subject, a message, and headers. This is not the most up-to-date or secure way to send an email, but it's a script that works so I'm going to use it for testing. I'm just going to save this as cron.php.

The path to your public facing folder will depend on the Linux distro, but for the sake of this tutorial I'll put it in /var/www/html/crontest. Therefore, the full path to my script will be /var/www/html/crontest/cron.php. You can test this script directly from your browser by hitting the file. If your website is example.com, it would most likely be example.com/crontest/cron.php. If you load the file and get an email, you know the script works.

localhost is not set up to send out PHP mail, so this script needs to be run from a live Linux environment.

Basic Cron Syntax

There's a useful site called Crontab Generator that will calculate the setup of a cron job for you, but first we'll go through a brief overview of what a syntax will look like.

Example cron job

* * * * * /usr/bin/php /var/www/html/crontest/cron.php > /dev/null 2>&1

Parts of a cron command

There are four main parts to a cron command.

Timing Execute PHP Path to script Output
* * * * * /usr/bin/php /var/www/html/crontest/cron.php > /dev/null 2>&1
  • Timing - set the minutes, hours, days, months, and weekday settings (more below).
  • Execute - the cron job needs to call upon PHP to run, which is located at /usr/bin/php.
  • Path to script - the full path of the file you plan to run.
  • Output - (optional) you can write the output to a file or discard it - > /dev/null 2>&1 will discard.

In the above example, the cron job is set to send every minute - or more specifically, every minute of every hour of every day of every month, every day of the week. An asterisk is a wildcard that stands for "all".

  • Minutes - set the minutes, from 0-59
  • Hours - set the hour, from 0-24
  • Days - set the day, from 1-31
  • Months - set the month, from 1-12 (January - December)
  • Week - set the day of the week, from 0-6 (Sunday - Saturday)

Here are a few basic examples to get an idea of how it works.

Syntax Explanation
0 * * * * run once an hour (every hour at minute zero)
0 0 * * * run once a day (every day at midnight and minute zero)
0 0 1 * * run once a month (on the first day of every month at midnight and minute zero)
0 0 1 1 * run once a year (on the first day of the first month every year at midnight and minute zero)

There are all sorts of settings, such as for odd/even days, every 5 minutes, etc, and plenty of resources to find the exact scheduling you're looking for.

Setting Up a Cron Job

If cron is set up under root, you'll need to run sudo before your code.

The cron command is known as crontab. We'll set our cron job to send out an email once a day, so here will be the full command:

0 0 * * * /usr/bin/php /var/www/html/crontest/cron.php >/dev/null 2>&1

Once you SSH into the server, you can check if there are any current jobs running with the following command to list all crontabs:

crontab -l
crontab: no crontab for user

Now, we'll edit the crontab.

crontab -e

At this point, you'll most likely be in the vi editor, if you've never changed the default editor for your server. It can be a little confusing and scary the first time you use it, so here's what to do:

  1. press esc.
  2. press i (for "insert") to begin editing the file.
  3. paste the cron command in the file.
  4. press esc again to exit editing mode.
  5. type :wq to save (w - write) and exit (q - quit) the file.

Now your crontab is saved, and an email should send out once per day at midnight. You can set it to */5 * * * * for every 5 minutes for a faster test.

You can also put multiple cron jobs in the file, just put each command on a separate line and they will all run.

Conclusion

There are more tutorials that will go into deeper explanation of cron jobs, such as this one from Envato Tuts, but this is a simple article to just get familiar with the concept.

I usually like to get at least one article per month, but I was away from computers on vacation in Sweden, so I haven't had a chance to write anything lately. Don't worry, I still have plenty of ideas for guides and tutorials in the works. Meanwhile, here's a picture from Sweden!

sweden