Scheduling Tasks with Cron Jobs

Cron Jobs are used for scheduling tasks to run on the server. They're most commonly used for automating system maintenance or administration. However, they are also relevant to web application development. There are many situations when a web application may need certain tasks to run periodically. Today we are going to explore the fundamentals of Cron Jobs.

Definitions

First let's familiarize ourselves with the terms related to this subject.

"Cron" is a time-based job scheduler in Unix-like operating systems (Linux, FreeBSD, Mac OS etc...). And these jobs or tasks are referred to as "Cron Jobs".

There is a cron "daemon" that runs on these systems. A daemon is a program that runs in the background all the time, usually initiated by the system. This cron daemon is responsible for launching these cron jobs on schedule.

The schedule resides in a configuration file named "crontab". That's where all the tasks and their timers are listed.

Why Use Cron Jobs?

Server admins have been using cron jobs for a long time. But since the target audience of this article is web developers, let's look at a few use cases of cron jobs that are relevant in this area:

  • If you have a membership site, where accounts have expiration dates, you can schedule cron jobs to regularly deactivate or delete accounts that are past their expiration dates.

  • You can send out daily newsletter e-mails.

  • If you have summary tables (or materialized views) in your database, they can be regularly updated with a cron job. For example you may store every web page hit in a table, but another summary table may contain daily traffic summaries.

  • You can expire and erase cached data files in a certain interval.

  • You can auto-check your website content for broken links and have a report e-mailed to yourself regularly.

  • You can schedule long-running tasks to run from a command line script, rather than running it from a web script. Like encoding videos, or sending out mass e-mails.

  • You can even perform something as simple as fetching your most recent Tweets, to be cached in a text file.

Syntax

Here is a simple cron job:

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10 * * * * /usr/bin/php /www/virtual/username/cron.php > /dev/null 2>&1

There are two main parts:

  1. The first part is "10 * * * *". This is where we schedule the timer.

  2. The rest of the line is the command as it would run from the command line.

The command itself in this example has three parts:

  1. "/usr/bin/php". PHP scripts usually are not executable by themselves. Therefore we need to run it through the PHP parser.

  2. "/www/virtual/username/cron.php". This is just the path to the script.

  3. "> /dev/null 2>&1". This part is handling the output of the script. More on this later.

Timing Syntax

This is the first part of the cron job string, as mentioned above. It determines how often and when the cron job is going to run.

It consists of five parts:

  1. minute

  2. hour

  3. day of month

  4. month

  5. day of week

Here is an illustration:

Asterisk

Quite often, you will see an asterisk (*) instead of a number. This represents all possible numbers for that position. For example, asterisk in the minute position would make it run every minute.

We need to look at a few examples to fully understand this Syntax.

Examples:

This cron job will run every minute, all the time:

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* * * * * [command]

This cron job will run at minute zero, every hour (i.e. an hourly cron job):

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0 * * * * [command]

This is also an hourly cron job but run at minute 15 instead (i.e. 00:15, 01:15, 02:15 etc.):

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15 * * * * [command]

This will run once a day, at 2:30am:

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30 2 * * * [command]

This will run once a month, on the second day of the month at midnight (i.e. January 2nd 12:00am, February 2nd 12:00am etc.):

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0 0 2 * * [command]

This will run on Mondays, every hour (i.e. 24 times in one day, but only on Mondays):

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0 * * * 1 [command]

You can use multiple numbers separated by commas. This will run three times every hour, at minutes 0, 10 and 20:

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0,10,20 * * * * [command]

Division operator is also used. This will run 12 times per hour, i.e. every 5 minutes:

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*/5 * * * * [command]

Dash can be used to specify a range. This will run once every hour between 5:00am and 10:00am:

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0 5-10 * * * [command]

Also there is a special keyword that will let you run a cron job every time the server is rebooted:

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@reboot [command]

Setting Up and Managing Cron Jobs

There are a few different ways to create and manage your cron jobs.

Control Panels

Many web hosting companies provide control panels for their customers. If you are one of them, you might be able to find a section in your control panel to manage your cron jobs.

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