Crontab Basics

by Marc Plotz

September 24, 2009

Cron What?

Cron Tab (CRON TABle) is a very cool little thing that allows you to execute scripts at certain times and dates without actually having to be around to do the work. Think of it as setting an alarm to go off at certain intervals, and every time that alarm goes off a certain script is run on your website, for instance. Now I have heard Cron Tab being called many things, often incorrectly. In the circles I travel in we usually refer to it as a Cron-Job. Anyone who has watched a few episodes of Star-Trek will tell you that Cronos is the Klingon Homeworld, although it is actually spelled Q'onos. Another--perhaps less geeky crowd--would tell you that there is a branch of Quantum Physics called Quantum-ChronoDynamics, which deals with the micro- relationships of time and space in quantum forms. These same guys might tell you that the particle hypothesized to be the very particle of time is called a Cronos. Any way you see it, Cronos means time. The TAB part simply comes from table. Hence by working with Cron Tabs we are simply writing Time Tables? Yup! That's about it.

Now you may ask yourself why would we want something like this to happen. Let's look at a real world example that I came across myself. About a year ago I built quite a cool e- commerce website for a certain customer who happened to be an optometrist. His website was designed to actually sell contact lenses online. Today it is a very well-run successful business, even in these harsh economic times. One of the reasons it is doing well, not the only reason, but ONE OF THEM, is that the website actually knows when its clients last purchase is complete, and it thus reminds them a few days earlier to order their next batch already. How this happens is that the website knows how many contact lenses are in a box (the optometrist sets this amount in the administration section of the site). So, every morning at 6AM sharp, a Cron Job runs which effectively executes a PHP script on the website. The PHP script simply checks which clients have not yet been reminded to refill their orders, and out of those it works out how long they have had their contact lenses and if they are due to re-order. If they are due, an email is sent to the client reminding them to re- order.

How this is good for business is that we know humans are creatures of habit, and humans forget. In the busy lives we live today very few of us would actually worry about how many contact lenses we have left. Being reminded by the website, which conveniently gives the client a link to click in order to start the re-order process, makes it so simple to do that re-orders are a major part of the business now.

Now, Cron Tab is actually the little system that allows you to edit cron tables without having to remember gut- wrenching things like where all the cron files are stored. Let's have a look how you can make Cron Tab work for you.

Working With Cron Tab

Crontab has 3 options

1. -l: lists the options for the current table file for your user-id
2. -r: removes a table file
3. -e: lets you edit a table file

Furthermore a single entry on a cron table consists of just a single line telling what job to run, and when to run that job.

There are six unique fields that can make up a single line, and these fields must be separated by a space or a tab (whitespace).

Lines that begin with hash (#) are comments.

The six required fields are:

1. Minute of the hour to run (0-59)
2. Hour of the day to run (0-24)
3. Day of the month to run (0-31)
4. Month of the year to run (1-12)
5. Day of the weak to run (0-6) (where sunday = 0)
6. Command to execute

So here you can see that the first five fields set the time to run, and the final field is what the cron actually must do.

It is important to note that an entry in the first five columns can consist of:

• A number in the Range specified in brackets
• A range of numbers in the Range specified in brackets eg: 2-6
• A list--separated by comma's--containing individual numbers or a range of numbers in the list eg: 1-2, 5, 7-10, 14.
• An asterisk which represents all valid values eg: *

Something important to remember is that since fields are separated by spaces, you must not include spaces or tabs in your comma separated number lists.

Let's create a simple cron script.

$crontab -e
0-59    *    *    *    *    echo `date` "Hello World" >>$HOME/test.txt
$

The sixth field tells the date to be printed along with Hello World in test.txt and the first field tell this to happen every minute.

Setting the cron to run while watching the test.txt file in your browser, you will see when the browser refreshes every minute that an extra line has been appended to the end of the file. That is simply your first "Hello World" with Cron Tabs!

Until next time.
Marc Steven Plotz

About the Author

Marc Steven Plotz is a Senior Software Developer for a major South African web development company specializing in the development of enterprise-class web applications and rapid application development frameworks. He is also a technical writer for various developer websites focusing on open source topics like PHP, CSS, HTML and Javascript. He lives in Pretoria, South Africa, with his wife and two children.