Cron Jobs - The Good, The Bad, the Ugly

Cron jobs are done in Linux/UNIX using the crontab file.

Reading it (and writing a entry) feels a little like transcribing an ancient spell. The file is a list of entries. Each entry represents a time to execute and a command to run. The time to execute may be one time, or repeating.

This is an example crontab entry:

1 0 * * * echo -n "" > /www/apache/logs/error_log

Here's what those bits all mean:

1 2 3 4 5 /sbin/ifconfig
1 represents the minute (0 - 59) to execute. So 1 = :01 of the hour.
2 represents the hour (0 - 23) to execute, So 2 = 2:00 am here
3 represents the day of the month, so 3 here is the 3rd.
4 represents the month of th year, so 4 here is April
5 represents the day of the week, starting at 0. 5 is Friday

Putting it all together, we have:
"Run /sbin/ifconfig on Friday, April 3rd @ 2:01am"

An asterisk (*) means that the field can have any value. Our first example:

1 0 * * * echo -n "" > /www/apache/logs/error_log

clears the Apache log at 00:01 every day, every month. This can get ugly in a hurry:

*/5 * * * * /home/user/test.pl

Runs test.pl every 5 minutes.

*/5 * * * 1-5 /home/user/test.pl

Runs test.pl every 5 minutes, Monday-Friday only. Ugh. Imagine a really complex one:

00 09-18 * * 1-5 /home/user/check-db-status

Runs check-db-status at the top of the hour, from 9am to 6pm, every Monday-Friday.

Will this work? Yes. Is it easy to remember? No, not even close. crontab is a very powerful tool and there circumstances where you must use it:

• You need to run a script on a local machine


• You need to run a script that shouldn't be accessed from the internet (e.g. administrator scripts)


• You don't have access to the internet to use an online cron service

These are all great examples when crontab is necessary.

Well, thankfully in the 21st Century, we can finally get away from this archaic 1979 syntax designation that a handful of people can repeat off the top of their head. Web developers today may or may not have access to cron jobs directly in their account.

An online cron service does exactly what a local cron job does, but with a twist--they execute a PHP, ASP, JSP or other CGI web script that is accessible on the internet at your scheduled time. You can run these just like you would a local script, but there are a few advantages:

• Whatever you can do in a complex PHP script, you can now run as a cron


• You can pass parameters to the same script to alter behavior at different times (e.g. a DB backup script that does incremental log backups nightly, and full backups once per week, using different parameters)


• Online cron services can have a simpler interface to configure jobs (more intuitive)


• For free web hosts that don't offer cron jobs locally, this is a great alternative to get crons cheaply or for free


• Some online cron services offer additional features like site monitoring, so you get two tools for the price of one

Online Cron Services are now available in many flavors--free (limiting you on the number of jobs you can run, or how often they execute), or paid services (ranging from $5 to $50+ per year). Some have additional services like site monitoring, some are just cron execution engines.

Many are built on the phpJobScheduler package, a popular PHP framework to run crons. The main downside to this framework is that it expects you to understand and input the cron syntax discussed above. That defeats the point of making things easier in most cases.