Perl for Web Site Management

Using cron

The Unix cron command is one of those things that makes you wonder how you ever got along without it. In brief, it lets you schedule a command or series of commands to be run periodically on the Unix server, whether you’re logged in or not.

The instructions on what commands to run and when to run them are stored in a special cron table, or crontab. On many systems only the superuser can use the crontab command to modify the entries in that table. On other systems ordinary users can do so, but the exact method sometimes varies depending on what flavor of Unix the server is running.

Tip

The following discussion assumes you are on a Linux server that has been configured to allow ordinary users to install their own crontab entries. If these instructions don’t work in your case, you should contact your system administrator. You should also try consulting the cron and crontab documentation available on your server (via man cron, man 1 crontab, and man 5 crontab) because some of the features described here could well be different or absent in the case of your particular cron installation.

A crontab file is simply a text file. You edit it using any text editor you like, then install it on your server using the crontab command, like this:

[jbc@andros jbc] crontab 
mycronfile.txt

The format of the crontab file is very specific. Blank lines are ignored, as are lines beginning with a hash sign (#). All other lines need to consist of the following fields, separated by spaces:

• Minute ...