The syntax of the crontab file is downright odd, but it's been this way in just about every Unix system or Linux system I've encountered. The first entry is the minute or minutes on which you want your cron job to run. For every 15 minutes, you would use "0,15,30,45, in the first field. The second field is the hour. It is expressed in 24-hour notation from 0 to 23. The third field is the day of the month you want to run the cron job. The fourth field is the month or months of the year to run the cron job. The fifth field is the day of the week from 0 for Sunday to 6 for Saturday. All of these fields can take a single value, a comma-separated list of values, a range, or an asterisk which means any. For example, if you wanted the time announced every 15 minutes, and you had a command called "saytime," you would put something like this in your crontab file. 0,15,30,45 * * * * /usr/local/bin/saytime >/dev/null 2>&1 It is generally a good idea to include the full path to commands since the path can be set up oddly when running a cron script. You also generally want to redirect both standard out and standard error somewhere. Otherwise, any output gets mailed to the user who has the cron job. This is a good thing in some cases, but not so good in a lot of others. Instead of /dev/null, you could use the ">> /var/log/saytime.log" to append output to that file. In the case of "saytime" it seldom produces output other than an error saying that the device is busy, so we don't really need to log it. Check out the default crontab file. It has some cron entries already set up. As I said, this applies to just about any Unix system you can think of. Some crontab programs will have special features like being able to understand symbolic names like MON instead of the numbers, but I stick with the most portable format. Here is another example. suppose you leave your computer on 24 hours a day. That Saytime program is keeping you up all night! How to shut it up while letting it run in the daytime? Try this. 0,15,30,45 8-20 * * * /usr/local/bin/saytime >/dev/null 2>&1 Now, the cron entry will only run from 8 A.M. to 8 P.M. I think you get the idea. These are just examples I made up off the top of my head. Lastly, always use "crontab -e" to edit cron files. If you don't like the editor it starts with, set the environment variable "EDITOR" to point to your favorite editor. For example "export EDITOR='pico -t'" or "export EDITOR=emacs." Since some programs use VISUAL instead of EDITOR, I have my .profile export both variables and give them the same value. This effects most news readers, and most other email readers besides Pine which has its own option for setting an editor. I hope this was helpful.
