cron

[Alex Snow]

Hi.
Thanks for the info!

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On Sat, 14 Jun 2003, Adam Myrow wrote:

> 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.
>
>
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