Tip:How to run a cron job every other week

A few weeks back, a customer asked me about running an automated task
every other week. Though most of us use cron as needed to run those nice
little tasks that clean up core files and evaluate the contents of log
files in the middle of the night, running a task every other week or
every other day presents a bit of a challenge. The cron command doesn't
have any way to express odd or even weeks.

The general "trick" that I use for tasks such as these is to ask cron to
run the task every week or every day and then insert logic into the
script itself to determine whether the week is odd or even.

Using this strategy, a cron entry that looked something like this:

8 8 * * 3 /usr/local/bin/send_msg

that would be executed every Wednesday might be calling a script that
examines the date and continues only when it's running on an odd or even
week.

A shell (/bin/sh) script to send a message on even weeks might look
something like this:

-------------
#!/bin/sh

WK=`date +%W`
ON_WK=`expr $WK % 2`

if [ $ON_WK = 1 ]; then
cat /opt/stats/msgs | mailx -s "stats report" someone@someplace.org
fi
-------------

This same strategy can be used for tasks that need to be performed every
other hour, every third week, every seven minutes or almost any other
interval that you might want to work with. For intervals that align
nicely with cron's timing fields (minutes after the hour, hour, day of
the month, month and day of the week, there's no good reason not to put
all of your timing logic into the cron file. When your needs don't align
well with these columns, on the other hand, or when you want to avoid
putting lines like these into the cron file:

0,4,8,12,16,20,24,28,32,36,40,44,48,52,56 * * * * /usr/local/bin/chk_log

constraining the time within the script itself is not such a bad idea.

The number 2 in the "ON_WK=`expr $WK % 2`" line of the scrip represents
the modulo operation. For anyone who isn't used to these, the result of
an "expr % " operation is what you'd be left with if
you removed the modulus as many times as you could. Because our modulus
is 2, the result is 0 or 1. Were the modulus 5, we could get any value
between 0 and 4.

The "WK=`date +%W`" command uses an argument to the date command to
obtain the number of the current week. You'd expect these to run from 1
to 52 or thereabouts. So the combination gives us a 1 if the current
week is odd and a 0 otherwise.

Other date command options that can be used with this kind of logic
include:

%d - date within the month (e.g., 21)
%m - month number (1-12)
%H - hour (0-23)
%M - minute (0-59)
%S - second (0-59)

To run a script every other day, you couldn't rely on the day of the
month. This would only work for a while. You'd soon find yourself moving
from one odd day to another. This would happen any time you got to the
end of a month with 31 days. Instead, you would use the value that
represents the day of the year. You'd expect these to run from 1 to 365
except, of course, on leap years. If the end-of-the-year problem
concerns you, you could probably perform some much more complex
calculation to be sure you're still running every other day but, for
most of us, an adjustment at the end of each calendar year is probably
not too big an issue. We could always switch our running from odd to
even days if the need for regularity was sufficiently important.