On Wed, Jul 21, 2004 at 09:38:48AM -0500, Dave Sherohman wrote:
> On Tue, Jul 20, 2004 at 05:13:46PM -0700, Mike Fedyk wrote:
> 
> > I want to have another cycle running every other Friday (two weeks), but 
> > cron doesn't seem to be able to handle that.
> 
> `date +%U` gives you the week number in the year.  If there's an easy way
> to test whether a number is even or odd in the shell, then that could be
> used to run something from cron on only even/only odd-numbered weeks,
> although there's still the possibility for anomalies at the end of the
> year (week 52 -> week 00 is two evens in a row - but if this happens
> every year, testing the evenness/oddness of year + week number would
> solve it)...
> 
> OK, somewhat convoluted way to do it:
> 
> [ $(echo "$(date +\%U) \% 2" | bc) == 0 ] && /usr/local/sbin/backup-script
> 
> This will run /usr/local/sbin/backup-script on all even-numbered
> weeks.  Change the 0 to 1 for odd weeks.  Also note that the %s need
> to be backslash-escaped to prevent cron from intepreting them as
> newlines/command separators.  (Only tested in bash, but, IIRC, the $()
> syntax works in csh and vanilla sh also.)
> 
> Anyone have a simpler even/odd week test?

In a 'real' bourne shell $(...) is not available, but `...` is.

However this can be simplified if you are working in a
posix-compatible shell.  The math operators ((...)) provide
a return code of 0 or 1 depending on the zero/non-zero value
of the result.  So you could do (ignoring cron required escapes)

  (( $(date +%U) % 2 )) && ...

As you point out, the test would have problems at the turn
of some years.

-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
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