4.0.1 crontab - good news and bad

[Norm Finn]

Ed Arnold's question about 4.0.1 running his crontab entries twice
prompted this submission.  The good news is that 4.0.1's cron facility
supports multiple users, each with his/her own crontab, and returns any
output from the jobs via mail.  The bad news is that they switched to SysV
semantics for interpreting the day-of-month and day-of-week fields in the
crontab files.

We had the following entries to perform disk maintenance on the first
Monday of the month (dumonthly), all other Mondays (duweekly) and all
other weekdays (dudaily):

45 4 1-7  * 1   /usr/local/bin/dumonthly
45 4 8-31 * 1   /usr/local/bin/duweekly
45 4 *    * 2-5 /usr/local/bin/dudaily

Under rev 3.3, dumonthly ran at 4:45 when the day-of-month (1-7) AND the
day-of-week (1) BOTH matched, which is the first Monday of the month.
Duweekly ran on all other Mondays (day-of-month in range 8-31 AND
day-of-week = 1).  Dudaily ran on any day of the month (*) that was a
Tuesday through Friday (2-5).

Under rev 4.0.1, as documented in the crontab(5) man page and textbooks on
SysV and as observed in action, this has been changed.  Now, the OR is
taken of the day-of-month and day-of-week fields.  Duweekly now runs on
every day that is a Monday OR is in the range 8-31.  On Wednesday, 12
April, both duweekly (8-31 was satisfied) and dudaily (2-5 was satisfied)
were run.

The net result is a reduction in functionality, since the Berkeley rules
can give the same results as SysV by fiddling with and adding more crontab
lines, while the SysV rules cannot perform the perfectly reasonable
scheduling we were enjoying under 3.5.  (Really.  I've gone through some
good examples I won't bore you with.  Think about it before you flame me.)
We now have to run the kludge:

30 4 1    * *   /usr/bin/touch /home/usr2/norm/size.trigger
45 4 *    * 2-5 /usr/local/bin/dudaily   / /home/usr2/norm/size 1024
45 4 *    * 1   /usr/local/bin/duweekly  / /home/usr2/norm/size 1024

Duweekly now checks for the presence of size.trigger, then deletes it and
calls dumonthly if the trigger file is present.

It's unfortunate that we have to retrogress from what is essentially a bug
fix to SysV's (mal)functionality, but I'm afraid that this kind of
annoyance is the price of compatibility.

Norm Finn                       domain:   norm at ultra.com
Ultra Network Technologies      Internet: ultra!norm at ames.arc.nasa.gov
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