11-27-2006
The first line works because it's running every minute of every day and the program being called accepts passed parameters that control what actually happens when. When I write programs that have the capability of checking date/time for acting on whatever, I usually have it do something very different from the schedule cron runs, ie:
If a program runs every Monday morning, but only if there's data to collect and act on, then Cron runs every Monday morning and the program checks for the data being present. Actually that example is a no-brainer.... It's Monday and I don't drink coffee. But, I hope that gives some semblance of understanding.
The above example, poor as it is, should also apply to your second line.
Oh, I just noticed the '&' at the end of the lines. If the called programs run once and then die, cron needs to be set to run often. If they run forever, then cron doesn't need to be run so often. In that case, the called programs should have a PID file in /var/run to check against. If the file is there and the PID is active, then the program isn't run, else it is run.
HTH,
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CRON(8) System Manager's Manual CRON(8)
NAME
cron - clock daemon
SYNOPSIS
auth/cron [-c]
DESCRIPTION
Cron executes commands at specified dates and times according to instructions in the files /cron/user/cron. It runs only on an authentica-
tion server. Option -c causes cron to create /cron/user and /cron/user/cron for the current user; it can be run from any Plan 9 machine.
Blank lines and lines beginning with # in these files are ignored. Entries are lines with fields
minute hour day month weekday host command
Command is a string, which may contain spaces, that is passed to an rc(1) running on host for execution. The first five fields are integer
patterns for
minute 0-59
hour 0-23
day of month 1-31
month of year 1-12
day of week 0-6; 0=Sunday
The syntax for these patterns is
time : '*'
| range
range : number
| number '-' number
| range ',' range
Each number must be in the appropriate range. Hyphens specify inclusive ranges of valid times; commas specify lists of valid time ranges.
To run the job, cron calls host and authenticates remote execution, equivalent to running rx host command (see con(1)). The user's profile
is run with $service set to rx.
Cron is not a reliable service. It skips commands if it cannot reach host within two minutes, or if the cron daemon is not running at the
appropriate time.
EXAMPLES
Here is the job that mails system news.
% cat /cron/upas/cron
# send system news
15 8-17, 21 *** helix /mail/lib/mailnews
%
SOURCE
/sys/src/cmd/auth/cron.c
SEE ALSO
con(1), rc(1)
CRON(8)