|CRON(8)||System Manager's Manual||CRON(8)|
cron — clock
cron daemon schedules commands to be
run at specified dates and times. Commands that are to be run periodically
are specified within
crontab(5) files. Commands
that are only to be run once are scheduled via the
batch(1) commands. Normally,
cron daemon is started from the
/etc/rc command script. Because it can execute
commands on a user's behalf,
cron should be run late
in the startup sequence, as close to the time when logins are accepted as
at(1) files when it starts up and
also when changes are made via the
at(1) commands. Additionally,
cron checks the modification time on the system
crontab file (/etc/crontab), the crontab spool
(/var/cron/tabs), and the at spool
(/var/cron/atjobs) once a minute. If the
modification time has changed, the affected files are reloaded.
Any output produced by a command is sent to the user specified in
MAILTO environment variable as set in the
crontab(5) file or, if no
MAILTO variable is set (or if this is an
batch(1) job), to the job's
owner. If a command produces no output or if the
MAILTO environment variable is set to the empty
string, no mail will be sent. The exception to this is
batch(1) jobs submitted with
-m flag. In this case, mail will be sent even if
the job produces no output.
Local time changes of less than three hours, such as those caused by the start or end of Daylight Saving Time, are handled specially. This only applies to jobs that run at a specific time and jobs that are run with a granularity greater than one hour. Jobs that run more frequently are scheduled normally.
If time has moved forward, those jobs that would have run in the interval that has been skipped will be run immediately. Conversely, if time has moved backward, care is taken to avoid running jobs twice.
Time changes of more than 3 hours are considered to be corrections to the clock or time zone, and the new time is used immediately.
The options are as follows:
cronwill detach from the current tty and become a daemon. The
-noption disables this behavior and causes it to run in the foreground.
Paul Vixie <email@example.com>
crontab(5) files will be ignored if they do not have the proper file mode. For user crontab files created by crontab(1), the mode must be 0400 or 0600. If the system crontab file is used, /etc/crontab must not be writable by any user other than root.
|November 12, 2015||OpenBSD-5.9|