CRON(8) System Manager's Manual CRON(8)
cron - clock daemon
cron [-n] [-l load_avg]
The 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 at(1) and batch(1) commands. Normally, the 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 possible.
cron loads crontab(5) and at(1) files when it starts up and also when
changes are made via the crontab(1) and 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 the
MAILTO environment variable as set in the crontab(5) file or, if no
MAILTO variable is set (or if this is an at(1) or 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 at(1) or batch(1) jobs submitted with the -m
flag. In this case, mail will be sent even if the job produces no
Daylight Saving Time and other time changes
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
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:
If the current load average is greater than load_avg, batch(1)
jobs will not be run. The default value is 1.5. To allow
batch(1) jobs to run regardless of the load, a value of 0.0 may
-n By default, cron will detach from the current tty and become a
daemon. The -n option disables this behavior and causes it to
run in the foreground.
/etc/crontab system crontab file
/var/cron/atjobs directory containing at(1) jobs
/var/cron/log cron's log file
/var/cron/tabs directory containing individual crontab files
/var/run/cron.sock used by crontab(1) to tell cron to check for crontab
at(1), crontab(1), syslog(3), crontab(5)
Paul Vixie <firstname.lastname@example.org>
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.
OpenBSD 5.9 November 12, 2015 OpenBSD 5.9
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