MirBSD manpage: cron(8)

CRON(8)                  BSD System Manager's Manual                   CRON(8)


     cron - clock daemon


     cron [-l load_avg] [-n] [-x [ext,sch,proc,pars,load,misc,test]]


     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 output.

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
     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 timezone, and the new time is used immediately.

     The options are as follows:

     -l load_avg
             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 be used.

     -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.

     -x debug_flags
             If cron was compiled with debugging support, a number of debug-
             ging flags are available to show what cron is doing. The follow-
             ing flags may be specified:

             ext     show extended information; used in conjunction with other
                     debug flags to provide even more information

             sch     print information related to scheduling jobs

             proc    print information related to running processes

             pars    print information related to parsing crontab(5) files

             load    print when loading the databases

             misc    show misc other debugging information

             test    test mode; don't actually execute commands

             Multiple flags may be specified, separated by a comma (','). Re-
             gardless of which flags were specified, the -x flag will cause
             cron to stay in the foreground and not become a daemon.


     SIGHUP  causes cron to close and reopen its log file. This is useful in
             scripts which rotate and age log files. On OpenBSD this has no
             effect because cron logs via syslog(3).


     /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/cron/tabs/.sock  used by crontab(1) to tell cron to check for cron-
                           tab changes immediately


     at(1), crontab(1), syslog(3), crontab(5)


     Paul Vixie <vixie@isc.org>


     All crontab(5) files must not be readable or writable by any user other
     than their owner, including /etc/crontab. In practice this means they
     should be mode 0600. This restriction is enforced automatically by
     crontab(1) but if /etc/crontab is used, the mode must be set manually on
     that file.

MirBSD #10-current               July 6, 2002                                1

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