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CRON(8)

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NAME    [Toc]    [Back]

     cron - clock daemon

SYNOPSIS    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

     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    [Toc]    [Back]
     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 debugging
 flags are available to show what cron is doing.
The following
 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 (`,').  Regardless
  of which flags were specified, the -x flag
will cause
             cron to stay in the foreground and not become a daemon.

SIGNALS    [Toc]    [Back]

     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).

FILES    [Toc]    [Back]

     /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
                           crontab changes immediately

SEE ALSO    [Toc]    [Back]

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

AUTHORS    [Toc]    [Back]

     Paul Vixie <vixie@isc.org>

CAVEATS    [Toc]    [Back]

     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.

OpenBSD      3.6                           July      6,      2002
[ Back ]
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