cron(8) - NetBSD Manual Pages

Command:
Section:
Arch:
Collection:
>>>
CRON(8)                 NetBSD System Manager's Manual                 CRON(8)


NAME

     cron -- daemon to execute scheduled commands (ISC Cron V4.1)


SYNOPSIS

     cron [-n] [-x debugflags]


DESCRIPTION

     cron is normally started during system boot by rc.d(8) framework, if cron
     is switched on in rc.conf(5).

     It will return immediately so you don't have to start it with `&'.

     cron searches /var/cron/tabs for crontab files which are named after
     accounts in /etc/passwd.  Crontabs found are loaded into memory.  cron
     also searches for /etc/crontab which is in a different format (see
     crontab(5)).  Finally cron looks for crontabs in /etc/cron.d if it
     exists, and executes each file as a crontab.

     When cron looks in a directory for crontabs (either in /var/cron/tabs or
     /etc/cron.d) it will not process files that:
           -   Start with a `.' or a `#'.
           -   End with a `~' or with ``.rpmsave'', ``.rpmorig'', or
               ``.rpmnew''.
           -   Are of zero length.
           -   Their length is greater than MAXNAMLEN.

     cron then wakes up every minute, examining all stored crontabs, checking
     each command to see if it should be run in the current minute.  When exe-
     cuting commands, any output is mailed to the owner of the crontab (or to
     the user named in the MAILTO environment variable in the crontab, if such
     exists).

     Events such as START and FINISH are recorded in the /var/log/cron log
     file with date and time details.  This information is useful for a number
     of reasons, such as determining the amount of time required to run a par-
     ticular job.  By default, root has an hourly job that rotates these log
     files with compression to preserve disk space.

     Additionally, cron checks each minute to see if its spool directory's
     modtime (or the modtime on /etc/crontab or /etc/cron.d) has changed, and
     if it has, cron will then examine the modtime on all crontabs and reload
     those which have changed.  Thus cron need not be restarted whenever a
     crontab file is modified.  Note that the crontab(1) command updates the
     modtime of the spool directory whenever it changes a crontab.

     The following options are available:

     -x      This  flag turns on some debugging flags.  debugflags is comma-
             separated list of debugging flags to turn on.  If a flag is
             turned on, cron writes some additional debugging information to
             system log during its work.  Available debugging flags are:
             sch   scheduling
             proc  process control
             pars  parsing
             load  database loading
             misc  miscellaneous
             test  test mode - do not actually execute any commands
             bit   show how various bits are set (long)
             ext   print extended debugging information

     -n      Stay in the foreground and don't daemonize cron.

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.


SIGNALS

     On receipt of a SIGHUP, the cron daemon will close and reopen its log
     file.  This is useful in scripts which rotate and age log files.  Natu-
     rally this is not relevant if cron was built to use syslog(3).


FILES

     /var/cron/tabs  cron spool directory
     /etc/crontab    system crontab file
     /etc/cron.d/    system crontab directory
     /var/log/cron   log file for cron events


SEE ALSO

     crontab(1), crontab(5)


AUTHORS

     Paul Vixie <vixie@isc.org>

NetBSD 8.1                     October 12, 2011                     NetBSD 8.1