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man/cron.8
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.\"/* Copyright 1988,1990,1993,1996 by Paul Vixie
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.\" * All rights reserved
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.\" */
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.\"
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.\" Copyright (c) 2004 by Internet Systems Consortium, Inc. ("ISC")
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.\" Copyright (c) 1997,2000 by Internet Software Consortium, Inc.
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.\"
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.\" Permission to use, copy, modify, and distribute this software for any
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.\" purpose with or without fee is hereby granted, provided that the above
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.\" copyright notice and this permission notice appear in all copies.
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.\"
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR
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.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
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.\" OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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.\"
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.\" Modified 2010/09/12 by Colin Dean, Durham University IT Service,
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.\" to add clustering support.
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.\"
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.\" $Id: cron.8,v 1.8 2004/01/23 19:03:32 vixie Exp $
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.\"
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.TH CRON "8" "2013-09-26" "cronie" "System Administration"
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.SH NAME
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crond \- daemon to execute scheduled commands
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.SH SYNOPSIS
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.B crond
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.RB [ -c " | " -h " | " -i " | " -n " | " -p " | " -P " | " -s " | " -m \fP\fI<mail command>\fP ]
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.br
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.B crond
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.B -x
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.RB [ext,sch,proc,pars,load,misc,test,bit]
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.br
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.B crond
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.B -V
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.SH DESCRIPTION
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.I Cron
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is started from
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.I /etc/rc.d/init.d
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or
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.I /etc/init.d
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when classical sysvinit scripts are used. In case systemd is enabled, then unit file is installed into
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.I /lib/systemd/system/crond.service
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and daemon is started by
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.I systemctl start crond.service
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command. It returns immediately, thus, there is no need to need to start it with
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the '&' parameter.
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.PP
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.I Cron
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searches
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.I /var/spool/cron
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for crontab files which are named after accounts in
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.I /etc/passwd;
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The found crontabs are loaded into the memory.
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.I Cron
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also searches for
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.I /etc/anacrontab
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and any files in the
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.I /etc/cron.d
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directory, which have a different format (see
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.BR crontab (5)).
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.I Cron
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examines all stored crontabs and checks each job to see if it needs to be
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run in the current minute. When executing commands, any output is mailed
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to the owner of the crontab (or to the user specified in the
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.I MAILTO
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environment variable in the crontab, if such exists). Any job output can
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also be sent to syslog by using the
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.B "\-s"
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option.
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.PP
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There are two ways how changes in crontables are checked. The first
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method is checking the modtime of a file. The second method is using the
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inotify support. Using of inotify is logged in the
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.I /var/log/cron
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log after the daemon is started. The inotify support checks for changes
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in all crontables and accesses the hard disk only when a change is
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detected.
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.PP
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When using the modtime option,
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.I Cron
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checks its crontables' modtimes every minute to check for any changes and
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reloads the crontables which have changed. There is no need to restart
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.I Cron
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after some of the crontables were modified. The modtime option is also
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used when inotify can not be initialized.
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.PP
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.I Cron
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checks these files and directories:
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.TP
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.IR /etc/crontab
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system crontab. Nowadays the file is empty by default. Originally it
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was usually used to run daily, weekly, monthly jobs. By default these
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jobs are now run through anacron which reads
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.IR /etc/anacrontab
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configuration file. See
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.BR anacrontab (5)
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for more details.
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.TP
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.IR /etc/cron.d/
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directory that contains system cronjobs stored for different users.
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.TP
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.IR /var/spool/cron
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directory that contains user crontables created by the
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.IR crontab
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command.
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.PP
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Note that the
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.BR crontab (1)
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command updates the modtime of the spool directory whenever it changes a
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crontab.
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.PP
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.SS Daylight Saving Time and other time changes
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Local time changes of less than three hours, such as those caused by the
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Daylight Saving Time changes, are handled in a special way. This only
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applies to jobs that run at a specific time and jobs that run with a
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granularity greater than one hour. Jobs that run more frequently are
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scheduled normally.
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.PP
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If time was adjusted one hour forward, those jobs that would have run in
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the interval that has been skipped will be run immediately. Conversely,
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if time was adjusted backward, running the same job twice is avoided.
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.PP
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Time changes of more than 3 hours are considered to be corrections to the
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clock or the timezone, and the new time is used immediately.
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.PP
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It is possible to use different time zones for crontables. See
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.BR crontab (5)
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for more information.
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.SS PAM Access Control
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.IR Cron
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supports access control with PAM if the system has PAM installed. For
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more information, see
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.BR pam (8).
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A PAM configuration file for
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.IR crond
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is installed in
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.IR /etc/pam.d/crond .
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The daemon loads the PAM environment from the pam_env module. This can
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be overridden by defining specific settings in the appropriate crontab
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file.
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.SH "OPTIONS"
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.TP
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.B "\-h"
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Prints a help message and exits.
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.TP
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.B "\-i"
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Disables inotify support.
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.TP
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.B "\-m"
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This option allows you to specify a shell command to use for sending
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.I Cron
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mail output instead of using
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.BR sendmail (8)
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This command must accept a fully formatted mail message (with headers) on
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standard input and send it as a mail message to the recipients specified
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in the mail headers. Specifying the string
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.I "off"
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(i.e., crond -m off)
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will disable the sending of mail.
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.TP
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.B "\-n"
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Tells the daemon to run in the foreground. This can be useful when
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starting it out of init. With this option is needed to change pam setting.
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.I /etc/pam.d/crond
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must not enable
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.I pam_loginuid.so
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module.
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.TP
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.B "\-f"
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the same as -n, consistent with other crond implementations.
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.TP
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.B "\-p"
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Allows
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.I Cron
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to accept any user set crontables.
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.TP
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.B "\-P"
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Don't set PATH. PATH is instead inherited from the environment.
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.TP
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.B "\-c"
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This option enables clustering support, as described below.
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.TP
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.B "\-s"
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This option will direct
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.I Cron
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to send the job output to the system log using
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.BR syslog (3).
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This is useful if your system does not have
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.BR sendmail (8)
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installed or if mail is disabled.
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.TP
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.B "\-x"
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This option allows you to set debug flags.
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.TP
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.B "\-V"
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Print version and exit.
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.SH SIGNALS
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When the
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.I SIGHUP
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is received, the
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.I Cron
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daemon will close and reopen its log file. This proves to be useful in
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scripts which rotate and age log files. Naturally, this is not relevant
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if
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.I Cron
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was built to use
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.IR syslog (3).
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.SH CLUSTERING SUPPORT
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In this version of
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.IR Cron
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it is possible to use a network-mounted shared
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.I /var/spool/cron
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across a cluster of hosts and specify that only one of the hosts should
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run the crontab jobs in this directory at any one time. This is done by
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starting
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.I Cron
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with the
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.B \-c
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option, and have the
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.I /var/spool/cron/.cron.hostname
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file contain just one line, which represents the hostname of whichever
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host in the cluster should run the jobs. If this file does not exist, or
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the hostname in it does not match that returned by
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.BR gethostname (2),
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then all crontab files in this directory are ignored. This has no effect
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on cron jobs specified in the
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.I /etc/crontab
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file or on files in the
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.I /etc/cron.d
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directory. These files are always run and considered host-specific.
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.PP
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Rather than editing
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.I /var/spool/cron/.cron.hostname
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directly, use the
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.B \-n
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option of
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.BR crontab (1)
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to specify the host.
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.PP
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You should ensure that all hosts in a cluster, and the file server from
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which they mount the shared crontab directory, have closely synchronised
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clocks, e.g., using
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.BR ntpd (8),
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otherwise the results will be very unpredictable.
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.PP
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Using cluster sharing automatically disables inotify support, because
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inotify cannot be relied on with network-mounted shared file systems.
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.SH CAVEATS
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All
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.BR crontab
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files have to be regular files or symlinks to regular files, they must
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not be executable or writable for anyone else but the owner. This
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requirement can be overridden by using the
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.B \-p
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option on the crond command line. If inotify support is in use, changes
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in the symlinked crontabs are not automatically noticed by the cron
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daemon. The cron daemon must receive a SIGHUP signal to reload the
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crontabs. This is a limitation of the inotify API.
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.PP
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The syslog output will be used instead of mail, when sendmail is not
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installed.
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.SH "SEE ALSO"
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.BR crontab (1),
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.BR crontab (5),
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.BR inotify (7),
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.BR pam (8)
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.SH AUTHOR
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.MT vixie@isc.org
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Paul Vixie
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.ME
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.br
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.MT mmaslano@redhat.com
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Marcela Mašláňová
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.ME
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.br
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.MT colin@colin-dean.org
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Colin Dean
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.ME
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.br
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.MT tmraz@fedoraproject.org
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Tomáš Mráz
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.ME
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