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