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CRON(8) System Administration CRON(8)
NAME
crond - daemon to execute scheduled commands
SYNOPSIS
crond [-c | -h | -i | -n | -p | -P | -s | -m<mailcommand>]
crond -x [ext,sch,proc,pars,load,misc,test,bit]
DESCRIPTION
Cron is started from /etc/rc.d/init.d or /etc/init.d when classical sysvinit scripts are
used. In case systemd is enabled, then unit file is installed into /lib/systemd/sys-
tem/crond.service and daemon is started by systemctl start crond.service command. It
returns immediately, thus, there is no need to need to start it with the '&' parameter.
Cron searches /var/spool/cron for crontab files which are named after accounts in
/etc/passwd; The found crontabs are loaded into the memory. Cron also searches for
/etc/anacrontab and any files in the /etc/cron.d directory, which have a different format
(see crontab(5)). 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 MAILTO environment variable in
the crontab, if such exists). Any job output can also be sent to syslog by using the -s
option.
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 /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.
When using the modtime option, 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 Cron after some of the crontables were modified. The modtime option is also used
when inotify can not be initialized.
Cron checks these files and directories:
/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 /etc/anacrontab configuration file. See anacrontab(5) for more
details.
/etc/cron.d/
directory that contains system cronjobs stored for different users.
/var/spool/cron
directory that contains user crontables created by the crontab command.
Note that the crontab(1) command updates the modtime of the spool directory whenever it
changes a crontab.
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 spe-
cific time and jobs that run with a granularity greater than one hour. Jobs that run more
frequently are scheduled normally.
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, run-
ning the same job twice is avoided.
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.
It is possible to use different time zones for crontables. See crontab(5) for more infor-
mation.
PAM Access Control
Cron supports access control with PAM if the system has PAM installed. For more informa-
tion, see pam(8). A PAM configuration file for crond is installed in /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.
OPTIONS
-h Prints a help message and exits.
-i Disables inotify support.
-m This option allows you to specify a shell command to use for sending Cron mail out-
put instead of using 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 off (i.e., crond
-m off) will disable the sending of mail.
-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. /etc/pam.d/crond must
not enable pam_loginuid.so module.
-p Allows Cron to accept any user set crontables.
-P Don't set PATH. PATH is instead inherited from the environment.
-c This option enables clustering support, as described below.
-s This option will direct Cron to send the job output to the system log using sys-
log(3). This is useful if your system does not have sendmail(8), installed or if
mail is disabled.
-x This option allows you to set debug flags.
SIGNALS
When the SIGHUP is received, the 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 Cron was built to use syslog(3).
CLUSTERING SUPPORT
In this version of Cron it is possible to use a network-mounted shared /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 Cron with the -c option,
and have the /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 gethostname(2), then all
crontab files in this directory are ignored. This has no effect on cron jobs specified in
the /etc/crontab file or on files in the /etc/cron.d directory. These files are always
run and considered host-specific.
Rather than editing /var/spool/cron/.cron.hostname directly, use the -n option of
crontab(1) to specify the host.
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 ntpd(8), oth-
erwise the results will be very unpredictable.
Using cluster sharing automatically disables inotify support, because inotify cannot be
relied on with network-mounted shared file systems.
CAVEATS
All 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 -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 dae-
mon must receive a SIGHUP signal to reload the crontabs. This is a limitation of the ino-
tify API.
The syslog output will be used instead of mail, when sendmail is not installed.
SEE ALSO
crontab(1), crontab(5), inotify(7), pam(8)
AUTHOR
Paul Vixie <vixie AT isc.org>
Marcela Malaova <mmaslano AT redhat.com>
Colin Dean <colin AT colin-dean.org>
cronie 2013-09-26 CRON(8)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/2.4.6 (CentOS)
Under GNU General Public License
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