|
NAME | SYNOPSIS | DESCRIPTION | OPTIONS | SIGNALS | CLUSTERING SUPPORT | CAVEATS | SEE ALSO | AUTHOR | COLOPHON |
CRON(8) System Administration CRON(8)
crond - daemon to execute scheduled commands
crond [-c | -h | -i | -n | -p | -P | -s | -m<mailcommand>]
crond -x [ext,sch,proc,pars,load,misc,test,bit]
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/system/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 specific 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, running 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 information.
PAM Access Control
Cron supports access control with PAM if the system has PAM
installed. For more information, 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.
-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 output 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 syslog(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.
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).
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), otherwise 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.
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 daemon must receive a SIGHUP signal to reload the
crontabs. This is a limitation of the inotify API.
The syslog output will be used instead of mail, when sendmail is not
installed.
crontab(1), crontab(5), inotify(7), pam(8)
Paul Vixie ⟨vixie@isc.org⟩
Marcela Mašláňová ⟨mmaslano@redhat.com⟩
Colin Dean ⟨colin@colin-dean.org⟩
This page is part of the cronie (crond daemon) project. Information
about the project can be found at
⟨https://github.com/cronie-crond/cronie⟩. If you have a bug report
for this manual page, see
⟨https://github.com/cronie-crond/cronie/issues⟩. This page was
obtained from the project's upstream Git repository
⟨https://github.com/cronie-crond/cronie.git⟩ on 2017-07-05. If you
discover any rendering problems in this HTML version of the page, or
you believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
cronie 2013-09-26 CRON(8)
Pages that refer to this page: crontab(1), pmie_check(1), pmlogger_check(1), crontab(5), passwd(5), hier(7), keyrings(7), persistent-keyring(7), user-keyring(7), anacron(8), warnquota(8)