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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | SIGNALS | CLUSTERING SUPPORT | CAVEATS | SEE ALSO | AUTHOR | COLOPHON |
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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]
crond -V
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.
-f the same as -n, consistent with other crond
implementations.
-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.
-V Print version and exit.
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⟩
Tomáš Mráz ⟨tmraz@fedoraproject.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 2024-06-14. (At
that time, the date of the most recent commit that was found in
the repository was 2024-05-31.) 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: cronnext(1), crontab(1), pmfind_check(1), pmie(1), pmie_check(1), pmlogger(1), pmlogger_check(1), pmlogger_daily(1), crontab(5), passwd(5), pmlogger.control(5), hier(7), keyrings(7), persistent-keyring(7), user-keyring(7), anacron(8), fstrim(8), warnquota(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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