CRON

Section: Cronie Users' Manual (8)
Updated: July 2009
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NAME

cron - daemon to execute scheduled commands  

SYNOPSIS

cron [-n | -p | -m<mailcommand>]
cron -x [ext,sch,proc,pars,load,misc,test,bit]
 

DESCRIPTION

Cron should be started from /etc/rc.d/init.d or /etc/init.d will return immediately, so you don't need to start it with '&'.

Cron searches /var/spool/cron for crontab files which are named after accounts in /etc/passwd; The founded crontabs are loaded into memory. Cron also searches for /etc/anacrontab and the files in the /etc/cron.d directory, which are in a different format (see crontab(5) ). Cron examines all stored crontabs, checking each command to see if it should be run in the current minute. When executing commands, any output is mailed to the owner of the crontab (or to the user named in the MAILTO environment variable in the crontab, if such exists).

There are two ways, how the changes are checked in crontables. The first is checking the modtime of file and the other is using inotify support. You can find out which of them are you using, if you check /var/log/cron where is (or isn't) inotify mentioned after start of daemon. The inotify support is watching for changes in all crontables and touch the disk only in case that something was changed.

In other case cron checks each minute to see if its crontables modtime have changes and reload those which have changes. There is no need to restart cron after some of the crontable is modified. The modtime option is used also when inotify couldn't be initialized.

Cron is checking those files or directories: /etc/anacrontab system crontab is usually for running daily, weekly, monthly jobs. .IR/etc/cron.d/ where are system cronjobs stored for different users. .IR/var/spool/cron that's mean spool directory for user crontables.

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 start or end of Daylight Saving Time, are handled specially. This only applies to jobs that run at a specific time and jobs that are run with a granularity greater than one hour. Jobs that run more frequently are scheduled normally.

If time has moved forward, those jobs that would have run in the interval that has been skipped will be run immediately. Conversely, if time has moved backward, care is taken to avoid running jobs twice.

Time changes of more than 3 hours are considered to be corrections to the clock or timezone, and the new time is used immediately.

It's possible to use different time zones for cron tables. More could be found in crontab(5).  

PAM Access Control

On Red Hat systems, crond now supports access control with PAM - see pam(8). A PAM configuration file for crond is installed in /etc/pam.d/crond. crond loads the PAM environment from the pam_env module, but these can be overriden by settings in the appropriate crontab file.  

OPTIONS

-m
This option allows you to specify a shell command string to use for sending cron mail output instead of sendmail(8). This command must accept a fully formatted mail message (with headers) on stdin and send it as a mail message to the recipients specified in the mail headers.
-n
This option changes default behavior causing it to run crond in the foreground. This can be useful when starting it out of init.
-p
Cron permit any crontab, which user set.
-x
With this option is possible to set debug flags.
 

SIGNALS

On receipt of a SIGHUP, the cron daemon will close and reopen its log file. This is useful in scripts which rotate and age log files. Naturally this is not relevant if cron was built to use syslog(3).  

CAVEATS

The crontab files have to be regular files or symlinks to regular files, they must not be executable or writable by anyone else than 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 to reload the crontabs. This is a limitation of inotify API.

 

SEE ALSO

crontab(1), crontab(5), inotify(7), pam(8)  

AUTHOR

Paul Vixie <vixie@isc.org>
Marcela Mašláňová <mmaslano@redhat.com>