cron - daemon to execute scheduled commands (ISC Cron V4.1)
Daylight Saving Time and other time changes
PAM Access Control
cron [-n | -p | -m<mailcommand>]
cron -x [ext,sch,proc,pars,load,misc,test,bit]
Cron should be started from /etc/rc.d/init.d or /etc/init.d
Cron searches /var/spool/cron for crontab files which are named after accounts in crontabs found are loaded into memory. Cron also searches for /etc/crontab and the files in the directory, which are in a different format (see crontab(5) ). Cron then wakes up every minute, examining 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).
Additionally, cron checks each minute to see if its spool directorys modtime (or the modtime on /etc/crontab) has changed, and if it has, cron will then examine the modtime on all crontabs and reload those which have changed. Thus cron need not be restarted whenever a crontab file is modified. Note that the crontab(1) command updates the modtime of the spool directory whenever it changes a crontab.
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.
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 crontab file.
-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.
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).
In this version of cron , without the -p option, /etc/crontab must not be writable by any user other than root, no crontab files may be links, or linked to by any other file, and no crontab files may be executable, or be writable by any user other than their owner.
crontab(1), crontab(5), pam(8)
Paul Vixie <email@example.com>
|Linux Programmers Manual||CRON (8)||10 January 2007|