cron - daemon to execute scheduled commands (ISC Cron V4.1)
cron [-l load_avg] [-n] [-p]
Cron should be started from /etc/rc or /etc/rc.local. It will return
immediately, so you don’t need to start it with ’&’. The -n option
changes this default behavior causing it to run in the foreground.
This can be useful when starting it out of init.
Cron searches /var/spool/cron for crontab files which are named after
accounts in /etc/passwd; crontabs found are loaded into memory. Cron
also searches for /etc/crontab and the files in the /etc/cron.d direc-
tory, 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 execut-
ing 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
Additionally, cron checks each minute to see if its spool directory’s
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.
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 fre-
quently are scheduled normally.
If time has moved forward, those jobs that would have run in the inter-
val 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.
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 crontab file.
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. Natu-
rally 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 <firstname.lastname@example.org>
4th Berkeley Distribution 10 January 1996" CRON(8)
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