cron - Online Manual Page Of Unix/Linux

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CRON(8) CRON(8)

NAME
cron - daemon to execute scheduled commands (ISC Cron V4.1)

SYNOPSIS
cron [-l load_avg] [-n] [-p]

DESCRIPTION
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 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 envi-
ronment variable in the crontab, if such exists).

Additionally, cron checks each minute to see if its spool directory's modtime (or the mod-
time 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 when-
ever 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 spe-
cific 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 time-
zone, 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 con-
figuration file for crond is installed in /etc/pam.d/crond . crond loads the PAM environ-
ment from the pam_env module, but these can be overriden by settings in the crontab file.

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

CAVEATS
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.