CRON - phpMan

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 behav-
ior 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 environment variable in the crontab, if such exists).

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 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.

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.

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 rele-
vant 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.