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cron - daemon to execute scheduled commands (ISC Cron V4.1)
cron [-l load_avg] [-n]
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 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 granu-
larity 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 imme-
diately. 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 SUSE LINUX 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 relevant if cron was built to use syslog(3).
CAVEATS
In this version of cron, /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. No crontab files may be executable, or be writable by any user other than
their owner.
RELATED
crontab(1), crontab(5), pam(8)
CATEGORY