Cron
should be started from
/etc/rc.d/init.d
or
/etc/init.d
will return immediately, so you don't need to start it with '&'.
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 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.
OPTIONS
-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.
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
, 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.
