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cron [-n | -p | -s | -m<mailcommand>]
cron -x [ext,sch,proc,pars,load,misc,test,bit]
Cron should be started from /etc/rc.d/init.d or /etc/init.d
Cron searches /var/spool/cron for crontab files which are named after
accounts in /etc/passwd; The founded crontabs are loaded into memory.
Cron also searches for /etc/anacrontab and the files in the /etc/cron.d
directory, which are in a different format (see crontab(5) ). Cron
examines 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). Job output can
also be sent to syslog by using the -s option.
There are two ways, how the changes are checked in crontables. The
first is checking the modtime of file and the other is using inotify
support. You can find out which of them are you using, if you check
/var/log/cron where is (or isn't) inotify mentioned after start of dae-
mon. The inotify support is watching for changes in all crontables and
touch the disk only in case that something was changed.
In other case cron checks each minute to see if its crontables modtime
have changes and reload those which have changes. There is no need to
restart cron after some of the crontable is modified. The modtime
option is used also when inotify couldn't be initialized.
Cron is checking those files or directories: /etc/anacrontab system
crontab is usually for running daily, weekly, monthly jobs.
/etc/cron.d/ where are system cronjobs stored for different users.
/var/spool/cron that's mean spool directory for user crontables.
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.
log using syslog(3). This is useful if your system has no send-
mail(8), or if mail is disabled using -m off.
-m This option allows you to specify a shell command string to use
for sending cron mail output instead of sendmail(8). This com-
mand must accept a fully formatted mail message (with headers)
on stdin and send it as a mail message to the recipients speci-
fied 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
-p Cron permit any crontab, which user set.
-x With this option is possible to set debug flags.
-P Don't set PATH. PATH is instead inherited from the environment.
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).
The crontab files have to be regular files or symlinks to regular
files, they must not be executable or writable by anyone else than the
owner. This requirement can be overridden by using the -p option on
the crond command line. If inotify support is in use, changes in the
symlinked crontabs are not automatically noticed by the cron daemon.
The cron daemon must receive a SIGHUP to reload the crontabs. This is
a limitation of inotify API.
crontab(1), crontab(5), inotify(7), pam(8)
Paul Vixie <firstname.lastname@example.org>
Marcela Maslanova <email@example.com>
Marcela Maslanova December 2009 CRON(8)