crond - daemon to execute scheduled commands
crond [-c | -h | -i | -n | -p |
-P | -s | -m<mailcommand>]
crond -x [ext,sch,proc,pars,load,misc,test,bit]
Cron is started from /etc/rc.d/init.d or /etc/init.d when
classical sysvinit scripts are used. In case systemd is enabled, then unit
file is installed into /lib/systemd/system/crond.service and daemon is
started by systemctl start crond.service command. It returns
immediately, thus, there is no need to need to start it with the '&'
Cron searches /var/spool/cron for crontab files
which are named after accounts in /etc/passwd; The found crontabs are
loaded into the memory. Cron also searches for /etc/anacrontab
and any files in the /etc/cron.d directory, which have a different
format (see crontab(5)). Cron examines all stored crontabs and
checks each job to see if it needs to be run in the current minute. When
executing commands, any output is mailed to the owner of the crontab (or to
the user specified in the MAILTO environment variable in the crontab,
if such exists). Any job output can also be sent to syslog by using the
There are two ways how changes in crontables are checked. The
first method is checking the modtime of a file. The second method is using
the inotify support. Using of inotify is logged in the /var/log/cron
log after the daemon is started. The inotify support checks for changes in
all crontables and accesses the hard disk only when a change is
When using the modtime option, Cron checks its crontables'
modtimes every minute to check for any changes and reloads the crontables
which have changed. There is no need to restart Cron after some of
the crontables were modified. The modtime option is also used when inotify
can not be initialized.
Cron checks these files and directories:
- system crontab. Nowadays the file is empty by default. Originally it was
usually used to run daily, weekly, monthly jobs. By default these jobs are
now run through anacron which reads /etc/anacrontab configuration
file. See anacrontab(5) for more details.
- directory that contains system cronjobs stored for different users.
- directory that contains user crontables created by the crontab
Note that the crontab(1) command updates the modtime of the
spool directory whenever it changes a crontab.
Local time changes of less than three hours, such as those caused by the
Daylight Saving Time changes, are handled in a special way. This only applies
to jobs that run at a specific time and jobs that run with a granularity
greater than one hour. Jobs that run more frequently are scheduled normally.
If time was adjusted one hour forward, those jobs that would have
run in the interval that has been skipped will be run immediately.
Conversely, if time was adjusted backward, running the same job twice is
Time changes of more than 3 hours are considered to be corrections
to the clock or the timezone, and the new time is used immediately.
It is possible to use different time zones for crontables. See
crontab(5) for more information.
Cron supports access control with PAM if the system has PAM installed.
For more information, see pam(8). A PAM configuration file for
crond is installed in /etc/pam.d/crond. The daemon loads the PAM
environment from the pam_env module. This can be overridden by defining
specific settings in the appropriate crontab file.
When the SIGHUP is received, the Cron daemon will close and reopen
its log file. This proves to be useful in scripts which rotate and age log
files. Naturally, this is not relevant if Cron was built to use
In this version of Cron it is possible to use a network-mounted shared
/var/spool/cron across a cluster of hosts and specify that only one of
the hosts should run the crontab jobs in this directory at any one time. This
is done by starting Cron with the -c option, and have the
/var/spool/cron/.cron.hostname file contain just one line, which
represents the hostname of whichever host in the cluster should run the jobs.
If this file does not exist, or the hostname in it does not match that
returned by gethostname(2), then all crontab files in this directory
are ignored. This has no effect on cron jobs specified in the
/etc/crontab file or on files in the /etc/cron.d directory.
These files are always run and considered host-specific.
- Prints a help message and exits.
- Disables inotify support.
- This option allows you to specify a shell command to use for sending
Cron mail output instead of using sendmail(8) This command
must accept a fully formatted mail message (with headers) on standard
input and send it as a mail message to the recipients specified in the
mail headers. Specifying the string off (i.e., crond -m off) will
disable the sending of mail.
- Tells the daemon to run in the foreground. This can be useful when
starting it out of init. With this option is needed to change pam setting.
/etc/pam.d/crond must not enable pam_loginuid.so
- Allows Cron to accept any user set crontables.
- Don't set PATH. PATH is instead inherited from the environment.
- This option enables clustering support, as described below.
- This option will direct Cron to send the job output to the system
log using syslog(3). This is useful if your system does not have
sendmail(8), installed or if mail is disabled.
- This option allows you to set debug flags.
- Print version and exit.
Rather than editing /var/spool/cron/.cron.hostname
directly, use the -n option of crontab(1) to specify the
You should ensure that all hosts in a cluster, and the file server
from which they mount the shared crontab directory, have closely
synchronised clocks, e.g., using ntpd(8), otherwise the results will
be very unpredictable.
Using cluster sharing automatically disables inotify support,
because inotify cannot be relied on with network-mounted shared file
All crontab files have to be regular files or symlinks to regular files,
they must not be executable or writable for anyone else but 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 signal to reload the crontabs. This is a limitation of the inotify
The syslog output will be used instead of mail, when sendmail is
crontab(1), crontab(5), inotify(7), pam(8)