The cron command starts a process that executes commands at specified dates and times. Regularly scheduled commands
can be specified according to instructions found in crontab files in the directory /var/spool/cron/crontabs. Users can submit their own crontab file using the crontab(1) command. Commands which are to be executed only once may be submitted using the at(1) command.
cron only examines crontab or at command files during its own process initialization phase and when the crontab or at command is run. This reduces the overhead of checking for new or changed files at
regularly scheduled intervals.
Since cron never exits, it should be executed only once. This is done routinely through /etc/rc2.d/S75cron at system boot time. The file /etc/cron.d/FIFO is used (among other things) as a lock file to prevent the execution of more than one
instance of cron.
cron captures the output of the job's stdout and stderr streams, and, if it is non-empty, mails the output to the user. If the job does not produce output, no mail is sent to the user (unless the job is an at(1) job and the -m option was specified when the job was submitted).
cron and at jobs will be not be executed if the user's account is locked. Only accounts which are not locked as defined in shadow(4)
will have their job or process executed.
Setting cron Defaults
To keep a log of all actions taken by cron, CRONLOG=YES (by default) must be specified in the /etc/default/cron file. If CRONLOG=NO is specified, no logging is done. Keeping
the log is a user configurable option since cron usually creates huge log files.
The PATH for user cron jobs can be set using PATH= in /etc/default/cron. The PATH for root cron jobs can be set using SUPATH= in /etc/default/cron. The security implications of setting PATH and SUPATH should be carefully considered.
Example /etc/default/cron file:
This example enables logging and sets the default PATH used by non-root jobs to /usr/bin:/usr/ucb:. Root jobs will continue to use /usr/sbin:/usr/bin.
/etc/cron.d/logchecker is a script that checks to see if the log file has exceeded the system ulimit. If so, the log file is moved to /var/cron/olog.