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

/etc/cron.d

main cron directory

/etc/cron.d/FIFO

used as a lock file

/etc/default/cron

contains cron default settings

/var/cron/log

cron history information

/var/spool/cron

spool area

/etc/cron.d/logchecker

moves log file to /var/cron/olog if log file exceeds system ulimit.

/etc/cron.d/queuedefs

queue description file for at, batch, and cron.

See attributes(5) for descriptions of the following attributes:

at(1), crontab(1), sh(1), queuedefs(4), shadow(4), attributes(5)

A history of all actions taken by cron is stored in /var/cron/log and (possibly) /var/cron/olog.