cron starts a process that executes commands at specified dates and times.
You can specify regularly scheduled commands to cron 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 can 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.
As cron never exits, it should be executed only once. This is done routinely by way of the /etc/rc2.d/S75cron file at system boot time. The file /etc/cron.d/FIFO file is used 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 not empty, mails the output to the user. If the job does not produce output, no mail is sent to the user. An exception is if the job is an at(1) job and the -m option was specified when the job was submitted.
cron and at jobs are not executed if your account is locked. Jobs and processses execute. The shadow(4) file defines which accounts are not locked and will have their jobs and processes executed.
The timezone of the cron daemon sets the system-wide timezone for cron entries. This, in turn, is by set by default system-wide using /etc/default/init.
If some form of daylight savings or summer/winter time is in effect, then jobs scheduled during the switchover period could be executed once, twice, or not at all.
To keep a log of all actions taken by cron, you must specify CRONLOG=YES in the /etc/default/cron file. If you specify CRONLOG=NO, no logging is done. Keeping the log is a user configurable option since cron usually creates huge log files.
You can specify the PATH for user cron jobs by using PATH= in /etc/default/cron. You can set the PATH for root cron jobs using SUPATH= in /etc/default/cron. Carefully consider the security implications of setting PATH and SUPATH.
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 continue to use /usr/sbin:/usr/bin.
The cron log file is periodically rotated by logadm(1M).
Main cron directory
cron default settings file
cron history information
Queue description file for at, batch, and cron
Configuration file for logadm
See attributes(5) for descriptions of the following attributes:
A history of all actions taken by cron is stored in /var/cron/log and possibly in /var/cron/olog.