overview of cron

Overview of the cron daemon
The cron daemon is where all timed events are initiated. It is executed upon system initialization and remains active while the system is operating in multi-user mode. Cron wakes up every minute and examines all the stored configuration files, called crontabs, to check each them for commands that may be scheduled to be executed at the current time. Some systems have limits to the number of tasks that can be scheduled during the one minute time period.

Most notably, because of the low number, is SGI's IRIX 5.3 which has a limit of 25 jobs.

Besides starting commands each minute, some cron daemons also check to see if its spool directory's last modified time has been updated. If it has, cron will check the modification time on all crontabs and reread the ones that have been modified. Other cron daemons examine new crontab files when first initialized and when the commands crontab or at are executed. This reduces the overhead of checking for new or changed files at regularly scheduled intervals.

Cron searches the crontab spool directory for crontab files. These files are named after user accounts. For instance, if the system administrator is logged into the root accounts creates a crontab file, it will be named root and will be placed in the crontab spool directory. If Bill Wilson, whose username is bill, creates a crontab file it is named bill in the crontab spool directory. 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).

The configuration files used to control the operation of cron are called crontab files or cron tables. These files contain information about the time, date and command to execute. Different versions of UNIX store cron and support files in different locations and may use a slightly different format.

IRIX 5.3

 /usr/lib/cron             main cron directory
 /usr/lib/cron/FIFO        used as a lock file
 /usr/lib/cron/log         accounting information
 /usr/spool/cron           spool area
 /usr/lib/cron/queuedefs   queue description file for at, batch and cron
 /usr/lib/cron/cron.allow  grant access to the cron facility
 /usr/lib/cron/cron.deny   revoke access to the cron facility

 

SunOS 5.X

 /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

/etc/cron.d/cron.allow   grant access to the cron facility 

/etc/cron.d/cron.deny    revoke access to the cron facility

SunOS 4.X
 /var/spool/cron             main cron directory

/var/spool/cron/FIFO        FIFO for sending messages to cron

/var/spool/cron/cron.allow  grant access to the cron facility 

/var/spool/cron/cron.deny   revoke access to the cron facility

Linux
 /var/spool/cron               main cron directory 

/var/spool/cron/cron.allow    grant access to the cron facility 

/var/spool/cron/cron.deny     revoke access to the cron facility

HP-UX
 /var/adm/cron                 main cron directory

/var/spool/cron/atjobs        Directory containing at and batch job files 

/var/spool/cron/crontabs      Directory containing crontab files

/var/adm/cron/log             Accounting information

/var/spool/cron/queuedefs     queue description  file for at, batch, and cron 

/var/adm/cron/cron.allow      grant access to the cron facility

/var/adm/cron/cron.deny       revoke access to the cron facility

jerry.lee | 제리
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