cron(1M)

cron -- clock daemon

Synopsis

/usr/sbin/cron [nofork]

Description

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 via the crontab command. Commands to be executed only once may be submitted via the at command.

cron normally forks itself and places itself into the background. When the nofork option is specified, this initial fork is not performed. This would allow cron to be started via inittab rather than during system boot time.

The commands are not executed if, at the time of execution, the requesting user ID or level ID has been deleted, or if the level is no longer a valid login level for the requesting user.

cron only examines crontab files and at command files during process initialization and when a file changes via the crontab or at commands. This reduces the overhead of checking for new or changed files at regularly scheduled intervals.

Because cron never exits, it should be executed only once, normally through /etc/rc2.d/S75cron at system boot time. The file /etc/cron.d/LCK_CRON is used as a lock file to prevent the execution of more than one cron.

Use the following files, described in cron(4), to control cron:

The variable MAXRUN in /etc/default/cron defines the number of concurrent cron jobs that are allowed to run. The default is 25.
To specify whether cron is to log all actions, set CRONLOG to YES or to NO in /etc/default/cron. cron usually creates huge log files.
Specify the number of concurrent jobs per user, their priority, and a retry interval in the file /etc/cron.d/queuedefs.

Files

/etc/cron.d: main cron directory
/etc/cron.d/queuedefs: concurrency, priority, retry options file
/etc/cron.d/LCK_CRON: lock file
/etc/default/cron: log options file
/var/cron/log: accounting information
/var/spool/cron: spool area
/usr/lib/locale/locale/LC_MESSAGES/uxcore: language-specific message file (see LANG in environ(5)).

Diagnostics

A history of all actions taken by cron is recorded in /var/cron/log if logging is turned on.

References

at(1), cron(4), crontab(1)

Notices

Changing the time of day clock on the system affects the execution of jobs specified in crontab files. If the clock is moved ahead, the jobs scheduled for the skipped time interval will not be run. If the clock is moved back, the jobs already executed in the time interval to be revisited will be run a second time. Adjusting the system time because of standard or alternate time zone changes, including daylight time changes, causes this problem.