NAME

cron − clock daemon

SYNOPSIS

/usr/sbin/cron

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 which are to be executed only once may be submitted via the at command. 

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. 

Since cron never exits, it should be executed only once.  This is done routinely through /etc/rc2.d/S75cron at system boot time.  /etc/cron.d/FIFO is used as a lock file to prevent the execution of more than one cron. 

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. 

FILES

/etc/cron.d main cron directory

/etc/default/cron used to maintain a log

/etc/cron.d/FIFO used as a lock file

/var/cron/log accounting information

/var/spool/cron spool area

SEE ALSO

at(1), crontab(1), sh(1). 

NOTES

Note that resetting the system time may cause a given clock time to not occur on a particular day or to occur more than once in a day, causing crontab(1) entries for those times to be omitted or repeated.  Note particularly that such omissions or repetitions may occur in places where seasonal time changes are observed.  cron also makes no provision for times that are skipped because, for instance, the system is shut down.  If it is critical that particular jobs be run regularly, a more robust mechanism should be used, such as scheduling them in cron to run hourly and including a mechanism in the application to determine whether each invocation is actually necessary. 

Note that if daylight savings time is observed, commands specified for a certain time are scheduled by cron to be executed in whichever time zone will be in effect at the time of execution, provided that that time exists in that time zone.  Otherwise, the time of execution is adjusted to an appropriate time in the new time zone.  For example, if daylight savings time ends at 2am and the job is scheduled during daylights savings time zone for 1:30am, then the job will actually run at 1:30am daylight savings time -- the first 1:30am of the night.  The same job will not be rescheduled again for the following hour in the new time zone.  It will be executed only once that night. 

If a particular job falls in the non-existent hour on the day that daylight savings time begins, it is scheduled by cron to be executed at the time that is nominally one hour later than requested, but in daylight savings time.  For example, if daylight savings time begins at 2am and the job is scheduled for 2:30am, then the job will actually run at 3:30am daylight savings time. 

DIAGNOSTICS

A history of all actions taken by cron are recorded in /var/cron/log. 

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