CRON(8) — UNIX Programmer’s Manual

NAME

cron − clock daemon

SYNOPSIS

/usr/sbin/cron

DESCRIPTION

Cron executes commands at specified dates and times according to instructions in the files stored in /var/spool/cron/crontabs and /var/spool/cron/atjobs.
A user can create a crontab file using the crontab(1) command. A command which only needs to be executed once can be submitted using at(1) or batch(1). Cron only updates its queues of jobs to be run when it starts up and when one of the job files is created or removed using at, batch or crontab. Since cron never exits, it should only be executed once. This is best done by running cron from the initialization process through the file /etc/rc; see init(8).

cron communicates with crontab and at using a FIFO, /var/spool/cron/crontabs/FIFO. This file is also used to make sure that only one instance of cron is running on a system, i.e. cron will fail if it finds that this file already exists.

FILES

/var/spool/cron/FIFO         FIFO used by cron, crontab and at
/var/spool/cron/queuedefs    queue description file
/var/spool/cron/.proto       used by at(1) for creating job files
/var/spool/cron/crontabs     directory containing crontab files
/var/spool/cron/atjobs       directory containing atjob files

DIAGNOSTICS

Error messages are handled by syslogd (facility name ‘cron’).

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