CRON(8) System Manager's Manual CRON(8)
cron - clock daemon
cron executes commands at specified dates and times. Regularly sched-
uled commands can be specified according to instructions found in
crontab files in the directory /var/spool/cron/crontabs. Users can
submit their own crontab files using the crontab(1) command. Commands
that are to be executed only once may be submitted using the at(1) com-
cron only examines crontab files and at command files during process
initialization and when a file changes using crontab or at. This
reduces the overhead of checking for new or changed files at regularly
Since cron never exits, it should only be executed once. This is nor-
mally done by running cron from the initialization process through the
file /etc/rc; see init(8). /var/spool/cron/FIFO is a FIFO file that
crontab and at use to communicate with cron; it is also used as a lock
file to prevent the execution of more than one cron.
/var/spool/cron main cron directory
/var/spool/cron/FIFO FIFO for sending messages to cron
/var/spool/cron/crontabs directory containing crontab files
at(1), crontab(1), sh(1), queuedefs(5), init(8), syslogd(8)
cron logs various errors to the system log daemon, syslogd(8), with a
facility code of cron. The messages are listed here, grouped by sever-
Can't create /var/spool/cron/FIFO: reason
cron was unable to start up because it could not create
Can't access /var/spool/cron/FIFO: reason
cron was unable to start up because it could not access
Can't open /var/spool/cron/FIFO: reason
cron was unable to start up because it could not open
Can't start cron - another cron may be running (/var/spool/cron/FIFO
cron found that /var/spool/cron/FIFO already existed when it was
started; this normally means that cron had already been started,
but it may mean that an earlier cron terminated abnormally with-
out removing /var/spool/cron/FIFO.
Can't stat /var/spool/cron/FIFO: reason
cron could not get the status of /var/spool/cron/FIFO.
Can't change directory to directory:reason
cron could not change to directory.
Can't read directory:reason
cron could not read directory.
error reading message: reason
An error occurred when cron tried to read a control message from
message received -- bad format
A message was successfully read by cron from
/var/spool/cron/FIFO, but the message was not of a form recog-
nized by cron.
received cron was told to terminate by having a SIGTERM signal
sent to it.
cron could not unlink /var/spool/cron/FIFO: reason
cron was told to terminate, but it was unable to unlink
/var/spool/cron/FIFO before it terminated.
******* CRON ABORTED ********
cron terminated, either due to an error or because it was told
Can't open queuedefs file file:reason
cron could not open a queuedefs file.
I/O error reading queuedefs file file:reason
An I/O error occurred while cron was reading a queuedefs file.
Using default queue definitions
An error occurred while trying to read a queuedefs file; the
default queue definitions will be used.
Can't allocate numberbytes of space
An internal error occurred in cron while trying to allocate mem-
queue queue max run limit reached
There were more jobs running or to be run in the queue queue
than the maximum number specified. cron will wait until one of
the currently-running jobs completes before starting to run a
MAXRUN (25) procs reached
There were more than 25 jobs running or to be run by cron. cron
will wait until one of the currently-running jobs completes
before starting to run a new one.
*** cron started ***
cron started running.
>> CMD: pid queue command job
A cron job was started, in queue queue, with process ID pid.
command is the command to be run. For at or batch jobs, job is
the job number.
>> user pid queue time job
A cron job was started for user user, in queue queue, with
process ID pid, at the date and time time. For at or batch
jobs, job is the job number.
<< user pid queue time job status
A cron job completed for user user, in queue queue, with process
ID pid, at the date and time time. For at or batch jobs, job is
the job number. If the command terminated with a non-zero exit
status or a signal, status indicates the exit status or signal.
An attempt to fork (2) to run a new job failed; cron will
attempt again after a 30-second delay.
Can't stat queuedefs file file:reason
cron could not get the status of a queuedefs file in order to
determine whether it has changed. cron will assume it has
changed and will reread it.
6 December 1988 CRON(8)