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man pages section 8: System Administration Commands

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Updated: Thursday, June 13, 2019
 
 

cron(8)

Name

cron - clock daemon

Synopsis

/usr/sbin/cron 

Description

cron starts a process that executes commands at specified dates and times.

You can specify regularly scheduled commands to cron according to instructions found in crontab files in the directory /var/spool/cron/crontabs. Users can submit their own crontab file using the crontab(1) command. Commands which are to be executed only once can be submitted using the at(1) command.

cron only examines crontab or at command files during its own process initialization phase and when the crontab or at command is run. This reduces the overhead of checking for new or changed files at regularly scheduled intervals.

As cron never exits, it should be executed only once. This is done routinely by way of the svc:/system/cron:default service. The file /etc/cron.d/FIFO file is used as a lock file to prevent the execution of more than one instance of cron.

cron captures the output of the job's stdout and stderr streams, and, if it is not empty, mails the output to the user. If the job does not produce output, no mail is sent to the user. An exception is if the job is an at(1) job and the –m option was specified when the job was submitted.

cron and at jobs are not executed if your account is locked or expired. Consult shadow(5) to determine which accounts are not locked and not expired.

Setting cron Jobs Across Timezones

The timezone of the cron daemon sets the system-wide timezone for cron entries. This, in turn, is by set by default system-wide using /etc/default/init. The timezone for cron entries can be overridden in a user's crontab file; see crontab(1).

If some form of daylight savings or summer/winter time is in effect, then jobs scheduled during the switchover period could be executed once, twice, or not at all.

Setting cron Defaults

To keep a log of all actions taken by cron, you must specify CRONLOG=YES in the /etc/default/cron file. If you specify CRONLOG=NO, no logging is done. Keeping the log is a user configurable option since cron usually creates huge log files.

You can specify the PATH for user cron jobs by using PATH= in /etc/default/cron. You can set the PATH for root cron jobs using SUPATH = in /etc/default/cron. Carefully consider the security implications of setting PATH and SUPATH.

Example /etc/default/cron file:


CRONLOG=YES
PATH=/usr/bin

This example enables logging and sets the default PATH used by non-root jobs to /usr/bin. Root jobs continue to use /usr/bin.

The cron log file is periodically rotated by logadm(8).

Files

/etc/cron.d

Main cron directory

/etc/cron.d/FIFO

Lock file

/etc/default/cron

cron default settings file

/var/cron/log

cron history information

/var/spool/cron

Spool area

/etc/cron.d/queuedefs

Queue description file for at, batch, and cron

/etc/logadm.conf

Configuration file for logadm

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Availability
system/core-os

See Also

at(1), crontab(1), sh(1), svcs(1), queuedefs(5), shadow(5), attributes(7), rbac(7), smf(7), smf_security(7), logadm(8), svcadm(8)

Notes

A cron job is executed in a separate invocation of the shell, running in a separate process group with no controlling terminal. Open file descriptors, traps and priority inherited from the invoking environment are lost.

The cron service is managed by the service management facility, smf(7), under the service identifier:

svc:/system/cron:default

Administrative actions on this service, such as enabling, disabling, or requesting restart, can be performed using svcadm(8). The service's status can be queried using the svcs(1) command. Most administrative actions may be delegated to users with the solaris.smf.manage.cron authorization (see rbac(7) and smf_security(7)).

Diagnostics

A history of all actions taken by cron is stored in /var/cron/log and possibly in /var/cron/olog.