CRON(8) System Manager's Manual CRON(8)

cron — clock daemon

cron [-n] [-l load_avg]

The cron daemon schedules commands to be run at specified dates and times. Commands that are to be run periodically are specified within crontab(5) files. Commands that are only to be run once are scheduled via the at(1) and batch(1) commands. Normally, the cron daemon is started from the /etc/rc command script. Because it can execute commands on a user's behalf, cron should be run late in the startup sequence, as close to the time when logins are accepted as possible.

cron loads crontab(5) and at(1) files when it starts up and also when changes are made via the crontab(1) and at(1) commands. Additionally, cron checks the modification time on the system crontab file (/etc/crontab), the crontab spool (/var/cron/tabs), and the at spool (/var/cron/atjobs) once a minute. If the modification time has changed, the affected files are reloaded.

Any output produced by a command is sent to the user specified in the MAILTO environment variable as set in the crontab(5) file or, if no MAILTO variable is set (or if this is an at(1) or batch(1) job), to the job's owner. If a command produces no output or if the MAILTO environment variable is set to the empty string, no mail will be sent. The exception to this is at(1) or batch(1) jobs submitted with the -m flag. In this case, mail will be sent even if the job produces no output.

Local time changes of less than three hours, such as those caused by the start or end of Daylight Saving Time, are handled specially. This only applies to jobs that run at a specific time and jobs that are run with a granularity greater than one hour. Jobs that run more frequently are scheduled normally.

If time has moved forward, those jobs that would have run in the interval that has been skipped will be run immediately. Conversely, if time has moved backward, care is taken to avoid running jobs twice.

Time changes of more than 3 hours are considered to be corrections to the clock or time zone, and the new time is used immediately.

The options are as follows:

load_avg
If the current load average is greater than load_avg, batch(1) jobs will not be run. The default value is 1.5. To allow batch(1) jobs to run regardless of the load, a value of 0.0 may be used.
By default, cron will detach from the current tty and become a daemon. The -n option disables this behavior and causes it to run in the foreground.

/etc/crontab
system crontab file
/var/cron/atjobs
directory containing at(1) jobs
/var/cron/log
cron's log file
/var/cron/tabs
directory containing individual crontab files
/var/run/cron.sock
used by crontab(1) to tell cron to check for crontab changes immediately

at(1), crontab(1), syslog(3), crontab(5)

Paul Vixie <vixie@isc.org>

crontab(5) files will be ignored if they do not have the proper file mode. For user crontab files created by crontab(1), the mode must be 0600. If the system crontab file is used, /etc/crontab must not be writable by any user other than root and must not have the execute, set-user-ID, set-group-ID or sticky bits set.

January 25, 2019 Debian