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
is started from the /etc/rc command script. Because
it can execute commands on a user's behalf,
should be run late in the startup sequence, as close to the time when logins
are accepted as possible.
crontab(5) and at(1) files when it starts up and also when changes are made
via the crontab(1) and
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
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
MAILTO environment variable is set to the
empty string, no mail will be sent. The exception to this is
or batch(1) jobs submitted with the
-m flag. In
this case, mail will be sent even if the job produces no output.
Daylight Saving Time and other time changes
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:
- 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,
cronwill detach from the current tty and become a daemon. The
-noption disables this behavior and causes it to run in the foreground.
- system crontab file
- directory containing at(1) jobs
- cron's log file
- directory containing individual crontab files
- used by crontab(1) to tell
cronto check for crontab changes immediately
at(1), crontab(1), syslog(3), crontab(5)
Paul Vixie <email@example.com>
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 0400 or 0600. If the system crontab file is used, /etc/crontab must not be writable by any user other than root.