daemon schedules commands to be run at
specified dates and times. Commands that are to be run periodically are
Commands that are only to be run once are scheduled via the
Normally, the cron
daemon is started from the
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
files when it
starts up and also when changes are made via the
checks the modification time
on the system crontab file (/etc/crontab
crontab spool (/var/cron/tabs
), and the at spool
) 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
environment variable as set in the
variable is set (or if this is
job), to the
job's owner. If a command produces no output or if the
environment variable is set to the
empty string, no mail will be sent. The exception to this is
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:
- If the current load average is greater than
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.
- system crontab file
- directory containing
- cron's log file
- directory containing individual crontab files
- used by
tell cron to check for crontab changes
be ignored if they do not have the proper file mode. For user crontab files
mode must be 0400 or 0600. If the system crontab file is used,
must not be writable by any user
other than root.