System cron
jobs are defined in crontab
-format files
in /etc/crontab
or in files in /etc/cron.d
. A
crontab
file usually consists of definitions for the
SHELL
, PATH
, MAILTO
, and
HOME
variables for the environment in which the jobs run, followed by the
job definitions themselves. Comment lines start with a #
character. Job
definitions are specified in the following format:
minute hour day month day-of-week user command
where the fields are:
minute
0-59.
hour
0-23.
day
1-31.
month
1-12 or
jan
,feb
,...,dec
.day-of-week
0-7 (Sunday is 0 or 7) or
sun
,mon
,...,sat
.user
The user to run the command as, or
*
for the owner of thecrontab
file.command
The shell script or command to be run.
For the minute
through day-of week
fields, you can use the following special characters:
*
(asterisk) All valid values for the field.
-
(dash) A range of integers, for example,
1-5
.,
(comma) A list of values, for example,
0,2,4
./
(forward slash) A step value, for example,
/3
in thehour
field means every three hours.
For example, the following entry would run a command every five minutes on weekdays:
0-59/5 * * * 1-5 * command
Run a command at one minute past midnight on the first day of the months April, June, September, and November:
1 0 1 4,6,9,11 * * command
root
can add job definition entries to /etc/crontab
,
or add crontab
-format files to the /etc/cron.d
directory.
If you add an executable job script to the /etc/cron.hourly
directory,
crond
runs the script once every hour. Your script should check that it
is not already running.
For more information, see the crontab(5)
manual page.