Cron - Program Scheduler
Your Virtual Server includes access to the cron daemon which is used
to automatically execute scheduled commands. The list of commands you
schedule cron to run is stored in a cron table, or "crontab" for
short. Using your own crontab, you can schedule your Virtual Server to
run a certain command at a predetermined date and time.
Creating a Crontab
Your Virtual Server crontab can be stored in any file you choose. For
example, you may wish to create or upload a file called cronjobs in
your Virtual Server home directory for your cron table. Each line in your
crontab will either be an environment variable setting, a cron table
entry, or a comment (beginning with the "#" character).
An environment variable setting in a crontab looks like this:
NAME = VALUE
Several environment variables are set up automatically by the cron
daemon, depending on your Virtual Server O/S.
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The cron environment variables include SHELL, LOGNAME, USER, and HOME. In addition
to these, there is a special MAILTO environment variable.
Any output generated by your cron jobs will be sent to the
address specified by MAILTO (if it is not specified it will be sent
to the owner of the crontab). If MAILTO is defined as an empty string
then no mail will be sent.
MAILTO = ""
The format of a cron table entry includes five (5) time fields followed
by a command. Commands are executed when the time specified by the
date fields matches the current time. The five time fields are as
Field Allowed Values
Day of Month 1-31
Month 1-12, jan, feb, mar, apr, may, jun, jul, aug, sep, oct,
Day of Week 0-7, sun, mon, tue, wed, thu, fri, sat (0 and 7 are "sun")
A field may be an asterisk (*), which indicates all values in the range
are acceptable. Ranges of numbers are allowed, i.e. "2-5" or "8-11", and
lists of numbers are allowed, i.e. "1,3,5" or "1,3,8-11". Step values
can be represented as a sequence, i.e. "0-59/15", "1-31/3", or "*/2".
The actual command you wish to execute, including any parameters to be passed to it,
is the sixth, and final field of a cron table entry.
NOTE: Each cron table entry must have a trailing line break
in order for the cron table entry to be
Some examples of complete cron table entries are show below, implementing
the vnukelog command as an example.
# Any output generated by the cron entries below is sent to the e-mail
# address assigned to the MAILTO environment variable.
# Execute the "vnukelog" command at 1:15 (15 1) AM every day.
15 1 * * * /usr/local/bin/vnukelog
# Execute the "vnukelog" command at 11:40 PM (40 23) on the first day (1)
# of each month.
40 23 1 * * /usr/local/bin/vnukelog
# Execute the "vnukelog" command every 10 minutes for for the first
# half-hour (0-30/10) of the 9:00 AM and 5:00 PM hours (9,17) on
# Monday-Friday (1-5).
0-30/10 9,17 * * 1-5 /usr/local/bin/vnukelog
# Execute the "vnukelog" command at 4:00 AM, 8:00 AM, 12:00 noon, 4:00 PM,
# and 8:00 PM (0 */4) on each Sunday (sun) every January (jan).
0 */4 * jan sun /usr/local/bin/vnukelog
# Execute the "vnukelog" command at 4:30 AM (30 4) on the first, fifteenth
# (1,15), and each Friday (fri) of every month.
30 4 1,15 * fri /usr/local/bin/vnukelog
# Execute the "vnukelog" command at 12:00 midnight (0 0) on August 19 (8)
0 0 19 8 * /usr/local/bin/vnukelog
0 0 19 aug * /usr/local/bin/vnukelog
Installing a Crontab
After you have defined the cron table entries in your cronjobs file, you
will need to register your crontab with the system. This can be done
by running the crontab command. For example, if you created
your crontab file as cronjobs in your Virtual Server home directory
then you would use the following command:
% crontab ~/cronjobs
This will register your crontab file with the cron system daemon.
If you ever need to review the current cron entries you have registered
with the cron system daemon, you need simply use this command:
% crontab -l
Issue the following commands to view the cron and crontab
% man cron
% man crontab
% man 5 crontab
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Last Modified: Wed Apr 11 15:43:30 2001 GMT
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