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. These 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
follows:
Field Allowed Values
----- --------------
Minute 0-59
Hour 0-23
Day of Month 1-31
Month 1-12, jan, feb, mar, apr, may, jun, jul, aug, sep, oct,
nov, dec
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 recognized. |
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.
MAILTO="webmaster@mycompany.com"
# 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 * /usr/local/bin/vnukelog
# Execute the "vnukelog" command at 12:00 midnight (0 0) on
August 19 (8)
# (aug).
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
Documentation
Issue the following commands to view the cron and crontab
man pages.
% man cron
% man crontab
% man 5 crontab
