Each Virtual Server can load its own cron job to execute scheduled tasks. The most effective way to use cron is to load the scheduled tasks into the cron daemon from a file that you have created and stored on the Virtual Server. Although it is possible to manipulate cron directly, loading cron jobs from pre-formatted files will ensure that you have a copy of the file around for editing and for archival purposes. A common place to put such a cron file is in a directory called cronfiles in your ~/etc directory.

1. Connect to your Virtual Server via Telnet.
2. Type:

   % cd ~/etc
   % mkdir cronfiles

   You can then store the file(s) holding your cron information in this directory. After you have made the cron file, you need to load it into the cron program (daemon).

% cd ~/etc/cronfiles

If you have placed a cron file in the directory named my_cron_file, load the file into the cron program by typing:

% crontab my_cron_file

A copy of the cron file you created is in memory in the cron program. To view cron 's copy in memory, you can call the cron program with the -l (list) option:

% crontab -l

cron has other command line options such as "edit" and "remove". These commands will allow you to manipulate the information that cron has in memory. For example, if you wanted to add another event to the cron information, you can use the crontab -e option:

% crontab -e

This will take the copy of the entry that is stored in the cron programs memory, and allow you to edit it. This is, however, a less preferable option than changing the physical file and re-loading it into cron , because the changes are not physically stored anywhere accept in cron 's memory.

% crontab -r

This removes the cron entry you just loaded.

Note: If you created a cron entry with crontab -e and your run crontab -r, you will lose your cron entry forever. This is a good reason to keep a physical copy of your cron file and load it into memory.

Creating cron Files
In a cron file, blank lines are ignored. Lines that have a pound sign (#) as the first character are considered comments. There are two types of cron entries: environment variables and cron commands.

Environment Variables
Environment variables have the form:

name = value

The spaces around the equal sign are optional and any spaces in the "value" will be included in the value being set. The value string may be placed in quotes (either single or double) to preserve leading or trailing spaces.

One environment variable that can be set is the MAILTO variable. If MAILTO is defined, any mail that is sent by cron , such as error notifications, are sent to the address assigned to the variable. If this value is not explicitly defined, error mail messages will be sent to the Virtual Server's administrator login name. For example, if your Virtual Server's administrator login name (i.e., Telnet login name) were "judy", administrative e-mail from the cron daemon would be sent to judy@yourcompany.com. An example MAILTO entry might look like:

MAILTO=johndoe@yourcompany.com

If MAILTO is defined as follows, no mail will be sent from cron :
MAILTO=""

An asterisk may be used as a wildcard meaning "first through last". The asterisk is used when you want an event to occur for every allowable value. For example, if you wanted to schedule your log files to be purged on a monthly basis you could place an asterisk in the Day of Month field. As you might imagine, it would be unwise to put an asterisk in the Minute field of the cron file as it may cause too much of a load on your Virtual Server.

Ranges such as two numbers separated with a hyphen ("-") are allowed. For example, if you wanted the cron to send you e-mail to warn you that your taxes are due April 15th, and you want to be warned starting in January until they are due in April, you could create a cron file with the value 1-4 in the month field, and the cron would run starting in January until April. You can specify a list of values by separating the numbers with a comma. For example, 1,7,9,10 would be the months January, July, September, and October. Skip values can be specified with the / sign. For example, 1-12/2 would be every other month. Names can also be used for the month and day of the week fields. The first three letters of the month or day can be used. This option is not allowable with ranges or lists.

Here are some additional examples of valid time/date values:

The sixth field in a cron file (i.e., rest of the cron line) are where you place the command you want to run. The entire command portion, up to the newline character or the % character will be executed by /bin/sh (or the shell you have specified with the SHELL environmental variable). Percent signs in the command, unless they are escaped with a backslash (\) will be changed into newline characters and all data after the first % will be sent to the command as standard input.

Example cron for mailing a notice about taxes:

# This is a comment.
SHELL=/bin/csh
MAILTO=johndoe@yourcompany.com
5 22 14 1-4 * mail -s "Your taxes are due on April 15th"
judy@yourcompany.com%Judy,%%Fill out your taxes!%

Note: Do not place hard returns in cron commands, because the line wraps on its own. Hard returns tell cron that the end of the cron command has occurred.

Example cron for deleting logs monthly:

MAILTO=johndoe@yourcompany.com
1 3 * * * /usr/local/bin/virtual /usr/local/bin/vnukelog -r

Notice the use of the virtual command in the above example. The virtual command is used to run scripts from the user's home directory. It should be pointed out here that CRON jobs do not run in the Virtual Server's environment. They run in the physical server's environment, but they run under the Virtual Server's User ID (a special number that keeps track of users, what files they own, and what processes they own). For this reason, when you try and run scripts or programs from cron , you must include the full path to the script. This includes the path to your home directory. For example, if my Telnet login were "judy", the path to my home directory would be /usr/home/judy/. This is the path from the physical server's root file structure.

Example cron for sending a notice to occasionally mail information to judy:

01 09 14,30 1,3,5,7,8,10,12 * cat $HOME/etc/ cron file/my_ cron _file | /usr/bin/mail -s "Message goes here" judy@yourcompany.com

Example cron for automating stats with getstats:

40 19 * * * /usr/local/bin/getstats -d -f | /usr/bin/mail -s "HTTP Daily stats" judy@yourcompany.com