12.6 Configuring the cron Utility

Contributed by Tom Rhodes.

One of the most useful utilities in FreeBSD is cron(8). The cron utility runs in the background and constantly checks the /etc/crontab file. The cron utility also checks the /var/cron/tabs directory, in search of new crontab files. These crontab files store information about specific functions which cron is supposed to perform at certain times.

The cron utility uses two different types of configuration files, the system crontab and user crontabs. These formats only differ in the sixth field and later. In the system crontab, cron will run the command as the user specified in the sixth field. In a user crontab, all commands run as the user who created the crontab, so the sixth field is the last field; this is an important security feature. The final field is always the command to run.

Note: User crontabs allow individual users to schedule tasks without the need for root privileges. Commands in a user's crontab run with the permissions of the user who owns the crontab.

The root user can have a user crontab just like any other user. The root user crontab is separate from /etc/crontab (the system crontab). Because the system crontab effectively invokes the specified commands as root there is usually no need to create a user crontab for root.

Let us take a look at /etc/crontab, the system crontab:

# /etc/crontab - root's crontab for FreeBSD
# $FreeBSD: src/etc/crontab,v 1.32 2002/11/22 16:13:39 tom Exp $
# (1)
PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin (2)
#minute	hour	mday	month	wday	who	command (3)
*/5	*	*	*	*	root	/usr/libexec/atrun (4)
Like most FreeBSD configuration files, lines that begin with the # character are comments. A comment can be placed in the file as a reminder of what and why a desired action is performed. Comments cannot be on the same line as a command or else they will be interpreted as part of the command; they must be on a new line. Blank lines are ignored.
First, the environment must be defined. The equals (=) character is used to define any environment settings, as with this example where it is used for the SHELL, PATH, and HOME options. If the shell line is omitted, cron will use the default, which is sh. If the PATH variable is omitted, no default will be used and file locations will need to be absolute. If HOME is omitted, cron will use the invoking users home directory.
This line defines a total of seven fields. Listed here are the values minute, hour, mday, month, wday, who, and command. These are almost all self explanatory. minute is the time in minutes the command will be run. hour is similar to the minute option, just in hours. mday stands for day of the month. month is similar to hour and minute, as it designates the month. The wday option stands for day of the week. All these fields must be numeric values, and follow the twenty-four hour clock. The who field is special, and only exists in the /etc/crontab file. This field specifies which user the command should be run as. The last field is the command to be executed.
This last line will define the values discussed above. Notice here we have a */5 listing, followed by several more * characters. These * characters mean “first-last”, and can be interpreted as every time. So, judging by this line, it is apparent that atrun is to be invoked by root every five minutes regardless of what day or month it is. For more information on atrun, see atrun(8).

Commands can have any number of flags passed to them; however, commands which extend to multiple lines need to be broken with the backslash “\” continuation character.

This is the basic setup for every crontab file, although there is one thing different about this one. Field number six, where we specified the username, only exists in the system crontab. This field should be omitted for individual user crontab files.

12.6.1 Installing a Crontab

Important: Do not use the procedure described here to edit and install the system crontab, /etc/crontab. Just use your favorite editor: the cron utility will notice that the file has changed and immediately begin using the updated version. See this FAQ entry for more information.

To install a freshly written user crontab, first use your favorite editor to create a file in the proper format, and then use the crontab utility. The most common usage is:

% crontab crontab-file

In this example, crontab-file is the filename of a crontab that was previously created.

There is also an option to list installed crontab files: just pass the -l option to crontab and look over the output.

For users who wish to begin their own crontab file from scratch, without the use of a template, the crontab -e option is available. This will invoke the selected editor with an empty file. When the file is saved, it will be automatically installed by the crontab command.

In order to remove a user crontab completely, use crontab with the -r option.