12.6 Configuring the cron Utility

Contributed by Tom Rhodes.

One of the most useful utilities in FreeBSD is cron(8). This utility runs in the background and regularly checks /etc/crontab for tasks to execute and searches /var/cron/tabs for custom crontab files. These 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 /etc/crontab. 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. This example has a */5 listing,followed by several more * characters. These * characters mean “first-last”, and can be interpreted as every time. In this example, atrun is invoked by root every five minutes regardless of the day or month. For more information on atrun, refer to 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, although there is one thing different about this one. Field number six, which specifies 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. Instead, use an editor: cron 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 an editor to create and save a file in the proper format. Then, specify the file name with crontab:

% crontab crontab-file

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

To list installed crontab files, pass -l to crontab.

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 crontab.

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