12.6. Configuring cron(8)

12.6. Configuring cron(8)
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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(5) files. These files store information about specific functions which cron(8) is supposed to perform at certain times.

Two different types of configuration files are used by cron(8): the system crontab and user crontabs. These formats only differ in the sixth field and later. In the system crontab, cron(8) runs 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 the system crontab, /etc/crontab. Because the system crontab invokes the specified commands as root, there is usually no need to create a user crontab for root.

Here is a sample entry from /etc/crontab:

# /etc/crontab - root's crontab for FreeBSD # # $FreeBSD: head/en_US.ISO8859-1/books/handbook/config/chapter.xml 42524 2013-08-08 17:21:07Z wblock $ #

# SHELL=/bin/sh PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin

# #minute hour mday month wday who command

# */5 * * * * root /usr/libexec/atrun

	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.
	The equals (`=`) character is used to define any environment settings. In this example, it is used to define the `SHELL` and `PATH`. If the `SHELL` is omitted, cron(8) will use the default of sh(1). If the `PATH` is omitted, no default will be used and file locations will need to be absolute.
	This line defines a total of seven fields: `minute`, `hour`, `mday`, `month`, `wday`, `who`, and `command`. These are almost all self explanatory. `minute` is the time in minutes when the specified command will be run. `hour` is the hour when the specified command will be run. `mday` stands for day of the month and `month` designates the month. The `wday` option stands for day of the week. These fields must be numeric values, representing the twenty-four hour clock, or a `*`, representing all values for that field. The `who` field only exists in the system crontab. This field specifies which user the command should be run as. The last field is the command to be executed.
	This last line defines 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(8) is invoked by `root` every five minutes, regardless of the day or month. 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(5). However, field number six, which specifies the username, only exists in the system crontab(5). This field should be omitted for individual user crontab(5) 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 and cron(8) 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(5), use an editor to create and save a file in the proper format. Then, specify the file name with crontab(1):

% crontab crontab-file

In this example, crontab-file is the filename of a crontab(5) that was previously created.

To list installed crontab(5) files, pass -l to crontab(1).

Users who wish to begin their own crontab file from scratch, without the use of a template, can use crontab -e. This will invoke the default editor with an empty file. When this file is saved, it will be automatically installed by crontab(1).

In order to remove a user crontab(5) completely, use crontab -r.

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