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.