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. The only difference between these two
formats is the sixth field. In the system crontab, the sixth field is the name of a user
for the command to run as. This gives the system crontab the ability to run commands as
any user. In a user crontab, the sixth field is the command to run, and all commands run
as the user who created the crontab; this is an important security feature.
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. This one is different from /etc/crontab (the system
crontab). Because of the system crontab, there is usually no need to create a user
crontab for root.
Let us take a look at the /etc/crontab file (the system
crontab):
Like most FreeBSD configuration files, the # character
represents a comment. 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. When a user installs his or
her crontab file, they will not have this option. Finally, the
command option is listed. This is the last field, so naturally
it should designate 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 the atrun
command is to be invoked by root every five minutes regardless
of what day or month it is. For more information on the atrun
command, see the atrun(8) manual
page.
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 set up 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 /etc/crontab file. This
field should be omitted for individual user crontab files.
Important: You must not use the procedure described here to edit/install the
system crontab. Simply 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.
If you later want to remove your user crontab completely,
use crontab with the -r option.
