The system startup files reside in the /etc directory. They are:

First, an overview of the control and support scripts (also documented in rc(8)).

Additional scripts outside of the rc.d directory:

rc.d scripts are controlled by a central configuration file, /etc/rc.conf, which loads its default settings from /etc/defaults/rc.conf. If you want to change a default setting, do not edit /etc/defaults/rc.conf; instead, apply the setting in /etc/rc.conf. This will override the default.

It is a good idea to read the rc.conf(5) man page to learn about the services that are available to you.

The following example shows how to enable the SSH daemon, which is disabled by default:

# cd /etc; grep ssh defaults/rc.conf
sshd=NO                 sshd_flags=""
# echo "sshd=YES" >> rc.conf

Now sshd(8) will be started automatically at system startup. The next section describes how to start and stop services at any time.

Last but not least, files can be created in the /etc/rc.conf.d/ directory to override the behavior of a given rc.d script without editing the script itself.

The actual scripts that control services are in /etc/rc.d. These scripts are automatically run at boot time, but they can be called manually if necessary, either through the service(8) alias or directly. The following example shows how to start the SSH daemon that we enabled in the previous section:

# service sshd start
Starting sshd.

Later, if you wish to stop the SSH daemon, run the following command:

# service sshd stop
Stopping sshd.
Waiting for PIDS: 123.

The rc.d scripts take one of the following arguments:

Some scripts may support other arguments (e.g., “reload”), but every script will support at least the above commands.

As an example, after adding a new record to a named(8) database, the daemon can be told to reload its configuration files with the following command:

# service named reload
Reloading named config files.

Note that all of the commands discussed above will only take action if the particular service is enabled in /etc/rc.conf. It is possible to bypass this requirement by prepending “one” to the command, as in:

# service httpd onestart
Starting httpd.

The above command will allow you to start the httpd(8) service one time. To stop a service that has been started in this manner, pass “onestop” to the script.

The startup system of every Unix system determines, in one way or another, the order in which services are started. On some Unix systems this is done by numbering the files and/or putting them in separate run level directories. Solaris relies on wildcards like /etc/rc[23].d/S* being sorted numerically when expanded. Some simply put all the commands that should be started into a single monolithic script (this is the traditional BSD method, and is what NetBSD did before the rc.d system). On modern NetBSD this is done by the rc.d scripts and their contents. Please note that NetBSD does not use multiple runlevels.

At the beginning of each rc.d script there is a series of commented out lines that have one of the following items in them:

These describe the dependencies of that particular script and allow rcorder to easily work either “up” or “down” as the situation requires. As an example, here is the ordering information contained in /etc/rc.d/nfsd:

...
 PROVIDE: nfsd
 REQUIRE: rpcbind mountd
...

Here we can see that this script provides the “nfsd” service and that it requires “rpcbind” and “mountd” to be running first. The rcorder(8) utility is used at system startup time to read through all the rc.d scripts and determine the order in which they should be run.

Luke Mewburn, one of the principal designers of the rc.d system, gave a presentation on the system at USENIX 2001. It is available in PDF format.