/etc/init.d is a directory containing initialization and termination scripts for changing init states. These scripts are linked when appropriate to files in the rc?.d directories, where `?' is a single character corresponding to the init state. See init(1M) for definitions of the states.
File names in rc?.d directories are of the form [SK]nn<init.d filename>, where S means start this job, K means kill this job, and nn is the relative sequence number for killing or starting the job.
When entering a state (init S,0,2,3,etc.) the rc[S0-6] script executes those scripts in /etc/rc[S0-6].d that are prefixed with K followed by those scripts prefixed with S. When executing each script in one of the /etc/rc[S0-6] directories, the /sbin/rc[S0-6] script passes a single argument. It passes the argument 'stop' for scripts prefixed with K and the argument 'start' for scripts prefixed with S. There is no harm in applying the same sequence number to multiple scripts. In this case the order of execution is deterministic but unspecified.
Guidelines for selecting sequence numbers are provided in README files located in the directory associated with that target state. For example, /etc/rc[S0-6].d/README. Absence of a README file indicates that there are currently no established guidelines.
When changing to init state 2 (multi-user mode, network resources not exported), /sbin/rc2 is initiated by the init process. The following steps are performed by /sbin/rc2.
In the directory /etc/rc2.d are files used to stop processes that should not be running in state 2. The filenames are prefixed with K. Each K file in the directory is executed (by /sbin/rc2) in alphanumeric order when the system enters init state 2. See example below.
Also in the rc2.d directory are files used to start processes that should be running in state 2. As in Step 1, each S file is executed.
Assume the file /etc/init.d/netdaemon is a script that will initiate networking daemons when given the argument 'start', and will terminate the daemons if given the argument 'stop'. It is linked to /etc/rc2.d/S68netdaemon, and to /etc/rc0.d/K67netdaemon. The file is executed by /etc/rc2.d/S68netdaemon start when init state 2 is entered and by /etc/rc0.d/K67netdaemon stop when shutting the system down.
/sbin/rc2 has references to the obsolescent rc.d directory. These references are for compatibility with old INSTALL scripts. New INSTALL scripts should use the init.d directory for related executables. The same is true for the shutdown.d directory.