An introduction to the /etc/init.d directory

If you use Linux you most likely have heard of the init.d directory.
But what exactly does this directory do? It ultimately does one thing but it does that one thing for your entire system, so init.d is
very important. The init.d directory
contains a number of start/stop scripts for various services on your system. Everything from acpid to x11-common is
controlled from this directory. Of course it's not exactly that simple.

If you look at the /etc directory
you will find directories that are in the form rc#.d (Where
# is a number reflects a specific initialization level - from 0 to 6). Within each of these directories is a number of other scripts that control processes. These scripts will either begin with a "K" or an "S". All "K" scripts are run before "S" scripts.
And depending upon where the scripts are located will determine when the scripts initiate. Between the directories the system services work together like a well-oiled machine. But there are times when you need to start or stop a process cleanly and without
using the kill or killall commands. That is where the /etc/init.d directory
comes in handy.

In order to control any of the scripts in init.d manually
you have to have root (or sudo) access. Each script will be run as a command and the structure of the command will look like: /etc/init.d/command
OPTION . Where command is
the actual command to run and OPTION can
be one of the following:

start

stop

reload

restart

force-reload
Most often you will use either start,
stop, or restart. So
if you want to stop your network you can issue the command: /etc/init.d/networking
stop . Or if you make a change to your network and need to restart it, you could do so with the following
command: /etc/init.d/networking restart .

About /etc/rc.local script.
This file runs after all other init level scripts have run, so it's safe to put various commands that you want to have issued upon startup.

转自：https://www.ghacks.net/2009/04/04/get-to-know-linux-the-etcinitd-directory/