When you boot your system
services and processes are started via shell scripts.
All shell scripts that could possibly get executed when you boot your system are stored in the /etc/init.d sub-directory. (Note that even directories can have periods in their name.)
In order to understand the boot-up process you have to be familier with runlevels.
Linux/UNIX systems can be set to run in different modes of functionality.
They can operate in a single-user mode, such as in the case of strictly being a "workstation" (desktop PC), or they can run in multi-user mode to operate as a server.
Each runlevel is identified by a single-digit number.
The runlevels worth remembering are:
0 - shut down the system
1 - single-user mode
2 - 5 multi-user mode
6 - reboot
Recall that any shell script that might be run at system startup are stored in the /etc/init.d directory. All these different runlevels do is run a different set of the scripts stored in this directory at startup. When the system boots up the first startup file it reads is the /etc/inittab text file. This file basically tells the system what the default run level is via the line
id:2:initdefault:
This is the line in the /etc/inittab file you need to edit if you want to change the default run level. Most distros default to runlevel 3 which is not a secure thing to do for Internet servers.
Recall that any shell script that might be run at system startup are stored in the /etc/init.d directory.
All these different runlevels do is run a different set of the scripts.
These different scripts are stored in/etc/init.d directory.
When the system boots up the first startup file it reads is the /etc/inittab text file.
This file basically tells the system what the default run level is via the line
id:2:initdefault:
This is the line in the /etc/inittab file you need to edit if you want to change the default run level. Most distros default to runlevel 3 which is not a secure thing to do for Internet servers.
Each runlevel also has it's own subdirectory under the /etc directory, and the subdirectory name contains the runlevel number. The naming convention for these subdirectories is:
rc2.d
with the runlevel represented as the number (in blue) above.
These runlevel subdirectories don't actually contain any scripts. Rather, they all contain symbolic links to the scripts in the /etc/init.d subdirectory.
This is so all the different runlevels can share common scripts eliminating the need to have multiple copies of the same script in multiple runlevel subdirectories. (That's why any script that could be run at startup is saved to the /etc/init.d subdirectory.) Here's a diagram of the process. Note that special single-user runlevel is always run at bootup to set up the basic system. The higher runlevel scripts are additionally run to provide the multi-user/server functionality.
There's another reason for using links in the individual runlevel subdirectories. These links use a special naming convention to indicate when they should be run and in what order they should be run. Links that have names starting with an upper-case S are called when the runlevel is entered (starting). Links that have names starting with an upper-case K are called when the runlevel is exited to Kill services. (Due to this naming convention it is more common to simply rename a symbolic link so that it begins with a dash (-) or underscore (_) rather than delete them when customizing a runlevel.)
The directories for the multi-user runlevels only contain links that start with an S. Only those run levels that deal with shutting down or restricting functionality of the system (0, 1, 6) have links that start with a K to kill services.
The numbers after the leading S or K determine the order in which the links are called, lowest number first. This is important for process dependancy reasons. For example, you wouldn't want to start up the Samba service if the networking service wasn't already running. For example:
S20thisservice
S30thatservice
S40anotherservice
S80yetanother
