4.2.4. Init
The kernel, once it is loaded, finds init in sbin and executes it.
When init starts, it becomes the parent or grandparent of all of the processes that start up automatically on your Linux system. The first thing init does, is reading its initialization file, /etc/inittab . This instructs init to read an initial configuration script for the environment, which sets the path, starts swapping, checks the file systems, and so on. Basically, this step takes care of everything that your system needs to have done at system initialization: setting the clock, initializing serial ports and so forth.
Then init continues to read the /etc/inittab file, which describes how the system should be set up in each run level and sets the default run level . A run level is a configuration of processes. All UNIX-like systems can be run in different process configurations, such as the single user mode, which is referred to as run level 1 or run level S (or s). In this mode, only the system administrator can connect to the system. It is used to perform maintenance tasks without risks of damaging the system or user data. Naturally, in this configuration we don't need to offer user services, so they will all be disabled. Another run level is the reboot run level, or run level 6, which shuts down all running services according to the appropriate procedures and then restarts the system.
Commonly, run level 3 is configured to be text mode on a Linux machine, and run level 5 initializes the graphical login and environment. More about run levels in the next section, see Section 4.2.5 .
After having determined the default run level for your system, init starts all of the background processes necessary for the system to run by looking in the appropriate rc directory for that run level. init runs each of the kill scripts (their file names start with a K) with a stop parameter. It then runs all of the start scripts (their file names start with an S) in the appropriate run level directory so that all services and applications are started correctly. In fact, you can execute these same scripts manually after the system is finished booting with a command like /etc/init.d/httpd stop or service httpd stop logged in as root , in this case stopping the web server.
Special case
Note that on system startup, the scripts in rc2.d and rc3.d are usually executed. In that case, no services are stopped. There are only services that are started.
None of the scripts that actually start and stop the services are located in /etc/rc<x>.d . Rather, all of the files in /etc/rc<x>.d are symbolic links that point to the actual scripts located in /etc/init.d . A symbolic link is nothing more than a file that points to another file, and is used in this case because it can be created and deleted without affecting the actual scripts that kill or start the services. The symbolic links to the various scripts are numbered in a particular order so that they start in that order. You can change the order in which the services start up or are killed by changing the name of the symbolic link that refers to the script that actually controls the service. You can use the same number multiple times if you want a particular service started or stopped right before or after another service, as in the example below, listing the content of /etc/rc5.d , where crond and xfs are both started from a linkname starting with "S90" . In this case, the scripts are started in alphabetical order.
[jean@blub /etc/rc5.d] ls
K15httpd@ K45named@ S08ipchains@ S25netfs@ S85gpm@
K16rarpd@ K46radvd@ S08iptables@ S26apmd@ S90crond@
K20nfs@ K61ldap@ S09isdn@ S28autofs@ S90xfs@
K20rstatd@ K65identd@ S10network@ S30nscd@ S95anacron@
K20rusersd@ K74ntpd@ S12syslog@ S55sshd@ S95atd@
K20rwalld@ K74ypserv@ S13portmap@
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