http://www.redhat.com/docs/manuals/linux/RHL-9-Manual/ref-guide/s1-boot-init-shutdown-process.html
The /sbin/init program (also called init) coordinates the rest of the boot process and configures the environment for the user.
When the init command starts, it becomes the parent or grandparent of all of the processes that start up automatically on a Red Hat Linux system. First, it runs the /etc/rc.d/rc.sysinit script, which sets the environment path, starts swap, checks the file systems, and takes care of everything the system needs to have done at system initialization. For example, most systems use a clock, so on them rc.sysinit reads the /etc/sysconfig/clock configuration file to initialize the hardware clock. Another example is if there are special serial port processes which must be initialized, rc.sysinit will execute the /etc/rc.serial file.
The init command then runs the /etc/inittab script, which describes how the system should be set up in each SysV init runlevel [3]. Among other things, the /etc/inittab sets the default runlevel and dictates that /sbin/update should be run whenever it starts a given runlevel [4].
Next, the init command sets the source function library, /etc/rc.d/init.d/functions, for the system. This spells out how to start or kill a program and how to determine the PID of a program.
The init program starts all of the background processes by looking in the appropriate rc directory for the runlevel specified as default in /etc/inittab. The rc directories are numbered to corresponds to the runlevel they represent. For instance, /etc/rc.d/rc5.d/ is the directory for runlevel 5.
When booting to runlevel 5, the init program looks in the /etc/rc.d/rc5.d/ directory to determine which processes to start and stop.
As illustrated in this listing, none of the scripts that actually start and stop the services are located in the /etc/rc.d/rc5.d/ directory. Rather, all of the files in /etc/rc.d/rc5.d/ are symbolic links pointing to scripts located in the /etc/rc.d/init.d/ directory. Symbolic links are used in each of the rc directories so that the runlevels can be reconfigured by creating, modifying, and deleting the symbolic links without affecting the actual scripts they reference.
The name of each symbolic link begin with either a K or an S. The K links are processes that are killed on that runlevel, while those beginning with an S are started.
The init command first stops all of the K symbolic links in the directory by issuing the /etc/rc.d/init.d/<command> stop command, where <command> is the process to be killed. It then starts all of the S symbolic links by issuing /etc/rc.d/init.d/<command> start.
After the system is finished booting, it is possible to log in as root and execute these same scripts to start and stop services. For instance, the command /etc/rc.d/init.d/httpd stop will stop the Apache Web server. |
Each of the symbolic links are numbered to dictate start order. The order in which the services are started or stopped can be altered by changing this number. The lower the number, the earlier it is started. Those symbolic links with the same number are started alphabetically
One of the last things the init program executes is the /etc/rc.d/rc.local file. This file is useful for system customization. See Section 1.3 Running Additional Programs at Boot Time for more on using the rc.local file. |
After the init command has progressed through the appropriate rc directory for the runlevel, the /etc/inittab script forks a /sbin/mingetty process for each virtual console (login prompts) allocated to the runlevel. Runlevels 2 through 5 get all six virtual consoles, while runlevel 1 (single user mode) gets only one and runlevels 0 and 6 get none. The /sbin/mingetty process opens communication pathways to tty devices [5], sets their modes, prints the login prompt, gets the user name, and initiates the login process for the user.
In runlevel 5, the /etc/inittab runs a script called /etc/X11/prefdm. The prefdm script executes the preferred X display manager — gdm, kdm, or xdm, depending on the contents of the /etc/sysconfig/desktop file.
At this point, the system is operating on runlevel 5 and displaying a login screen.