For each run level, scripts are run to start each individual service, instead of having a few large files to edit by hand. These scripts are located in /etc/rc.d/init.d, and most take as an option start or stop. This is to allow the specific programs to start (on bootup) or stop (on shutdown).
This setup involves a bunch of directories under /etc/rc.d/. These are:
rc0.d Contains scripts to run when the system shuts down. Technically, halt or shutdown bring the system to runlevel 0. This directory is mostly made up of kill commands.
rc1.d through rc3.d Scripts to run when the system changes runlevels. Runlevel 1 is usually single-user mode, runlevel 2 is for multi-user setup without NFS, and runlevel 3 is full multi-user and networking.
Runlevel 4 is typically unused.
rc5.d Scripts to start the system in X11 mode. This is the same as runlevel 3, with the exception that the xdm program starts, which provides a graphical login screen.
rc6.d Scripts to run when the system reboots. These scripts are called by a reboot command.
init.d Actually contains all of the scripts. The files in the rc?.d directories are really links to the scripts in the init.d directory.
Now that we know where files are located, let's look at what happens in a normal Red Hat boot sequence.
Once the system boots, /etc/rc.d/rc.sysinit is run first. The starting runlevel (specified in /etc/inittab) is found, and the /etc/rc.d/rc script is run, with the sole option being the runlevel we want to go to. For most startups, this is runlevel 3.
The rc program looks in the /etc/rc.d/rc3.d directory, executing any K* scripts (of which there are none in the rc3.d directory) with an option of stop. Then, all the S* scripts are started with an option of start. Scripts are started in numerical order—thus, the S10network script is started before the S85httpd script. This allows you to choose exactly when your script starts without having to edit files. The same is true of the K* scripts.
Let's look at what happens when we switch runlevels—say from runlevel 3 (full networking and multi-user mode) to runlevel 1 (single-user mode).
First, all the K* scripts in the level to which the system is changing are executed. My Caldera Preview II (Red Hat 2.0) setup has seven K scripts and one S script in the /etc/rc.d/rc.1/ directory. The K scripts shut down nfs, sendmail, lpd, inet, cron, and syslog. The S script then kills off any remaining programs and executes init -t1 S, which tells the system to really go into single-user mode.
Once in single-user mode, you can switch back to full multi-user mode by typing init 3.
There are two additional points I can make here.
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