Chapter 3. Boot Process, Init, and Shutdown

57

If you need to alter the command line arguments to the kernel, see Chapter 4. For information on

changing the runlevel at the GRUB or LILO prompt, see Section 3.5.

Once the second stage boot loader has determined which kernel to boot, it locates the corresponding

kernel binary in the

/boot/

directory. The proper binary is the

/boot/vmlinuz 2.4.x xx

file that

corresponds to the boot loader's settings. Next the boot loader places the appropriate initial RAM

disk image, called an initrd, into memory. The

initrd

is used by the kernel to load any drivers not

compiled into it that are necessary to boot the system. This is particularly important if you have SCSI

hard drives or are using the ext3 file system

1

.

Warning

Do not remove the /initrd/ directory from the file system for any reason. Removing this directory

will cause your system to fail with a kernel panic error message at boot time.

Once the kernel and the initrd image are loaded into memory, the boot loader hands control of the

boot process to the kernel.

3.2.3. The Kernel

When the kernel loads, it immediately initializes and configures the computer's memory. Next it con 

figures the various hardware attached to the system, including all processors and I/O subsystems, as

well as any storage devices. It then looks for the compressed

initrd

image in a predetermined lo 

cation in memory, decompresses it, mounts it, and loads all necessary drivers. Next it initializes file

system related virtual devices, such as LVM or software RAID before unmounting the

initrd

disk

image and freeing up all the memory it once occupied.

After the kernel has initialized all the devices on the system, it creates a root device, mounts the root

partition read only, and frees unused memory.

At this point, with the kernel loaded into memory and operational. However, with no user applications

to give the user the ability to provide meaningful input to the system, not much can be done with it.

To set up the user environment, the kernel starts the

/sbin/init

command.

3.2.4. The

/sbin/init

Program

The

init

program 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 your environment path, starts swap, checks the file systems, and so on. Basically,

rc.sysinit

takes care of everything that your 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 clock. Another example is if you have 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

2

. Among other things, the

/etc/inittab

sets the default runlevel

and dictates that

/sbin/update

should be run whenever it starts a given runlevel

3

.

1. For details on making an initrd, see the section about converting to an ext3 file system in the Official Red

Hat Linux Customization Guide.

2. For more information on SysV init, see Section 3.5. For more about init runlevels, see Section 3.6.

3. The update command is used to flush dirty buffers back to disk.