The boot process in Redhat Linux and CentOS involves several stages, each responsible for initializing different components of the system. Here's a breakdown of the boot process:
BIOS (Basic Input/Output System):
- Performs system integrity checks.
- Searches, loads, and executes the boot loader program.
- Detects and loads the MBR (Master Boot Record) boot loader.
MBR (Master Boot Record):
- Located in the 1st sector of the bootable disk (/dev/hda or /dev/sda).
- Contains information about the GRUB (Grand Unified Bootloader) boot loader.
GRUB (Grand Unified Bootloader):
- Displays a splash screen and waits for user input.
- Loads the default kernel image specified in the grub configuration file (/boot/grub/grub.conf).
- Has knowledge of the filesystem.
- Loads and executes the kernel and initrd images.
Init:
- Reads the /etc/inittab file to determine the default run level.
- Defines the available run levels (0-6) and their corresponding modes (halt, single user, multiuser, etc.).
- Sets the default run level based on the configuration.
- Loads all appropriate programs based on the default run level.
Runlevel Programs:
- Services and programs started during bootup.
- Located in directories corresponding to each run level (/etc/rc.d/rc*.d/).
- Symbolic links are available under /etc/rc*.d/ for convenience.
- Programs starting with 'S' are executed during startup, while those starting with 'K' are executed during shutdown.
- The sequence number in the program names determines the order of execution.
For example:
- S12syslog starts the syslog daemon with a sequence number of 12.
- S80sendmail starts the sendmail daemon with a sequence number of 80.
- Programs with lower sequence numbers are executed before those with higher numbers.
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