RedHat Linux Boot Sequence

Boot sequence

  1. BIOS
  2. POST
  3. MBR
  4. GRUB (stage 1)
  5. GRUB (stage 2)-- /etc/grub.conf
  6. KERNEL

Components in the file grub.conf

  • Default
  • Time out
  • Splash image : The wall paper
  • Title : Windows, linux, dos, etc

Few parameters in grub.conf
root (0,0) : This means the root is in hard disk 1, partition 1

kernel: The kernel to be used

Rhgb quiet: This will hide the kernel messages

Initrd: will have image of “necessary” drivers while booting.

If another OS (Windows) is also installed the grub.conf file will have another block with

  • title Dos
  • rootnoverify (hd0,1)
  • deviceloader +1

Grub prompt
At the booting time, when the menu to choose the OS appears, pressing “c” will bring us the grub prompt (GRUB >).
Here various commands could be executed. Example:

GRUB> root (hd0, “press tab key”
This will show the various partitions

If the / is in (hd0,4), we could type “cat /etc/groups” to see the contents of the “groups” file.

If grub cannot find the kernel to be used do the following:

  1. GRUB> root(hd0,4)
  2. kernel /vmlinuz ro root=LABEL=/
  3. initrd /initrd?.img

Run level:

  • 0 -- Halt
  • 1 -- single user mode
  • 2 -- no nfs
  • 3 -- Text mode with NW
  • 4 -- user definable
  • 5 -- XII
  • 6 -- Reboot

If the system do not bootup following could be tried:

  1. GRUB> (hd0,0)
  2. GRUB> kernel /kmlinyz-2.6.9.5.EL ro root=LABEL
  3. GRUB> initrd /initrd-?..

Init
Init -> inittab -> will call for many other files including rc0 to rc6

The rc0 will have only kill commands. All the links in rc6 will start with ‘K’ by norm

No 1 -- 6 in the inittab runs getty. These are definitions for the virtual terminals.
If we add a line : “20:2345:respawn:/sbin/migetty tty 20” a new terminal will be added. This new terminal will work only after reboot.

Note: /etc/security will have info about the terminals root can log in.

# init -q
# chvt 20 is to switch terminal using CLI.
Change run level at bootup/grub
If we want to switch to a different run level at the bootup itself, the following has to be done at the boot prompt at startup.

  1. At the startup, at the grub menu press ‘e’
  2. Select the kernel entry and press “e”
  3. After rhgb quiet enter 1 after a space and press the ENTER key.
  4. Now system will reboot in run level 1

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