👢 Boot

The Boot - starting process

- The BIOS is started automatically and detects the peripherals.
- Loads the boot routine from the MBR (Master Boot Record) - it is the boot disk, located on the first sector of the hard disk.
- The MBR contains a loader that loads the "second stage loader": this is the "boot loader" specific to the system being loaded.
	-> Linux uses LILO (Linux Loader) or GRUB (Grand Unified Bootloader).
- LILO loads the kernel into memory, decompresses it, and passes it the parameters.
- The kernel mounts the `/` filesystem (from there, the commands in `/sbin` and `/bin` are available).
- The kernel runs its first process: `init`.

LILO configuration

LILO can offer several kernels as choices. The default choice: “Linux”. /etc/lilo.conf : configuration of the kernel parameters. /sbin/lilo : to write the new parameters to disk. -> creates the /boot/map file, which contains the physical blocks where the boot program is located.

Possible parameters " name = value " - Boot : location of LILO. - Install : path of the “loader” - default /boot/boot.b. - Prompt : if present, displays a prompt and waits for user input. - Timeout : used with Prompt, the delay before booting the default kernel. - Default : name of the image to load by default. - Image : path of the kernel. - Label : name given to the system. - Root : partition containing the root filesystem. - append : parameters passed to the system at boot.

GRUB configuration

/sbin/grub : to modify the config in /boot/grub/grub.conf or /boot/grub/menu.lst, read by /sbin/grub-install at startup.

EFI efibootmgr : Boot Order efibootmgr -o 0012,0013,0002,0000,0001,0003,0004,0005,0006,0007,0008,0009,000C,0010,000E,000B,000F,000D,000

FS vfat /boot/efi

GRUB

cat /proc/cmdline : shows the parameters passed to the kernel at boot. cat /etc/grub.conf : parameters to pass to the kernel at boot. /!\ grub-install /dev/sda

/etc/grub.d/00_header => loads /etc/default/grub (the rest of /etc/grub.d/ is loaded in alphabetical order).

Any change in [ vi /etc/default/grub ] must be taken into account in /boot : BIOS based : grub2-mkconfig -o /boot/grub2/grub.cfg EFI based : grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

1# List all kernels available at boot
2grubby --info=ALL | grep vmlinuz
3kernel=/vmlinuz-2.6.32-754.3.5.el6.x86_64
4kernel=/vmlinuz-2.6.32-696.18.7.el6.x86_64
5kernel=/vmlinuz-2.6.32-696.13.2.el6.x86_64
6
7# Show the kernel that will boot
8grubby --default-kernel
9/boot/vmlinuz-3.10.0-957.1.3.el7.x86_64

Initrd

1/etc/ecsi_linux_version
2dracut /dev/sda