Linux Learning Notes-system startup process

CentOS startup process

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POST--Boot Sequence (BIOS)--Boot Loader (MBR)--Kernel (RAMDisk)--rootfs--Switchroot In/init---/etc/inittab,/etc/init/*.conf--Set default run level--system initialization script--Shut down or start service at the corresponding level--start terminal

POST

Power-on self-test

BOOT Sequence

In order to find the boot device, the first device with a boot program is the device for this boot

MBR boot

MBR records are generally in Disk 0 tracks 1 sectors, a total of 512 bytes. The first 446 bytes are bootloder, after 4*16 64 bytes is the storage partition information, the last 2 bytes is the checksum information, is generally 55AA

Start Bootloder

Grub: Provides a menu that allows the user to select a system to boot or a different kernel version, to load a user-selected kernel into a specific space in memory, unzip, expand, and transfer control of the system to the kernel

Load kernel

After the kernel is loaded, the operating system takes over the hardware and then initializes itself

Detect all hardware devices that can be identified

Load Hardware Drivers

Mount the root file system as read-only

The first application running user space:/sbin/init

Where Init has different types in different CentOS versions

Sysv:init, CentOS 5 configuration file:/etc/inittab

Upstart:init, CentOS 6 configuration file:/etc/inittab,/etc/init/*.conf

SYSTEMD:SYSTEMD, CentOS 7 configuration file:/usr/lib/systemd/system,/etc/systemd/system

Another/sbin/init is divided into 7 operating levels

0: Turn off the machine

1: Single user mode (root, no login required), one, maintenance mode

2: Multi-user mode, will start the network function, but will not start NFS; Maintenance mode

3: Multi-user mode, normal mode, text interface

4: Reserved level, can be same as 3 level

5: Multi-user mode, Normal mode, graphical interface

6: Restart

The INIT process first reads the file/etc/inittab configuration file

Each row defines an action and the process that corresponds to it

Id:runlevel:action:process

Action

Wait: Switch to this level to run once

Respawn: This process terminates and the restart is

Initdefault: Setting the default runlevel; process omitted

Sysinit: Sets the system initialization mode, where the specified/etc/rc.d/rc.sysinit is typically

/etc/rc.d/rc.sysinit: System Initialization script

(1) Set host name

(2) Setting welcome information

(3) Activate Udev and SELinux

(4) Mount the file system defined in the/etc/fstab file

(5) Detecting the root file system and re-mounting the root filesystem in read and write mode

(6) Setting the system clock

(7) Activate swap device

(8) Set kernel parameters according to/etc/sysctl.conf file

(9) Activating LVM and software RAID devices

(10) Loading drivers for additional devices

(11) Cleaning operation

Each run level under/etc directory has a corresponding subdirectory that specifies the program to load

/etc/rc0.d
/etc/rc1.d
/etc/rc2.d
/etc/rc3.d
/etc/rc4.d
/etc/rc5.d
/etc/rc6.d

Set up a login terminal

Linux Learning Notes-system startup process