CentOS 5 and 6 start-up process

Linux main components: kernel+rootfs+ library + Program

Kernel (kernel): Process management, memory management, network management, drivers, file systems, security features, etc.

Rootfs (Root file System): glibc (library file), etc.

Libraries: function sets, functions, calling interfaces

Procedure: Procedure call, no return value

Functions: Function call, with return value

Kernel design Genre:

Single Core design: All functions are integrated into the same program. For example: Linux

Microkernel Design: Each function is implemented with a single subsystem. Example: Windows,solaris

Linux kernel Features:

Support modularity:. Ko

Dynamic loading and unloading of support modules

Linux Kernel components:

Core file:/boot/vmlinux-version-release

RAMDisk: Secondary mount HDD Drive

CENTOS5: Analog to Hard drive/boot/initrd-version-resease.img

CENTOS6: Analog to File system/boot/Initramfs-version-resease.img

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Module file:/lib/modules/version-release/

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CentOS 5 System Startup process

POST--Bootsequence (BIOS)--bootloader (MBR)--Kernel (ramdisk)--ROOTFS (read-only)--/sbin/init- Etc/inittab)--Set the default run level (Id:3:initdefault:)--run the system initialization script, complete the system initialization (Si::sysinit:/etc/rc.d/rc.sysinit --Shut down and start the corresponding level write the service that needs to be shut down (L#:#:WAIT:/ETC/RC.D/RC #)--Set the login terminal (Tty1:2345:respawn:/usr/sbin/mingetty tty1)

(1) POST: Power-on self-test

(2) BIOS: Basic Input and Output System

boot Sequence: In order to find the boot device, the first device with a boot program is the device used for this start;

(3) bootloder: Boot loader

Linux contains two Bootloder, Lilo (Linux LOader) and GRUB (Grand Uniform Bootloader)

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

(4) kernel: ( must be read-only when loading the root and re-hung in the system initialization script/etc/rc.d/rc.sysinit )

Self-initialization:

detect all hardware devices that can be identified

Load the hardware driver (it is possible to load the driver with RAMDisk)

mount the root file system as read-only (Rootfs read-only)

The first application running user space:/sbin/init

Types of init programs:

sysv:init (used in CentOS5, configuration file:/etc/inittab)

upstart:init (used in CentOS6 and compatible with CENTOS5, configuration file:/etc/inittab,/etc/init/*.conf)

systemd:systemd (centos7 configuration file:/usr/lib/systemd/system,/etc/systemd/system)

RAMDisk Type:

RAMDISK:INITRD (CENTOS5 system is generated by MKINITRD after installation

Ramfs:initramfs (generated by Dracut after CENTOS6 system is installed)

(5) /sbin/init

Operating level: Set for application purposes such as operation or maintenance of the system

0-6:

# 0-halt (do not set Initdefault to this)

# 1-single User mode

# 2-multiuser, without NFS (the same as 3, if you don't have networking)

# 3-full Multiuser mode

# 4-unused

# 5-x11

# 6-reboot (do not set Initdefault to this)

0: Turn off the machine

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

2: Multi-user mode: Network function is started, but NFS is not started, maintenance mode

3: Multi-user mode: Normal mode, default to Text interface

4: Reserved level: Available with 3 levels

5: Multi-user mode: Normal mode: Graphical interface

6: Restart

Toggle Level: Init #

View level: Runlevel,who-r

Configuration file:/etc/inittab

Id:3:initdefault:

si::sysinit:/etc/rc.d/rc.sysinit

L0:0:WAIT:/ETC/RC.D/RC 0

L1:1:WAIT:/ETC/RC.D/RC 1

    ...

L6:6:WAIT:/ETC/RC.D/RC 6

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: Set default Run Level: process omitted

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

...

NOTE:RC 0--> means reading the/etc/rc.d/rc0.d/[k*| S*]

k*:k##*

s*:s##* linked files, source files located in/etc/rc.d/init.d/

# #: The smaller the number, the higher the priority, depending on the service dependency

terminate K start service in order, start the S start service

for SRV in/etc/rc.d/rc0.d/k*;d o

$srv stop

Done

For SRV in/etc/rc.d/rc0.d/s*;d o

$srv Start

Done

Chkconfig: Managing Services

--list: View all services on/off status at all levels

--add Name: Add a Service script SysV type placed in/ETC/RC.D/INIT.D (/ETC/INIT.D)

--del Name: Delete

--level # # # # NAME On/off/reset: Modify the link type at the specified level

--level: omitted as 2345

7 levels of S or K depending on the Chkconfig line in the service script

#! /bin/bash

    #

#chkconfig: LLL nn nn

LLL: Which level is S (0-6)

The order in the Nn:s

The order in the Nn:k

Note: At normal level, a service s99local that was last started is not a service script that is linked to/ETC/RC.D/INIT.D, but instead points to the/etc/rc.d/rv/local script; therefore, it is inconvenient or not required to be written as a service script placed in the/etc/rc.d/ INIT.D directory, and a command to automatically run the boot, can be placed directly in the/etc/rc.d/rc.local file

Tty1:2345:respawn:/usr/sbin/mingetty tty1

Tty2:2345:respawn:/usr/sbin/mingetty Tty2

Tty3:2345:respawn:/usr/sbin/mingetty Tty3

Tty4:2345:respawn:/usr/sbin/mingetty Tty4

Tty5:2345:respawn:/usr/sbin/mingetty Tty5

Tty6:2345:respawn:/usr/sbin/mingetty Tty6

Mingetty will automatically invoke the login program

Tips

/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) Detect the root file system and re-mount the root filesystem in read/write mode (kernel mount is read-only and re-mounted here)

(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

The system boot process for CentOS 6 is roughly the same as 5, but its init program is upstart and its configuration file is/etc/inittab,/etc/init/*.conf, not a single/etc/inittab file.

[email protected] init]# LL

Total 68

-rw-r--r--. 1 root root 412 Apr 9 control-alt-delete.conf

-rw-r--r--. 1 root root init-system-dbus.conf

-rw-r--r--. 1 root root 463 Apr 9 kexec-disable.conf

-rw-r--r--. 1 root root 560 Apr 9 plymouth-shutdown.conf

-rw-r--r--. 1 root root 357 APR 9 prefdm.conf

-rw-r--r--. 1 root root 505 APR 9 quit-plymouth.conf

-rw-r--r--. 1 root root 417 Apr 9 rc.conf

-rw-r--r--. 1 root root 1046 Apr 9 rcs.conf

-rw-r--r--. 1 root root 430 APR 9 rcs-emergency.conf

-rw-r--r--. 1 root root 725 Apr 9 rcs-sulogin.conf

-rw-r--r--. 1 root root 2915 Nov readahead-collector.conf

-rw-r--r--. 1 root root 1559 Nov readahead.conf

-rw-r--r--. 1 root root 726 Nov readahead-disable-services.conf

-rw-r--r--. 1 root root 1302 Apr 9 serial.conf

-rw-r--r--. 1 root root 791 Apr 9 splash-manager.conf

-rw-r--r--. 1 root root 473 Apr 9 start-ttys.conf

-rw-r--r--. 1 root root 335 Apr 9 tty.conf

[Email protected] init]#

This article from the "Smart Young Planck" blog, declined to reprint!

CentOS 5 and 6 start-up process