Detailed description of the linux system startup process and system initialization

I. linux Startup Process

The linux Startup Process can be divided into the following steps:

POST power-on self-check --> BIOS (Boot Sequence) --> load the MBR on the corresponding Boot (bootloader) --> the main boot settings load its BootLoader --> Kernel initialization --> initrd->/etc/init process load/etc/inittab. The process flow chart is as follows:

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Ii. Detailed Startup Process Analysis

(1,

 
 

  
  
POST power-on self-check: when the computer host powers on, it will then hear a drop, the system starts the power-on self-check POST-power on self
  
  
Test). In this process, it is mainly used to detect the faulty storage of computer hardware devices such as CPU, memory, motherboard, video card, and CMOS.
  
  
In the case of hardware faults, there are two possible reasons: serious faults (fatal faults) are shut down. At this time, various initialization operations have not been completed,
  
  
No prompts or signals are provided. If there is no serious fault, a prompt or sound alarm signal is provided, waiting for the user to handle it .)
  
  
To the BIOS.
 
 

(2,

 
 

  
  
BIOS: BIOSBasic Input Output System (Basic Input Output System) is the first place to be read after self-check is completed)
  
  
The BIOS records the chip set and related settings of the motherboard, such as the communication frequency between the CPU and the interface device, the search sequence of the boot device, and the size of the hard disk.
  
  
Type, system time, external bus, I/O address of various interface devices, IRQ interrupt information that has been communicated with the CPU, so to start smoothly,
  
  
First, read the BIOS settings.
 
 

(3,

 
 

  
  
Start the Boot on the first device according to the system Boot process set by BIOS. If the detection succeeds, start the Boot Sequence according to the Boot Sequence.
  
  
Program, our boot device mainly includes hard disk, USB, SD, etc. We usually use hard disk, and then read the first device is the hard disk, the first
  
  
To read the MBRMaster Boot Record of the hard disk), and then the system can install
  
  
Boot Loader) to start Core identification. In this example, the MBR program finds only the front of the hard disk partition.
  
  
446 bytes of Boot Loader] and then find the relevant configuration and definition.
 
 

(4,

 
 

  
  
Boot Loader loads Grub Program
  
  
In this process, Grub is mainly started with Grub guidance. Grub is divided into two phases:
  
  
Stage1: Mainly Boot loader
  
  
Stage 1.5: Transition
  
  
Stage2: Mainly/boot/grub
 
 

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We can see that the linux kernel VMLnuz, grub, and initrd are all in the/boot directory.

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In/boot/grub/, we can see the familiar stage1, stage2 and grub tool configuration files grub. conf. What are defined in grub. conf ??

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Kernel and Kernel

 
 

  
  
According to the definition in Grub, after reading grub, submit the following work to the kernel. Kernel is mainly used to test the system hardware and initialize the hardware driver.
  
  
And mount the root file system root switch in read/write mode), then here there is a "first chicken or first egg file", what is it?
  
  
So?
  
  
To access the real root file system rootfs), you must load the device in the root file system. At this time, the root file system is not mounted,
  
  
What should I do if I need to mount the root file system and load the driver in the root file system? To solve this problem, the initrd file is used.
  
  
In this section, the content of the kernel initialization task is summarized as follows:
  
  
Test hardware-> load driver initrd)-> mount the root file system-> rootfs (/sbin/init)
 
 

Average,

 
 

  
  
At this point, the related work of the kernel space has been completed. The task of the kernel space begins to be transferred to the user space.
  
  
Linux) to the user space/sbin/init excessive, so gurb started to guide the kernel to initrd.
  
  
Initrd: A Virtual File System with directories such as lib, bin, sbin, usr, proc, sys, var, dev, and boot,
  
  
In fact, you will find the directory inside is a bit like true/right, so we call it a virtual root file system, which is used to convert the kernel and the real root file system
  
  
Establish associations so that the kernel can load the driver required by the root file system in initrd, mount the root file system in read/write mode, and run
  
  
Init is the first process among users.
 
 

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Let's take a look at the content in the init script:

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From the above script content, we can see the main work of the init process:

 
 

  
  
Mount: Mount/proc,/sys/dev in initrd to the corresponding directory in the current primary partition.
  
  
Create directory:/dev/mapper
  
  
Create block or character special files using mknod
  
  
Mounting related modules
  
  
Create a root device
  
  
Mount/sysroot
  
  
Final root Switch
 
 

After the initialization and init operations are completed, the/etc/inittab file in the system is started to initialize the system. Next we will introduce the details in the inittab configuration file.

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Definition of each level:

Default running level

 
 

  
  
0: halt // Shutdown
  
  
1: single user mode // single user maintenance mode)
  
  
2: multi user mode, without NFS // NFS function is not supported
  
  
3: multi user mode, text mode // character Interface
  
  
4: reserved // retained by the System
  
  
5: multi user mode, graphic mode // graphical interface
  
  
6: reboot // restart
 
 

/Etc/inittab format and syntax (:)

 
 

  
  
[Option]: [runlevel]: [Behavior]: [command]
  
  
Behavior:
  
  
Initdefault: indicates the default running level.
  
  
Sysinit: indicates the system initialization operation option.
  
  
Ctrlaltdel: indicates the restart settings.
  
  
Wait: indicates that the following operations can be performed only after the last command is executed.
  
  
Respawn: indicates that the following fields can be regenerated without limit (reboot)
  
  
Command Options
  
  
Some commands, but usually scripts
 
 

The initialization script defined in inittab is rc. sysinit --/etc/rc. d/rc. sysinit.

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As shown in: the rc. sysinit script defines some definitions with system initialization.
  
  
Set Host Name
  
  
Detect and mount other file systems in/etc/fstab
  
  
Start swap Partition
  
  
Set Kernel Parameters in/etc/sysctl. conf
  
  
Load key ing --> function of each key on the keyboard
  
  
Then run the related service script at the system running level:/etc/rc. d/init. d/script and/etc/rc. d/rc # d
 
 

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Rc0-rc6 directory script:
  
  
K * ## stop all files starting with K
  
  
S * ## as long as all files starting with S execute start
  
  
0-99 (execution order, the smaller the number, the first execution)
  
  
User-Defined boot Startup Program (/etc/rc. d/rc. local)
  
  
You can write some execution commands or scripts to/etc/rc. d/rc. local as needed. It can be loaded when it is started.
 
 

Iii. Summary

The general content of system initialization is summarized as follows:

 
 

  
  
Hardware initialization, image interface startup initialization if the default startup basics are set)
  
  
Initialize host RAID settings, device er, and related initialization,
  
  
Checks the root file system and mounts it in read-only mode.
  
  
Activate udev and selinux
  
  
Set the Kernel Parameter/etc/sysctl. conf
  
  
Set system clock
  
  
Enable swap partition and set Host Name
  
  
Load keyboard ing
  
  
Activate RAID and LVM logical volumes
  
  
Mount additional file systems/etc/fstab
  
  
Finally, call login according to the mingetty program to log on to the user-> log on to the user to complete the system startup)
  
  
The main scripts and directories during system startup include:
  
  
Boot
  
  
/Grub
  
  
/Boot/grub. conf
  
  
/Boot/initrd + kernel version
  
  
Mount the/proc // sys // dev/directory in the/initrd file and switch the root
  
  
/Etc/inittab script
  
  
/Etc/rc. d/rc. sysinit script, etc.
 
 

These important scripts and directories include other important directories, files, and scripts. From this article, we can have a detailed understanding of the linux system startup and initialization process, and then we can DIY a micro linux system according to the linux system startup process and the commands used, we will post a blog post about the "DIY micro-linux System" in the next two to three days. At that time, we will discuss more details about the components of the linux system.
Information !!!

The content I summarized is not rigorous enough. I hope you can correct me ~~~

This article is from the "Chris On the way" blog, please be sure to keep this source http://chrinux.blog.51cto.com/6466723/1192004