CentOS6 system Start-up process

CentOS 6 Start-up process | Root Password Reset | Adding system Services

First, the CentOS 6 startup process:

1) Post power -on self-test

Power-on-self-test, the motherboard in the power supply, the system first by the BIOS program to the CPU, motherboard, memory, hard disk subsystem, display subsystem, serial parallel interface, keyboard, CD-ROM drive and other hardware conditions detection.

If an error is found in the self-test, it will be handled in two cases: for a serious fault, or for a non-critical fault, a prompt or audible alarm signal is waiting for the user to process.

Motherboard ROM: Saved BIOS program RAM: Save various parameter settings

The BIOS locates the boot device in a set order, and the first device with the boot program is the boot device.

2) Read MBR

Read the first boot device MBR bootloader(first 446 bytes), CentOS 6 is the Grub program , bootloader Space Limited only saved the first phase of Grub Stage1.

3) boot loader GRUB

stage1: exists in the first 446 bytes of MBR, loading stage1_5,stage2,initramfs*.img

stage1_5: In the 62 sectors after the MBR, the necessary file system drivers are saved, so that stage1 can identify the file system on the Stage2 partition;

stage2: Provides a menu interface that allows the user to load the kernel with options, change parameters, modify options, and get the contents of files such as grub.conf and Menu.lst, to provide authentication protection mechanisms.

4) Loading the kernel

Self-initialization:

Detect all hardware devices that can be identified

Load the hardware driver (possibly with RAMDisk load driver)

The kernel does not recognize the device on the drive on the hard disk, with the initramfs-version-release.img file in the recognized boot partition (a small, compact root directory containing the driver modules that are necessary for the start-up phase, Executable files and startup scripts),

Mount it as a temporary root directory, execute a startup script, load the real file system to mount the real root directory, and switch the root directory.

Mount the root file system as read-only

The first application running user space:/sbin/init

5) Start init

The kernel is loaded, the INIT program is run, and the system-initiated control is transferred to the INIT process

The init process is the parent process for all processes, and it reads the configuration file/etc/inittab to do the following:

1. Execute the System initialization script (/etc/rc.d/rc.sysinit), Carry on basic configuration to the system, mount the root file system and other file system in read-write mode, carry on the determination of operation level and start the corresponding service;
2. Determine the operating level entered after startup, a total of 7, the default 3, 5;
3. Implementation/ETC/RC.D/RC, the order in which services are started is first k after S, and the service state for each runlevel is placed in the/etc/rc.d/rc#.d (#=0~6)directory, all files are linked to the corresponding file under/ETC/INIT.D.
4. Settings for key sequence
5. Script definitions for UPS
6. Start the virtual terminal/sbin/mingetty
7. Run X on Run level 5

System initialization:

Post-->bootsequence (BIOS)-->bootloader (MBR)-->kernel (RAMDisk)-->rootfs-->/sbin/init

To add a custom boot run script in Rc.loacl:

At normal level, the last service s99local is not linked to a script under/ETC/INIT.D, but is linked to/etc/rc.d/rc.local (/etc/ rc.local) script, so that a program that is inconvenient or does not need to be written as a service script can be placed directly in this script file when it is expected to run automatically.

Second, root Password reset

1. Enter single-user mode

Type E at power-on, enter the Grub menu page, type A, add 1,s,s,sigle any one at kernel end, that is, single user mode, enter boot system

2. Change your password directly with passwd

You can change your password by typing passwd directly after entering the command line

Third, add system services

The system service scripts in CentOS 6 are placed in the/ETC/INIT.D directory, and when a user operates on a service using the Services command, the script is actually called, so we can add our own service scripts.

1. Writing Service Scripts

First three lines basic format: VI testsrv

#!bin/bash

#chkconfig: 35 91 09

#description: Testsrv.

Script first Behavior Shellbang

The second line chkconfig followed by 3 digits, the 1th number is the level to start , the 35 table runs at 3 or 5 o'clock, and the 2nd number is the start priority , the higher the number, the lower the priority, the more it starts at the back, and the 3rd number is the off priority , the smaller the number, the higher the priority, the more the first is closed;

General after two number of the sum of 100, the service is not high dependent can be started after the start, after the first shutdown; the next two bits are one digit (0-9), should be written in 08 format

The third behavior describes the information, which can be modified as needed.

The service script should be able to recognize the basic parameters [Start|stop|restart|status]

Put the written script in the/ETC/INIT.D directory.

2. Add As System service

Chkconfig--add/etc/init.d/testsrv

This command adds the script as a system service, allows Chkconfig to manage it, and creates a corresponding symbolic link k/s entry in each run-level startup service (/ETC/RC.D/RC#.D) directory, the K-Start table closes, and the S-Start table starts

3. Service Operation

Chkconfig--level testsrv on modify default boot level for service

Service Testsrv Restart Restart services

This article is from the "mediocre" blog, please be sure to keep this source http://zzjasper.blog.51cto.com/9781564/1850571

CentOS6 system Start-up process