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This article mainly explains "what is the process of starting the CentOS system". Interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Now let the editor take you to learn "what is the process of starting the CentOS system"?

When we press the boot button, do we know the secret behind the system? Here, I'll take you to explore the secret behind booting the linux system.

1. Power-on self-test

After the motherboard is connected to the power supply, the system will first check each internal equipment by the POST program. If errors are found in the self-test, they will be dealt with in two cases: for serious faults (fatal faults), the machine will be stopped, because various initialization operations have not yet been completed, no hints or signals can be given; for non-serious faults, prompt or sound alarm signals will be given, waiting for users to deal with.

2.BIOS boot phase

After passing the self-test, look for each boot device in order, and the first device with a boot program is the device to be used for this startup. Generally, the computer presses the "F2" or "Del" keys to enter the BIOS, in which we can adjust the boot order of the hard disk.

After finding the first device with a bootstrap program, BIOS finds the MBR in the device and reads the Bootloader in it

3.GRUP boot phase

At this time you can see our Centos system, but generally do not need to operate, will automatically start the system. In fact, we can also start the system manually.

3.1 manually start the system on the grub command line interface:

Grub > root (hd#,#)

Grub > kernel / vmlinuz-VERSION-RELEASE ro root=/dev/DEVICE

Grub > initrd / initramfs-VERSION-RELEASE.img

Grub > boot

3.2 Command line interface of grub:

E: edit mode for editing menu

C: command mode, interactive interface

Help: get a list of help

Help KEYWORD: detailed help

Find (hd#,#) / PATH/TO/SOMEFILE:

Root (hd#,#) sets the root device of grup

Kernel / PATH/TO/KERNEL_FILE: set the kernel file used for this startup; in addition, you can add many cmdline parameters that the kernel supports.

For example: init=/path/to/init, selinux=0

Initrd / PATH/TO/INITRAMFS_FILE: set the ramdisk to provide additional files for the selected kernel

Boot: boot the selected kernel

3.3Profiles: / boot/grub/grub.conf

Configuration items:

Default=#: sets the menu item to be launched by default; the title number starts from 0

Timeout=#: specifies the length of time for menu items to wait for options to be selected

Splashimage= (hd#,#) / PATH/TO/XPM_PIC_FILE: indicates the path of the menu background picture file

Hiddenmenu: hiding menus

Password [--md5] STRING: menu Editor Certification

Title TITLE: defines the "title" of menu items, which can appear multiple times; used to boot multiple kernels or operating systems

Root (hd#,#): grub finds the device partition where the stage2 and kernel files are located; is the "root" of grub

Kernel / PATH/TO/VMLINUZ_FILE [PARAMETERS]: boot kernel

Initrd / PATH/TO/INITRAMFS_FILE: kernel matching ramfs file

Password [--md5] STRING: authenticates when starting a selected kernel or operating system

4. Loading kernel phase

The kernel initializes itself:

Detect all recognizable hardware devices

Load hardware drivers; (it is possible to load drivers with ramdisk)

Mount the root file system read-only

Run the first application in user space: / sbin/init

5.init initialization phase

After the kernel is loaded, the init program will be run. After the init process starts, the control of system startup will be transferred to the init process.

The / sbin/init process is the parent of all processes, and when init is up, it first reads the configuration file / etc/inittab and does the following:

1. Execute the system initialization script (/ etc/rc.d/rc.sysinit), make the basic configuration of the system, and mount the root file system and other file systems in a read-write manner. After that, the system is basically running, and then the running level and the corresponding service startup are needed.

two。 Determine the level of operation entered after startup

3. Execute / etc/rc.d/rc, which defines that the order in which services are started is K followed by S, while the service status of each runlevel is placed in the / etc/rc.d/rcn.d (nasty 0running 6) directory, and all files are linked to the corresponding files under / etc/init.d.

4. About the settings of key sequence

5. Script definition for UPS

6. Start the virtual terminal / sbin/mingetty

7. Run X on runlevel 5

Configuration file: / etc/inittab

Each line defines an action and its corresponding process

Id:runlevels:action:process

Id: the identifier of a task

Runlevels: at which level to start this task; #, #, which can be empty, indicating all levels

Action: under what conditions to start this task

Process: task; (script or program)

Action:

Wait: execute once when waiting to switch to the level of this task

Respawn: once this task is terminated, restart it automatically

Initdefault: sets the default run level; at this point, process omits

Sysinit: sets the initialization mode of the system. The / etc/rc.d/rc.sysinit script is usually specified here.

Knights: the service to be stopped; the smaller the number, the more priority it is to shut down; the dependent service shuts down first, then the dependent service shuts down

Slots: the service to be started. The smaller the number, the more priority it is to start. The dependent service starts first, while the dependent service starts later.

Chkconfig command: controls / etc/init.d/ the startup or shutdown status of each service script at all levels

View: chkconfig-- list [name]

Add: chkconfig-- add name

Delete: chkconfig-- del name

Modify the specified link type:

Chkconfig [--level LEVELS] name

-- level LEVELS: specifies the level to control; default is 2345

Init command:

Level switching: init #

Level view: who-r

Note:

Normally, the last service S99local started is not linked to a script under / etc/init.d, but to a / etc/rc.d/rc.local (/ etc/rc.local) script; therefore, when a program that is inconvenient or does not need to be written as a service script expects to boot and run automatically, it can be placed directly in this script file.

At this point, I believe you have a deeper understanding of "what is the process of starting the CentOS system". You might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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