Shulou(Shulou.com)06/03 Report--

Starting from the CentOS7 version, the system startup and the service manager are handed over to systemd to manage.

1. Overview of the guidance process:

1. Post:

After the server is powered on, the CPU, memory, graphics card, keyboard and other devices will be initially tested according to the settings of the motherboard BIOS (basic input system), and some of the hardware will be initialized.

2. MBR boot:

When booting the system from the local hard disk, first transfer the system control to the partition containing the system operation boot file according to the setting of the MBR (master boot record) in the first sector of the hard disk, or call the boot menu directly according to the boot information in the MBR record.

3. GRUB menu:

GRUB is the most widely used multi-system bootstrap program for linux systems. After system control is passed to GRUB, it will be displayed to provide options and load linux kernel files based on options.

4. Load linux kernel and memory file system:

The system bootstrap program loads the kernel and the memory file system from the local hard disk (initramfs is used in centos7). The memory file system initramfs is archived by gzip's cpio, which contains all necessary hardware kernel modules, initialization scripts, etc.

5. Load hardware driver and initialize process:

The kernel initializes all the hardware that finds the driver in initramfs, then copies it as PID 1 from initramfs execution / sbin/init,CtnOS7 to systemd,systemd to start all units in initrd.target, and mounts the root file system / sysroot, the kernel and files switch from the memory file system to the system root file system, and rerun / sysroot.systemd. Systemd starts the default target (graphics or character terminal) and finally waits for the user to log in.

2. Init process:

1. / sbin/init program is loaded and run by Linux kernel.

2. It is the first process in the system

3. The PID (process tag) number is always 1.

III. Systemd process:

1. Introduction to systemd:

A process (a running program) in a linux system that is marked with numbers, and the identity tag of each process is called PID. Starting with the CentOS7 version of the system, systemd calls the initialization process PID constant 1, which is the first program loaded by the kernel.

The systemd process is not allowed to be terminated easily, and when we need to switch different systems, we can send the correct command to the systemd process.

The purpose of systemd is to start more services concurrently, so as to improve the startup speed of the system. The biggest advantage is that it has the ability to provide on-demand startup services.

2. Systemd unit type:

3. The Systemd target corresponding to the runlevel:

Among them, 3 and 5 are the two levels commonly used in our study and work.

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