Centos7 notes | operating system startup process, Linux users and permissions
Centos Service Manager: systemd and init run in parallel. (Systemctl and service)
MBR (Master Boot Record) only occupies 446 bytes, and the other 64 bytes are Disk Partition Table (DPT)
**1.1 Operating system startup process: **
1.1.1 Centos6 startup process
(1) Load BIOS
(2) Read MBR (Master Boot Record)
(3) GRUB boot
(4) Load Kernel
(5) Set the run level (init0-init6)
(6) Load rc.sysinit (linux system initialization)
(7) Load the kernel module
(8) Start the run-level program
(9) Read the rc.local file
(10) Execute /bin/login program
1.1.2 Centos7 startup process
The startup process of CentOS7 and CentOS6 is similar, except that the init program is changed to systemd, so I will explain in detail the startup process after systemd
1.1.3 uefi or BIOS initialization, start post POST
1.1.4 Load mbr to memory
1.1.5 GRUB stage
1.1.6 Load kernel and inintamfs module
1.1.7 The kernel starts to initialize, use systemd to replace the init program before centos6
(1) Execute initrd.target
Including mounting the system in the /etc/fstab file, after mounting at this time, you can switch to the root directory
(2) Switch from the initramfs root file system to the disk root directory####
(3) systemd executes the default target configuration####
On the surface of centos7, there is the concept of "run level". In fact, it is to be compatible with previous systems. Each so-called "run level" has a corresponding soft connection point. The default startup level is /etc/systemd/system/default. target, according to its point, you can find which mode the system will enter:
· 0 ==> runlevel0.target, poweroff.target
· 1 ==> runlevel1.target, rescue.target
· 2 ==> runlevel2.target, multi-user.target
· 3 ==> runlevel3.target, multi-user.target
· 4 ==> runlevel4.target, multi-user.target
· 5 ==> runlevel5.target, graphical.target
· 6 ==> runlevel6.target, reboot.target
(4) systemd executes sysinit.target
Is it familiar? Yes, it is called rc.sysint program on CentOS6, which initializes the system and basic.target prepares the operating system
(5) systemd starts the local and server services under multi-user.target####
(6) systemd executes /etc/rc.d/rc.local #### under multi-user.target
1.1.8 Systemd executes getty.target and login service under multi-user.target
Getty.target is also familiar to us. It is the systemd object that starts the terminal. If at this step, the system has not been designated to start the graphical desktop, you can end here, if you want to start the graphical interface, you need to start the desktop program on this basis
1.1.9 systemd executes services required by graphical
CentOS6, 7 startup difference
System startup and server daemon process manager, which is different from centos5's Sysv init, centos6's Upstart (made by Ubuntu). Systemd was first proposed by an employee of Redhat. After the kernel is started, all services and everything will be systemd. Takeover, the kernel is only used to manage hardware resources, which is equivalent to the kernel being emptied, so Linus is very dissatisfied with Redhat.
1.2 Linux users and permissions
Root is also called super administrator. Only users with Uid of 0 can be called super administrators.
LinuxCentos6 users are divided into three types, as follows:
- root user (ID 0)
- System user (ID 1-499)
- Ordinary users (above ID 500)
LinuxCentos7 users are divided into three types, as follows:
- root user (ID 0)
- System user (ID 1-999)
- Ordinary user (ID 1000 and above)
Linux user regulations
- Each user has a UserID, the operating system actually reads the UID, not the user name;
- Each user belongs to a main group and one or more subsidiary groups. A user has 31 subsidiary groups at most;
- Each group has a GroupID;
- Each process runs as a user, who can have resource control authority over the process;
- Each login user has a designated Shell environment.
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