Linux systems have 7 runlevels (runlevels)

Linux system has 7 runlevels
Runlevel 0: System shutdown state, the system default runlevel cannot be set to 0, otherwise it cannot start normally
Runlevel 1: Single-user working state, root authority, used for system maintenance , remote login is prohibited
Run Level 2: Multi-user state (without NFS)
Run Level 3: Complete multi-user state (with NFS), enter the console command line mode after logging in
Run Level 4: The system is not used, reserved
Run level 5: X11 console, enter the graphical GUI mode after logging in.
Run level 6: The system shuts down and restarts normally. The default run level cannot be set to 6, otherwise it cannot start normally.

Principle of run level:
1. There are many server script programs in the directory /etc/rc.d/init.d, generally called services
2. There are 7 directories named rcN.d under /etc/rc.d, corresponding to the 7 operating levels of the system
3. There are some symbolic link files in the rcN.d directory. These link files point to the service script file in the init.d directory. The naming rule is K+nn+service name or S+nn+service name, where nn is a two-digit number.
4. The system will enter the corresponding rcN.d directory according to the specified run level, and retrieve the link files in the directory in order of file names. For files starting with K, the system will terminate the corresponding service. For files starting with S, the system will Start the corresponding service
5. To check the run level, use: runlevel
6. To enter other run levels use: init N
7. In addition, init0 is to shut down the system, and init 6 is to restart the system