7 Types of Linux operating levels

Linux has 7 runlevel (runlevel)
RunLevel 0: System down state, system default RunLevel cannot be set to 0, otherwise it will not start normally
RunLevel 1: Single User working status, root privilege, for system maintenance, remote login Forbidden
RunLevel 2: Multi-User State (no NFS)
RunLevel 3: Full multiuser State (with NFS), enter console command line mode after login
Run Level 4: System not used, reserved
Run level 5:x11 console, login to GUI mode
RunLevel 6: The system shuts down gracefully and restarts, the default runlevel cannot be set to 6, otherwise it will not start normally

The operating level principle:
1. There are many server scripts under directory/ETC/RC.D/INIT.D, commonly referred to as services (service)
2. There are 7 directories named RCN.D under/ETC/RC.D, corresponding to the 7 RunLevel of the system
3. RCN.D directories are symbolic link files that point to a service script file under the INIT.D directory, with a naming convention of 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 RunLevel, and retrieve the linked files under the directory in the order of file name.
For files starting with K, the system terminates the corresponding service
For files beginning with S, the system will start the corresponding service
5. View run levels by: RunLevel
6. Go to other RunLevel by: Init N
7. Also init0 for shutdown, init 6 reboot system

7 Types of Linux operating levels