Detailed explanation of the seven runlevels of the Centos System

Linux has 7 runlevels)
Running level 0: The system is down. The default running level cannot be set to 0. Otherwise, the system cannot start normally.
Running Level 1: single-user working status, root permission, used for system maintenance, remote login prohibited
Running Level 2: multi-user status (no NFS)
Running Level 3: full multi-user status (with NFS). log on to the console and go to the command line mode.
Operation Level 4: The system is not used and retained
Running Level 5: X11 console. log on to the GUI
Running level 6: The system is shut down and restarted normally. The default running level cannot be set to 6. Otherwise, the system cannot start normally.

Running principle:
1. There are many server script programs in the/etc/rc. d/init. d directory, which are generally called services)
2. Under/etc/rc. d, there are 7 directories named rcN. d, corresponding to 7 running LEVELS OF THE SYSTEM
3. RcN. the d directory contains some symbolic link files that all point to init. the service script file under the d directory. The naming rule is K + nn + service name or S + nn + service name. The nn is a double digit.
4. The system will enter the corresponding rcN. d directory based on the specified running level, and retrieve the linked files under the directory in the order of file names
For Files starting with K, the system terminates the corresponding service.
For Files starting with S, the system starts the corresponding service
5. Run runlevel
6. For other running levels, use init.N
7. In addition, init0 is shutdown, init6. restart the system.