Run level
Boot----"BIOS----"/boot----"init process----"run level----"services corresponding to run level
View the default level: vi/etc/inittab
The Linux system has 7 operating levels, the most commonly used are level 3 and 5
--0: The system is stopped, the default running level of the system cannot be set to 0, otherwise it cannot start normally
--1: Single user working status, root authority, user system maintenance, remote login is prohibited
--2: Multi-user status (no NFS), no network support
--3: Fully multi-user state (with NFS), enter the console command line mode after logging in
--4: The system is not used, reserved
--5: X11 console, enter image GUI mode after login
--6: The system shuts down and restarts normally, the default run level cannot be set to 6, otherwise it cannot start normally
Command line: chkconfig --level 5 service name on
The difference between centos6 and centos7
