Run level
Many programs need to be started up. They are called "services" in Windows and "daemons" in Linux.
A major task of the init process is to run these boot-up programs.
However, different programs need to be started on different occasions. For example, when used as a server, Apache needs to be started, but not when used as a desktop.
Linux allows to assign different boot-up programs for different occasions, which is called "runlevel". In other words, according to the "run level" at startup, determine which programs to run.
The Linux system has 7 runlevels:
Run level 0: The system is in shutdown state. The default run level of the system cannot be set to 0, otherwise it cannot be started normally.
Run level 1: Single user working state, root privileges, used for system maintenance, remote login is prohibited.
Run level 2: Multi-user state (no NFS)
Run level 3: Complete multi-user status (with NFS), enter the console command line mode after login
Run level 4: The system is not used, reserved
Run level 5: X11 console, enter the graphical GUI mode
Run level 6 after login : The system shuts down and restarts normally, the default run level cannot be set to 6, otherwise it cannot start normally