There are several levels of linux operation

Linux operation has 7 levels: 1. "0" refers to the system shutdown state; 2. "1" refers to the single-user working state; 3. "2" refers to the multi-user state ( (without NFS); 4. "3" refers to the complete multi-user mode (with NFS); 5. "4" refers to the system not being used; 6. "5" refers to the graphical mode; 7. "6 ” refers to a graceful shutdown and restart of the system.

#The operating environment of this tutorial: linux7.3 system, Dell G3 computer.

What is the Linux run level?

Linux can support run level settings. The run level is the functional level of the operating system currently running. Level is from 0 to 6. Previous versions of the Centos7 system defined the system through the /etc/inittab file, while CentOS7 used /lib/systemd/system/runlevel*.

The default runlevel of most desktop Linux systems is 5, which is a graphical interface when users log in, while the default runlevel of most server versions of Linux systems is 3, which is a character interface when users log in.

Runlevel 1 and 2 are rarely used except for debugging. Runlevel s and S are not directly used by users, but are used to prepare for Single user mode.

There are seven levels of Linux operation

##LevelDescription0 In shutdown state, the system default run level cannot be set to 0, otherwise the system cannot start normally. Use the init 0 command to shut down the system. 1Single-user state, in this state only the root user can log in. Used for system maintenance, remote login is prohibited, equivalent to the safe mode under Windows. 2Multi-user state (without NFS). There is no internet service. 3Full multi-user state (with NFS). There is a network service. After logging in, enter the console command line mode. 4The system is not used and is reserved for general use. It can be used to do some things in some special circumstances. For example, when the laptop's battery is exhausted, you can switch to this mode to make some settings. 5 Graphical mode, after logging in, enter the graphical GUI mode or GNOME, KDE graphical interface, such as X Window system. 6The system shuts down and restarts normally. The default run level cannot be set to 6, otherwise it cannot start normally and will always be turned on and restarted.

The principle of Linux run level

• In the directory /etc/rc.d/ There are many server script programs under init.d, generally called services.

• There are 7 directories named rcN.d under /etc/rc.d, corresponding to the 7 operating levels of the system.

• rcN.d directory are some symbolic link files, 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.

• 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.

How to check the Linux run level

Use the runlevel command to check the current run level, enter the following command:

runlevel

The running results are as follows:

The above figure shows that the running level at this time is 3.

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