How many run levels does Linux have?

Linux has 7 operating levels, which are: 1. "0", system shutdown state; 2. "1", single-user working state; 3. "2", multi-user state; 4. " 3", complete multi-user mode; 5. "4", the system is not used; 6. "5", graphical mode; 7. "6", the system shuts down and restarts normally.

#The operating environment of this tutorial: linux5.9.8 system, Dell G3 computer.

Linux running level:

init is one of the indispensable programs in Linux system operation. init process, which is a user-level process started by the kernel. The kernel looks for init in several places where it has been used in the past. Its correct location (for Linux systems) is /sbin/init. If the kernel cannot find init, it will try to run /bin/sh. If this fails, the system will fail to start.

Linux has 7 running levels (0: shutdown, shutdown mode, 1: single-user mode, 2: multi-user mode, 3: complete multi-user text mode, 4: system is not used, reserved and generally not used , 5: Graphical mode, 6: Restart mode), the run level is the functional level of the operating system currently running. This level goes from 0 to 6 and has different functions.

Level description:

• 0: System shutdown (shutdown) mode, the system default running level cannot be set to 0, otherwise it will not start normally and will automatically shut down as soon as it is turned on. .

• 1: Single-user mode, root authority, used for system maintenance, remote login is prohibited, just like safe mode login under Windows.

• 2: Multi-user mode, no NFS network support.

• 3: Complete multi-user text mode, with NFS, enter the console command line mode after logging in.

• 4: The 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.

• 6: The 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 standard Linux run level is 3 or 5

Run level principle:

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

2. There are 7 directories named rcN.d under /etc/rc.d, corresponding to the system The 7 run levels

3.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.

4. 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 (Kill), the system will terminate the corresponding Service; for files starting with S (Start), the system will start the corresponding service

5. To check the running level, use: runlevel

6. To enter other running levels, use: initN, if init3 Then enter the terminal mode, and init5 logs into the graphical GUI mode

7. In addition, init0 is to shut down, and init6 is to restart the system

The standard Linux running level is 3 or 5. If it is 3, The system is in multi-user state; if it is 5, it is running the XWindow system. Different run levels have different uses and should be set according to your different situations. For example, if you lose your root password, you can boot the machine into a single-user state to set it up. Enter:

init=/bin/shrw

at the lilo prompt after startup. This will make the machine enter run level 1 and mount the root file system as read and write. It will pass all system authentication, let you use the passwd program to change the root password, and then boot to a new runlevel.

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