A detailed introduction to Linux system operation levels

1. Introduction to Linux system operation levels

Linux has 7 run levels by default, and the Linux system runs at a specified run level at any time. The programs and services run at different operating levels are different, and the tasks to be completed and the goals to be achieved are also different.

0: Shut down.

1: Single-user mode, which can be imagined as the safe mode of the Windows system, is mainly used for system repair.

2: Incomplete command line mode (command line mode is the command interface), does not include NFS service (a service for sharing files between Linux systems).

3: Complete command line mode, which is a standard character interface (the command line mode we have been using before is this mode).

4: System reserved.

5: Graphics mode.

6: Restart.

2. Check the run level

In Linux system, you can use runlevel command to view the system's run level.

The command is as follows:

# N represents the previous level before entering this level. 3 represents the current level [root@localhost ~]# runlevel 
N 3

illustrate:

N means None , which means that the system enters run level 3 directly when it is turned on, and there is no previous run level. If you switch from the graphical interface to the character interface and check the run level, it should be 5 3 .

3. Modify the current system's operating level

Just use the init command. Note: this is not the init process.

The command is as follows:

Enter the graphical interface. Of course, you can only enter it if the Linux system has installed the graphical interface.

[root@localhost ~]# init 5 shutdown
[root@localhost ~]# init 0 restart
[root@localhost ~]# init 6

illustrate:

If we have not installed the graphical interface and execute the init 5 command, there will be no response on the remote terminal, but in the virtual machine, the following prompt will be displayed: The attempt to start the graphical interface run level failed.

However, when we query the current Linux system's run level and execute the runlevel command, the result is 3 5 Remember that this is an illusion and we have not entered the graphical interface level.

However, please note that using the init command to shut down and restart is not very safe and can easily cause data loss. Therefore, it is recommended that you use shutdown command to shut down and restart.

4. System default run level

Now that we have learned what the operating level of the Linux system is, let's turn our attention back to the system startup process.

The system's default run level is the run level that the system enters directly after startup. What is said above is to check the current running level of the system, rather than the default system running level.

The default operation level of the Linux system is configured in the /etc/inittab configuration file. The contents of the file are as follows:

As you can see from the picture above, a lot of the information inside is put into other different files.

illustrate:

In previous Linux distributions, such as Red Hat 5 or earlier versions, the /etc/inittab /etc/inittab configuration file is a very important configuration file during the system startup process, and the data in it needs to be loaded. However, after Red Hat 6, the function of the /etc/inittab configuration file has been weakened.

Therefore, in Red Hat 6, the only function of the /etc/inittab configuration file is to configure the default run level of the Linux system. id：3：initdefault： : The number 3 in the middle is used to configure the default running level of the Linux system. The number you write is the corresponding level.

Notice:

The run level description in the figure below notes that levels 0 and 6 cannot be set as the system default run level, and the system default run level can only be selected from 1-5. However, it is recommended that the default operating level of the system be 3 or 5. Other levels are either shutdown and restart, or reserved or single user, and cannot be used as the default operating level of the system.

5. /etc/rc.d/rc.local file description

The /etc/rc.d/rc.local configuration file is a very important file. This configuration file will be read before the user logs in. Whatever commands are written in this file will be executed every time the system starts. In other words, if I have any work that needs to be run when the system starts, I only need to write it to the configuration file /etc/rc.d/rc.local . For example, if some services need to be started when the computer starts, the complete service startup command can be configured here. (Note that it is best not to write the service startup method, but to start it by writing the /etc/init.d/ path)

The contents of the /etc/rc.d/rc.local configuration file are as follows:

You can see that the /etc/rc.d/rc.local file is a shell script, touch a very hidden file /var/ lock/ subsys/local . ( touch command: if the file does not exist, it will be created; if the file exists, the access time of the file will be modified.)

In other words, every time the system starts up, it looks for this very hidden file, so the modification time of this file is the system startup time. In other words, the purpose of this command is to obtain the system startup time.

Also note that the /etc/rc.d/rc.local configuration file also has a soft link file, which is the /etc/rc.local configuration file. (The two files are equivalent to one file, and you can modify either one.

The relationship between /etc/rc.d/rc.local configuration file and /etc/rc.local configuration file is as follows:

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