What steps do you need when Linux starts? This article will describe the role of different running levels in the startup. Linux seems to be a monster for advanced users under the DOS / WIN9X / NT platform. No confi g.sys, no autoexec.bat, machine configuration with personal characteristics, don't know where you start. It should be noted that many people think that Linux is a clone of UNIX, but this statement is unfair for Linux. Linux is more open than UNIX, more powerful. We should call GNU / Linux. When Linux is started, run a program called InIT, then start the following tasks, including multi-user environments, networks, etc. So what is the run level? Simply put, the run level is the functional level of the operating system is currently running. This level has different functions from 1 to 6. These levels are specified in the / etc / inittab file. This file is the main file of the init program, and the first running service is those that are placed in the /etc/rc.d directory. In most Linux release, the startup script is located in /etc/rc.d/init.d. These scripts are connected to the /etc/rc.d/rcn.d directory by the ln command. (N is running grade 0-6), for example /etc/rc.d/rc2.d, the S10Network below is connected to the NetWork script under /etc/rc.d/init.d. Therefore, we can know that the files below RC2.D are related to running level 2. The START starting is that START is the meaning of the startup service, and the number 10 after the latter is the order of the startup. For example, in the same directory, you can also see the file of the S80Postfix, 80 is the order in the order, because there is no way to start the network, starting postfix does not have any effect. Look at /etc/rc.d/rc3.d, you can see the file S60NFSLOCK, but this file does not exist in / etc / rc.d / rc2.d directory. NFS To use this file, it is generally used in a multi-user environment, so put it in the RC3.D directory. In addition, in /etc/rc.d/rc2.d, you can also see files that start with K, such as /etc/rc.d/rc2.d/k45named, and k standby KILL. Standard Linux runs 3 or 5, if it is 3, the system is in multi-user status. If it is 5, it is running the X WINDOW system. If you are currently 3 or 5, then you reduce the running level to 2, init will execute the K45named script. Different run levels are defined as follows: (can refer to the / etc / inittab inside Red Hat Linux) # 缺 时级 运级, the level used by RHS is as follows: # 0 - Stop (Do not set the initDefault to 0) # 1 - Single user mode # 2 - Multiuser, but no NFS # 3 - Complete Multi-User Mode # 4 - No Use # 5 - X11 # 6 - Restart (Do not set the initDefault to 6) # Detailed explanation: 0 is downtime, machine close. 1 is a single user mode, just like the security mode under Win9x. 2 For multi-user mode, but there is no NFS support. 3 is a complete multi-user mode, which is a standard operating level. 4 Generally, it can be used to do something in some special cases.
