Linux run level and boot process

1. Linux Run Level:

1)0: Shutdown

2)1: Single user

3)2: Multi-user status No NETWORK service

4)3: Network Service with multi-user status

5)4: The system is not used reserved to the user

6)5: Graphical interface

7)6: System Restart

Note: The Common runlevel is 3 and 5, to modify the run-level modifiable file /etc/inittab ID:5:initdefault the number in the.

2. How to resolve the root password:

when the machine starts, enter the Grub boot program, press e, then select the line with label= (usually the second row), then click an e, enter a 1, (To change the running level to 1) and then enter, you can go to the stand-alone mode, and then reboot to modify the original configuration file.

3.linux boot process:

1) load The BIOS, when you turn on the computer power, the computer will first load the BIOS information,the BIOS contains information about the CPU , device boot sequence information, hard disk information, Memory information,PnP features, and so on. So the computer knows which hardware device to read.

    2) read mbr 0 The first sector of the track is called mbr ( Master Boot Record 512 bytes, with pre-boot information, Partition table information. The system finds the Bios The specified hard disk mbr 0x7c00 Boot loader and specifically to your computer, that's lilo or grub

    3) Boot loader: A small program that precedes the operating system kernel. Usually this small program, we can initialize the hardware device, set up a map of the memory space, so as to bring the system's hardware and software environment to a suitable state, in order to finally call the operating system kernel ready to do everything. Boot loader Grub lilo and spfdisk grub grub configuration information (typically menu.lst grub.lst), and follow this configuration information to start a different operating system.

    4)   uncompressing linux" hint. When the decompression core is complete, the screen output " ok star_kernel linux linux program should work correctly.

    5)   inittab File to set the RunLevel: After the kernel is loaded, the first program that runs is /sbin/init/etc/inittab linux id:5:initdefault: need to run on the level 5. operating level See 1

6) init Process Execution rc.sysinit: After setting the operating level,the first user layer file executed by the Linux system is /etc/rc.d/rc.sysinit Scripting It does a lot of work, including setting Path, set network configuration (/etc/sysconfig/network), start swap partition, set /proc, and so on.

7) boot kernel module: The kernel module is loaded according to the files in the/etc/modules.conf file or the/ETC/MODULES.D directory.

8) perform different runlevel scripts: Depending on the runtime, the system runs rc0.d to the corresponding script in RC6.D to perform the corresponding initialization and start the corresponding service.

9) execute /etc/rc.d/rc.local:rc.local is the place where Linux is left to the user to personalize after all initialization work . You can put the things you want to set up and start up here.

Execute /bin/login program, enter login status: At this time, the system has entered into waiting for user input username and password , you can already log into the system with your own account.

Reference article: http://blog.chinaunix.net/uid-26495963-id-3066282.html

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Linux run level and boot process