Linux study note _ 5_Linux Boot Process Analysis _ 2_inittab File Analysis and System Startup Process Analysis

Inittab file profiling

[Inittab file format]: id: runlevels: action: process

[Filter out rows starting with #: grep-v "^ #"/etc/inittab | more]

Id: identifier, which is unique and can contain two digits or letters.

Runlevels: indicates the running level. You can specify multiple runlevels. If this parameter is left blank, the value ranges from 0 ~ 6. Execute all running levels

Action: Specifies the running status

Process: Specifies the script or command to run.

Body Analysis:

Statement 1: id: 5: initdefault:

Initdefault: Specifies the default startup running level and the default startup command.

5: You can change it to 3. The default startup character interface, instead of starting the graphic interface.

Statement 2: si: sysinit:/etc/rc. d/rc. sysinit

/Etc/rc. d/rc. sysinit: starts the system service program, such as setting the system clock in the system environment variable, loading the font, checking the file system, and generating the system startup information log... [Shell script, run at once boot time]

Sysinit: the system starts and executes the commands in process. No matter which running level, the service starts as long as the system starts.

Statement 3:/etc/rc. d/rc

Purpose: based on the previous article, determine the default running level to call the/etc/rc. d/rc Script, execute the service program in the directory of the corresponding running level, and complete the initialization settings for the corresponding running level.

Statement 4:/etc/rc. d/rc [0123456]. d // [/etc/rcN. d In Ubuntu]

Stores soft symbolic links corresponding to running-level service program scripts,

Link to the script in the init. d directory

S-start: these services are loaded in sequence at system startup.

K-kill: Shut down these services in turn when the system shuts down [if the file name is changed, it will not start]

Number: indicates the startup sequence. The smaller the number, the higher the startup priority.

Script Name: for example, vmware

If you do not want to start this service, you only need to rename it, for example:

Rm S03-vmware-tools s03-vmware-tools // It does not start automatically

Common Values of action:

1. initdefault, sysinit

2. wait: Execute the specified command in process and wait until it ends before running other commands.

Once: Execute the specified command in process without waiting for it to end

3. ctrlaltdel: Press Ctrl + Alt + Del to specify the process

4. respawn: 1. If the command specified by process is terminated, run the command again.

5. powerfail: When a power error occurs, run the command specified by process without waiting for it to end.

Powerokwait: Execute the command specified by process when the power supply is restored

/Etc/rc. d/init. d [in Ubuntu:/etc/init. d]

This directory stores the startup scripts of various running levels. In this way, we can manually start/close any service in the system.

For example, manually start ssh/Etc/rc. d/init. d/sshd# Prompt options

# In Ubuntu:/etc/init. d/ssh

Set the self-starting program:

Method 1: generate a soft link ln-s

For example, write a script program vim/etc/rc. d/init. d/msg. sh.

Data>/root/msg. today

Who>/root/msg. today

Free>/root/msg. today

Chmod u + x/etc/rc. d/init. d/msg. sh

Ln-s/etc/rc. d/init. d/msg. sh/etc/rc. d/rc4.d/S100msg. sh // generate soft link

Method 2: chkconfig: only services installed by default can be changed.

Directly hitting chkconfig will list its usage. The usage is very intuitive and simple and requires installation!

Chkconfig -- list lists all service programs in the default state at each running level.

Chkconfig -- list sshd only displays the sshd startup status

...

Method 3: ntsysv

If ntsysv is hit, the startup setting of the current running level is changed.

Other running levels: ntsysv -- levels 3

Simple and intuitive. installation is required! [Many UNIX systems do not have this service]

Command-related applications:

1. dmesg: view all startup information and check whether the hardware is recognized by the kernel.

Dmesg | grep eth0 // view the load information

Dmesg | grep sda // view the loading information of the first hard disk

2. All directory files of the system are saved in the/var/log directory.

Linux Startup Process Overview:

Firmware [CMOS/BIOS] -- POST home appliance self-check [software/hardware clock synchronization, NTP]

|

BootLoader [GRUB] [in Windows: NTLDR]: load the kernel

|Root --/boot

|Kernel

|Initrd

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Kernel? Hardware

| Init PID = 1 [parent-child process relationship]

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/Etc/inittabFile Format id: runlevels: action: process

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Default running level of the initdefault system: 1. single-user mode [security mode ]...

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/Etc/rc. d/rc. sysinit: system initialization, regardless of the running level, will run this file

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/Etc/rc. d/rc

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/Etc/rc. d/rcN. d N = 0 ~ 6 S-start K-kill

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Username:

PassWd:

[Also: Be sure to develop the habit of using help !]