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This article mainly introduces "how to understand the daemons in the Linux system". In the daily operation, I believe many people have doubts about how to understand the daemons in the Linux system. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts about "how to understand the daemons in the Linux system". Next, please follow the editor to study!

The daemon daemon is a process with a long lifetime. They are often started when the system is bootstrapped and terminated only when the system is shut down. Because they do not control the terminal, they are running in the background. The UNIX system has many daemons that perform daily transaction activities.

1. System bootstrap

The word bootstrapping comes from the idea that people stand up by their own "bootstrap" institutions. The computer must have the ability to bootstrap to activate all its components so that it can load the operating system, and then the operating system can undertake more complex tasks that cannot be accomplished by bootstrapping code alone.

Bootstrap has only two functions: power-on self-test and disk boot.

Power-on self-test: when we press the computer power switch, the machine does not seem to respond for the first few seconds, in fact, the computer is undergoing a power-on self-test to determine that all its components are working correctly. If there is something wrong with a component, an alarm message will appear on the monitor (if the monitor does not work properly, alarm with a series of beeps). Since most computers are very reliable, the power-on self-test alarm is very rare.

Disk boot: find the disk drive with the operating system installed. There are two reasons for loading an operating system from disk, one is that it is easy to upgrade the operating system, and the other is that it gives users the freedom to choose the operating system.

When the power-on self-test and disk boot are complete, the bootstrap operation starts a process of reading and writing operating system files and copying them to random access memory, and the machine is the real computer. There are two ways to start a computer: cold start and hot start. the difference between them is that hot start does not carry out the self-test of the machine (the check and test of the configuration of the machine itself). When the computer crashes for some reason in the course of use, the computer can be hot-started.

2. The concept of daemon

The daemon can be viewed through the ps axj command:

Parameter a lists not only the processes of the current user, but also those of all other users, parameter x lists not only processes with control terminals, but also processes without control terminals, and parameter j lists information related to job control.

The code is as follows:

PPID PID PGID SID TTY TPGID STAT UID TIME COMMAND

01 1 1?-1 Ss 0 0:01 / sbin/init

0 200?-1 S < 00: 00 [kthreadd]

2 300?-1 S < 00: 00 [migration/0]

2 400?-1 S < 00: 00 [ksoftirqd/0].

1 2373 2373 2373?-1 S

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