Linux process concept

前文我们了解了，进程的基本概念，在课本上被描述为，正在执行的程序，在linux内核上，认为进程是担当分配系统资源（CPU时间，内存）的实体。

Process = kernel description data structure (PCB) + code and data

在Linux中，PCB为task_struct,它会被装载到RAM(内存)里并且包含着进程的信息。

In this article, we mainly understand some concepts of the process.

view progress

查看进程的方式有很多种，前文我们介绍了在/proc 系统文件中查看，在该文件目录下，进程就是一个又有一个数字目录。

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use of ps

语法：ps ajx 可以查看当前所有进程

ps ajx | grep 程序/文件名来查看程序/文件的进程信息

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explain:

ps ajx | grep proc can find the process information of proc in all process information

ps ajx | head -1 means to display the description information of the attributes of the process

ps ajx | head -1 && ps ajx | grep proc Display the relevant information of the process proc, and bring the description of the process attributes

ps ajx | head -1 && ps ajx | grep proc | grep -v grep does not display grep process information

&& can be replaced by a semicolon, so that the two instructions can be executed together, and the results of the execution will be displayed together.

kill process

语法：kill -9 PID

向执行的PID进程发送9号信号，正在运行的PID进程显示killed，被杀死
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process identifier

PID：进程ID

PPID：父进程ID

获取标识符的方式：getpid（）、getppid（）

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注意：getpid()和getppid()都是系统调用函数，来通过该函数来访问操作系统，得到自己的进程信息，重新运行程序子进程的PID会发生变化，但是父进程ID不会发生变化。

Bash is the parent process of all programs and treats other programs as its child processes. A child has only one parent, but a parent can have multiple children

How to manually create a process

我们知道两种创建进程的方式，第一种就是./proc执行程序，以此来创建进程，第二种，使用fork函数来创建进程。

There are two ways to create a process:

./Run program instruction level

fork function code level

The fork function creates a process

fork函数用来创建子进程。

two outcomes

The child process is successfully created, the child process returns 0, and the parent process returns the PID of the child process

Creation failed, parent process returns -1

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#include<stdio.h>
#include<unistd.h>
#include<sys/types.h>
int main()
{
    
    
    
    printf("这是一个进程，PID为：%d,PPID为：%d\n",getpid(),getppid());
    sleep(1);
    //fork（）创建子进程
    pid_t id =fork();
    
    if(id == 0)
    {
    
    
        //表示子进程
        while(1)
        {
    
    
            printf("子进程，PID为：%d,PPID为：%d\n",getpid(),getppid());
            sleep(1);
        }
    }
    else if(id > 0)
    {
    
    
        //父进程，PID是一个非负数
        while(1)
        {
    
    
            printf("父进程, PID为：%d,PPId为：%d\n",getpid(),getppid());
            sleep(1);
        }
    }
    else{
    
    
        //创建子进程失败
        printf("创建子进程失败\n");
    }
    return 0;
}

question:

Why does fork return 0 to the child process and the PID of the child process to the parent process?

How can a function return twice? How to understand?

How can a variable have different content? How to understand?

What exactly is the fork function doing? What did you do?

order of process calls

How bash was created and its relationship to fork