How to learn the process and child_process modules in node (detailed tutorial)

This article mainly introduces the node process and child_process module study notes. Now I share them with you and give them a reference.

After struggling with the process for a week, I finally understood the obscure and difficult document, and am ready to share my understanding with everyone. I also hope that everyone can point out some opinions

Process The concept of

1. #In Node.js every application is an instance object of the process class.

2. Use the process object to represent the application. This is a global object through which you can obtain the properties of the Node.jsy application and the user and environment running the program. methods and events.

Several important attributes in the process

1. stdin standard input readable stream

2. stdout standard input writable stream

3. stderr standard error output stream

4. argv terminal input parameter array

5. env operating system environment information

6. pid application process id

stdin and stdout

process.stdin.on('data', (chunk) => {
 process.stdout.write('进程接收到数据' + chunk)
})

Run result

argv

console.log(process.env)

##env: Enter export NODE_ENV=develop

console.log(process.env.NODE_ENV) //develop

# in the mac terminal

##Methods of process

process.memoryUsage() View memory usage information
process.nextTick() Current The callback function is executed after the eventloop is executed
process.chdir() The chdir method is used to modify the current working directory used in the Node.js application
process.cwd() The current working directory of the process
process.kill() Kill the process
process.uncaughtException() When the application The uncaughtException event of the process object is triggered when an uncaught exception is thrown

6. say() //方法不存在
   process.on('uncaughtException',function(err){
    console.log('捕获到一个未被处理的错误:',err);
   });

child_process

The child process is the focus of today, I also have some things that I don’t quite understand, and I hope I can communicate with you more

The background of child_process

In Node.js, there is only one thread to perform all operations. If a certain When an operation consumes a lot of CPU resources, subsequent operations need to wait.

In Node.js, a child_process module is provided, through which multiple child processes can be started, memory space can be shared between multiple child processes, and information exchange can be achieved through mutual communication of child processes.

The child_process module gives node the ability to create child processes at will. The official document of node gives four methods for the child_proces module. Mapping to the operating system actually creates child processes. But for developers only, the APIs of these methods are a little different

child_process.exec(command[, options][, callback]) starts the

child process to execute the shell command, which can be done through the callback parameter Get the script shell execution result

child_process.execfile(file[, args][, options][, callback])

Different from the exec type, it does not execute a shell command but an executable File

child_process.spawn(command[, args][, options]) only executes a shell command and does not need to obtain the execution results

child_process.fork(modulePath[, args][, options] ]) You can use node

to execute the .js file, and there is no need to obtain the execution results. The child process coming out of fork must be the node process

spawn

Syntax: child_process.spawn(command, [args], [options])

command Parameters that must be specified, specify the command to be executed
args array, which stores all the parameters required to run the command
The options parameter is an object, used to specify the options used when starting the child process

3. const { spawn } = require('child_process')
   const path = require('path')
   
   let child1 = spawn('node', ['test1.js', 'yanyongchao'], {
    stdio: ['pipe', 'pipe', 'pipe'], // 三个元素数组 下面会详解
    cwd: __dirname, 子进程工作目录
    env: process.env, 环境变量
    detached: true // 如果为true，当父进程不存在时也可以独立存在
   })

In fact, the above is easy to understand except for the sdtio array. Let’s analyze stdio

stdio

stdio is an array used to set standard input, standard output, and error output. Personal understanding

pipe: Establish a pipe between the parent process and the child process

Main process code

const path = require('path')
const { spawn } = require('child_process')

let p = spawn('node', ['childs_t.js'], {
 cwd: path.join(__dirname, 'childs'),
 stdio: ['pipe', 'pipe', process.stderr]
})

p.stdout.on('data', (data) => {
 console.log(data.toString())
}) 
// 这里用stdout原因： 子进程的数据流与常规理解的数据流方向相反，
// stdin：写入流，stdout、stderr：读取流。

Child process code

process.stdout.write('asd')

If you put it in stdio A stream, process.stdout, process.stdin

Main process code

const { spawn } = require('child_process')
const path = require('path')

// 如果放的是一个流，则意味着父进程和子进程共享一个流
const p = spawn('node', ['child_t.js'], {
 cwd: path.join(__dirname, 'childs'),
 stdio: [process.stdin, process.stdout, process.stderr]
})

Sub process code

process.stdout.write('asd') //控制台会输出asd

ipc

Main Process code

const path = require('path')
const { spawn } = require('child_process')

let p = spawn('node', ['child_t.js'], {
 cwd: path.join(__dirname, 'childs'),
 stdio: ['ipc', 'pipe', 'pipe']
})

p.on('message', (msg) => {
 console.log(msg)
})

p.send('hello chhild_process')

Child process code

process.on('message', (msg) => {
 process.send('子进程' + msg)
})
// child.send(message,[sendHandle]);//在父进程中向子进程发送消息
// process.send(message,[sendHandle]);//在子进程中向主进程发送消息

detached mode

const { spawn } = require('child_process')
const fs = require('fs')
const path = require('path')
let out = fs.openSync(path.join(__dirname, 'childs/msg.txt'), 'w', 0o666)

let p = spawn('node', ['test4.js'], {
 detached: true, //保证父进程结束，子进程仍然可以运行
 stdio: 'ignore',
 cwd: path.join(__dirname, 'childs')
})

p.unref()

p.on('close', function() {
 console.log('子进程关闭')
})

p.on('exit', function() {
 console.log('子进程退出')
})

p.on('error', function(err) {
 console.log('子进程1开启失败' + err)
})

fork starts a child process

Spawn a new Node.js process and call a specified module by establishing an IPC communication channel, which allows the parent process and the child process to send information to each other
The fork method returns an implicitly created ChildProcess object representing the child process
After the input/output operation of the child process is completed, the child process will not automatically exit and must Explicitly exit using the process.exit() method
Child process code

const { fork } = require('child_process')
const path = require('path')
let child = fork(path.join(__dirname, 'childs/fork1.js'))

child.on('message', (data) => {
 console.log('父进程接收到消息' + data)
})

child.send('hello fork')

child.on('error', (err) => {
 console.error(err)
})

Child process code

process.on('message', (m, setHandle) => {
 console.log('子进程接收到消息' + m)
 process.send(m) //sendHandle是一个 net.Socket 或 net.Server 对象
})

exec starts the child process

// exec同步执行一个shell命令
let { exec } = require('child_process')
let path = require('path')

// 用于使用shell执行命令, 同步方法

let p1 = exec('node exec.js a b c', {cwd: path.join(__dirname, 'childs')}, function(err, stdout, stderr) {
 console.log(stdout)
})

execFile opens the child process

let { execFile } = require('child_process')
let path = require('path')

let p1 = execFile('node', ['exec.js', 'a', 'b', 'c'], {
 cwd: path.join(__dirname, 'childs')
}, function(err, stdout, stderr) {
 console.log(stdout)
})

上面是我整理给大家的，希望今后会对大家有帮助。

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