NodeJs concise tutorial

Keywords: node.js shell

NodeJs concise tutorial will start from scratch to learn about NodeJs and help JS developers build a full stack development technology stack!

Focus on getting more NodeJs articles

This article is the eighth chapter of NodeJs concise tutorial, which will introduce the basic operation of NodeJs sub-process module.

The child_process module provides the ability to derive child processes in a similar but different way than popen(3).

Although NodeJs JS threads are single-threaded, can not use multi-core CPU, and can not perform CPU-intensive tasks, but through the form of derived sub-processes plus IPC (interprocess communication), can make full use of multi-core CPU.

spawn

spawnExecutableDesignated commands，spawnThe prototype of the function is as follows:
child_process.spawn(command[,args][,options])

Command < string > command to execute
Args < string []> parameter list passed to the command
Options < Object > Additional Options
- CWD < string > subprocess workdir
- Env < Object > child process environment variable

const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh', '/usr']); // Command Configuration

ls.stdout.on('data', (data) => { // Standard output for listening for command execution
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => { // Standard error output for listening for command execution
  console.log(`stderr: ${data}`);
});

ls.on('close', (code) => { // Listener child process exit
  console.log(`Subprocess exit, using exit code ${code}`);
});

The above routine output (different machine output may be different)

stdout: total 0
drwxr-xr-x  970 root  wheel    30K  7 19 23:00 bin
drwxr-xr-x  306 root  wheel   9.6K  7 12 22:35 lib
drwxr-xr-x  249 root  wheel   7.8K  7 19 23:00 libexec
drwxr-xr-x   15 root  wheel   480B  4  1 14:15 local
drwxr-xr-x  239 root  wheel   7.5K  7 12 22:35 sbin
drwxr-xr-x   46 root  wheel   1.4K  9 21  2018 share
drwxr-xr-x    5 root  wheel   160B  9 21  2018 standalone

//Subprocess exit, using exit code 0

exec

Exc can also execute a specified command, unlike span, where the execution result is notified by a callback, and span is notified by an event. The prototype of exec function is as follows:

exec(command[,options][,callback])

Command < string > command parameters are separated by spaces
Options < Object > Additional Options
- CWD < string > subprocess workdir
- Env < Object > child process environment variable
- Timeout < number > subprocess execution timeout
Callback < Function > Result Callback
- Error < Error > Execution error (not the error output of the child process)
- Stdout < string | Buffer > standard output for subprocesses
- Stderr < string | Buffer > standard error output for subprocesses

const exec = require('child_process').exec;

exec('ls -lh /usr',function(err,stdout,stderr) {
    if(err) {
        console.log('Execution error', err);
    }
    console.log('stdout', stdout);
    console.log('stderr', stderr);
});

Output of the above routine

stdout: total 0
drwxr-xr-x  970 root  wheel    30K  7 19 23:00 bin
drwxr-xr-x  306 root  wheel   9.6K  7 12 22:35 lib
drwxr-xr-x  249 root  wheel   7.8K  7 19 23:00 libexec
drwxr-xr-x   15 root  wheel   480B  4  1 14:15 local
drwxr-xr-x  239 root  wheel   7.5K  7 12 22:35 sbin
drwxr-xr-x   46 root  wheel   1.4K  9 21  2018 share
drwxr-xr-x    5 root  wheel   160B  9 21  2018 standalone

//Subprocess exit, using exit code 0

execFile

ExcFile is similar to exec, but does not derive a shell by default. Instead, the specified execFile is directly derived as a new process, making it slightly more efficient than exec.

Support the same options as exec. Because there is no derived shell, I/O redirection and file wildcards are not supported. ExcFile prototype:

execFile(file[,args][,options][,callback])

File < string > command to execute or executable file path
Args < string []> Array parameter list
Options < Object > Additional Options
- CWD < string > subprocess workdir
- Env < Object > child process environment variable
- Timeout < number > subprocess execution timeout
Callback < Function > Result Callback
- Error < Error > Execution error (not the error output of the child process)
- Stdout < string | Buffer > standard output for subprocesses
- Stderr < string | Buffer > standard error output for subprocesses

const execFile = require('child_process').execFile;
execFile('ls', ['--version'], function(error, stdout, stderr) {
    if(err) {
        console.log('Execution error', err);
    }
    console.log('stdout', stdout);
    console.log('stderr', stderr);
});

The output of the above routine is the same as exec

fork

Fork is a special case of span that is dedicated to deriving new NodeJs processes. Spawn can spawn any process. The prototype of fork method is as follows:

fork(modulePath[,args][,options])

ModulePath < string > JS path to execute
Args < string []> Array parameter list
Options < Object > Additional Options
- workdir of CWD < string > subprocess
- Env < Object > environment variable
- Silent < Boolean > If true, the stdin, stdout and stderr of the child process will be transported to the parent process, otherwise they will inherit from the parent process. Default false

b.js

const fork = require('child_process').fork;

const child = fork('./a.js',{silent:true}); // When silent is true, it can listen for standard output and standard error output of subprocesses.
child.stdout.on('data',function(data){ // Standard output of listener subprocesses
    console.log('child stdout', data.toString('utf8'));
});
child.stderr.on('data', function(data){ // Standard error output for listener subprocesses
    console.log('child stderr', data.toString('utf8'));
});
child.on('close', function(){
    console.log('child exit');
});

a.js

console.log('I'm a child process');

The terminal executes node b.js, and the above routine output:

child stdout I am a child process

child exit

epilogue

This is the end of the introduction of the sub-process module, usually using span and execFile. If you have any questions, please do not hesitate to share them.

Posted by lauriedunsire on Tue, 23 Jul 2019 00:08:43 -0700

Programmer Group