node.js - node的child_process.spawn(...[, options])怎么写多个options?

Question

如果有多个grep，怎么写到上面的语句中？例如cat /dev/urandom |od -x|tr -d ' '|head -n 1

在网上找了下，发现用以下的方法也行，使用spawn和exec有什么区别呢？

const exec = require('child_process').exec;
exec('cat /dev/urandom |od -x|tr -d ' '|head -n 1', (error, stdout, stderr) => {
  if (error) {
    console.error(`exec error: ${error}`);
    return;
  }
  console.log(`stdout: ${stdout}`);
  console.log(`stderr: ${stderr}`);
});

Answer 1

If not encapsulated, you need to listen to multiple events, for example cat /dev/urandom |od -x|tr -d ' '|head -n 1

const spawn = require('child_process').spawn;
const cat = spawn('cat', ['/dev/urandom']);
const od = spawn('od',['-x']);
const tr = spawn('tr',['-d'," "]);
const head = spawn('head', ['-n',1]);

cat.stdout.on('data', data => od.stdin.write(data));
cat.on('close', (code) => od.stdin.end());

od.stdout.on('data', data => tr.stdin.write(data));
od.on('close', (code) => tr.stdin.end());

tr.stdout.on('data', data => head.stdin.write(data));
tr.on('close', (code) => head.stdin.end());

head.stdout.on('data', data => console.log(`${data}`));
head.stdin.on('error',err=>head.stdin.end());

You can also specify a pipe when spwan creates a child process, such as this

const spawn = require('child_process').spawn;
const cat = spawn('cat', ['/dev/urandom'], {stdio: 'pipe'});
const od = spawn('od',['-x'], {stdio: [cat.stdout, 'pipe', 'pipe']});
const tr = spawn('tr',['-d',' '], {stdio: [od.stdout, 'pipe', 'pipe']});
const head = spawn('head', ['-n',1], {stdio: [tr.stdout, 'pipe', 'pipe']});

head.stdout.on('data', data => console.log(`${data}`));
head.stdin.on('error',err=>head.stdin.end());

In actual environment, the information on stderr must be processed

Answer 2

The example you gave doesn’t have grep?

https://nodejs.org/api/child_...

Multiple options are passed as the second parameter in the form of an array: ls -lh /usr

const spawn = require('child_process').spawn;
const ls = spawn('ls', ['-lh', '/usr']);

The example you gavecat /dev/urandom |od -x|tr -d ' '|head -n 1

Just follow the pipeline you screenshot and do it separately:

// cat /dev/urandom
const cat = spawn('cat',['/dev/urandom']);
//od -x
const od = spawn('od',['-x']);
//tr -d ' '
const tr = spawn('tr', ['-d',"' '"]);
//head -n 1
const head = spwan('head', ['-n','1']);

As for the pipeline, see how it is written in the example you screenshot, it is how the next command is executed in the callback