如何使用模块child_process在Node.js中实现一进一出

Question

I try to make R and nodejs communicate with each other, but I run into some problems. I will explain it more with this chunk of code

var cp = require('child_process')
var spawn = cp.spawn

var r = spawn("rterm",["--ess" ,"--slave"])

r.stdout.on('data',function(data){
     console.log(data.toString())
    })

I send a command to the child process, the child process then generates a lot of output(within a reasonable length), the output is automatically read back by Nodejs because of 'data' listener.

But for the child process, the output is one chunk, then when read back by Nodejs, it becomes two or more chunks SOMETIMES. And that is not what I wanted.

How do I fix that in my code?

Answer 1

When you spawn a child process , stdout is a Readable Stream . When you attach a ' data ' event, the stream goes into 'flowing mode', meaning that it will return the data as soon as it is available. When you have a lot of data, it will return your data one chunk at a time.

There's a couple of ways to fix that :

You could buffer all the chunks, and output them when the ' end ' event is fired, like below :

var output="";
r.stdout.on('data',function(data){
  output+=data
}).on('end',function(){
  console.log(output);
});

Or, you can remove the data event listener altogether and call stdout.read() .

stdout.read() will read all the content of the stream at any moment, so you must call it when your process won't output anything anymore.