Node.js spawn child process and get terminal output live

Question

I have a script that outputs 'hi', sleeps for a second, outputs 'hi', sleeps for 1 second, and so on and so forth. Now I thought I would be able to tackle this problem with this model.

var spawn = require('child_process').spawn,
temp    = spawn('PATH TO SCRIPT WITH THE ABOVE BEHAVIOUR');

temp.stdout.pipe(process.stdout);

Now the problem is that the task needs to be finished in order for the output to be displayed. As I am understanding it, this is due to the fact that the newly spawned process takes execution control. Obviously node.js does not support threads so any solutions? My idea was to possibly run two instances, first one for the specific purpose of creating the task and have it pipe the output to process of the second instance, considering this can be achieved.

Answer 1

It's much easier now (6 years later)!

Spawn returns a childObject, which you can then listen for events with. The events are:

• Class: ChildProcess
  • Event: 'error'
  • Event: 'exit'
  • Event: 'close'
  • Event: 'disconnect'
  • Event: 'message'

There are also a bunch of objects from childObject, they are:

• Class: ChildProcess
  • child.stdin
  • child.stdout
  • child.stderr
  • child.stdio
  • child.pid
  • child.connected
  • child.kill([signal])
  • child.send(message[, sendHandle][, callback])
  • child.disconnect()

See more information here about childObject: https://nodejs.org/api/child_process.html

Asynchronous

If you want to run your process in the background while node is still able to continue to execute, use the asynchronous method. You can still choose to perform actions after your process completes, and when the process has any output (for example if you want to send a script's output to the client).

child_process.spawn(...); (Node v0.1.90)

var spawn = require('child_process').spawn;
var child = spawn('node ./commands/server.js');

// You can also use a variable to save the output 
// for when the script closes later
var scriptOutput = "";

child.stdout.setEncoding('utf8');
child.stdout.on('data', function(data) {
    //Here is where the output goes

    console.log('stdout: ' + data);

    data=data.toString();
    scriptOutput+=data;
});

child.stderr.setEncoding('utf8');
child.stderr.on('data', function(data) {
    //Here is where the error output goes

    console.log('stderr: ' + data);

    data=data.toString();
    scriptOutput+=data;
});

child.on('close', function(code) {
    //Here you can get the exit code of the script

    console.log('closing code: ' + code);

    console.log('Full output of script: ',scriptOutput);
});

Here's how you would use a callback + asynchronous method:

var child_process = require('child_process');

console.log("Node Version: ", process.version);

run_script("ls", ["-l", "/home"], function(output, exit_code) {
    console.log("Process Finished.");
    console.log('closing code: ' + exit_code);
    console.log('Full output of script: ',output);
});

console.log ("Continuing to do node things while the process runs at the same time...");

// This function will output the lines from the script 
// AS is runs, AND will return the full combined output
// as well as exit code when it's done (using the callback).
function run_script(command, args, callback) {
    console.log("Starting Process.");
    var child = child_process.spawn(command, args);

    var scriptOutput = "";

    child.stdout.setEncoding('utf8');
    child.stdout.on('data', function(data) {
        console.log('stdout: ' + data);

        data=data.toString();
        scriptOutput+=data;
    });

    child.stderr.setEncoding('utf8');
    child.stderr.on('data', function(data) {
        console.log('stderr: ' + data);

        data=data.toString();
        scriptOutput+=data;
    });

    child.on('close', function(code) {
        callback(scriptOutput,code);
    });
}

Using the method above, you can send every line of output from the script to the client (for example using Socket.io to send each line when you receive events on stdout or stderr).

Synchronous

If you want node to stop what it's doing and wait until the script completes, you can use the synchronous version:

child_process.spawnSync(...); (Node v0.11.12+)

Issues with this method:

• If the script takes a while to complete, your server will hang for that amount of time!
• The stdout will only be returned once the script has finished running. Because it's synchronous, it cannot continue until the current line has finished. Therefore it's unable to capture the 'stdout' event until the spawn line has finished.

How to use it:

var child_process = require('child_process');

var child = child_process.spawnSync("ls", ["-l", "/home"], { encoding : 'utf8' });
console.log("Process finished.");
if(child.error) {
    console.log("ERROR: ",child.error);
}
console.log("stdout: ",child.stdout);
console.log("stderr: ",child.stderr);
console.log("exist code: ",child.status);