Node.js: child_process.spawn no output standard output unless 'inherit'

Question

I'm trying to capture standard output from a spawned child_process in Node.js (0.10.29).

Right now I'm just trying with ping.

The following code doesn't print (but does ping)

var exec = require('child_process').exec;
var spawn = require('child_process').spawn;
var util = require('util')

var ping = spawn('ping', ['127.0.0.1'], {stdio: 'pipe'});

ping.stdout.on('data', function(data){
    util.print(data);
})

ping.stderr.on('data', function(data){
    util.print(data);
})

If I change stdio: 'pipe' to stdio: 'inherit' and get rid of the stdout/stderr hooks like so:

var ping = spawn('ping', ['127.0.0.2'], {stdio: 'inherit'});

// ping.stdout.on('data', function(data){
//     util.print(data);
// })

// ping.stderr.on('data', function(data){
//     util.print(data);
// })

I get

PING 127.0.0.2 (127.0.0.2): 56 data bytes
Request timeout for icmp_seq 0
Request timeout for icmp_seq 1

If I change the address from 127.0.0.2 to 127.0.0.1, which I know will respond to the pings and use the original code I get

PING 127.0.0.1 (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.060 ms
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.063 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.152 ms
64 bytes from 127.0.0.1: icmp_seq=3 ttl=64 time=0.124 ms

Why are stdout/stderr not firing data events when the ping isn't pinging or inheriting?

Aureole

what do you mean by ` doesn't print (but does ping)` ? – Ordure

The ping process runs and pings localhost but the node program doesn't print to stdout. – Aureole

but I copied your code and it's working fine I think. – Ordure

@Ordure - which section of code? The trouble code is the first bit of code with 127.0.0.2 – Aureole

sorry I just copied & pasted first part with 127.0.0.1 – Ordure

but for that also it's working. I'm using node v0.11.13 unstable. – Ordure

Anyone try with v0.10.25 or 29? – Aureole

I changed it to var ping = spawn('ping', ['-v', '-c', '10', '127.0.0.2'], {stdio: 'pipe'}); and after the ping runs through all ten requests and ends it outputs PING 127.0.0.2 (127.0.0.2): 56 data bytes Request timeout for icmp_seq 0 Request timeout for icmp_seq 1 Request timeout for icmp_seq 2 Request timeout for icmp_seq 3 Request timeout for icmp_seq 4 Request timeout for icmp_seq 5 Request timeout for icmp_seq 6 Request timeout for icmp_seq 7 Request timeout for icmp_seq 8 --- 127.0.0.2 ping statistics --- 10 packets transmitted, 0 packets received, 100.0% packet loss – Aureole

Also @Ordure I just tried it on 0.11.13 and couldn't get it to print – Aureole

are you getting warning like util.print: Use console.log instead. Try with console.log once. – Ordure

Even with console.log. Basically I've narrowed it down - Node doesn't seem to get stdout until the process has finished in some cases - others it can get a live update. Not sure what the difference between these two are. – Aureole

Did you ever find a solution to this? A way to force stdout to be parsed in a stream instead of waiting until the command finishes? – Infelicity

It actually works fine in latest node version. Probably, you'd just better update yours! – Brainless

Can you update you node.js version? – Groundhog

Answer 1

There have been a lot of issue fixes as well as feature improvements with respect to console printing, on the lines of chunking and buffering. As the issue isn't reproducible any more, I would assume that this could be due to one of the underdocumented behaviors of the then Node.js, based on how many bytes of data is available for printing.

Volley