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?

The `ping` process runs and pings localhost but the node program doesn't print to stdout. — dbenny, Jul 08 '14 at 13:51
@Mritunjay - which section of code? The trouble code is the first bit of code with 127.0.0.2 — dbenny, Jul 08 '14 at 14:12
but for that also it's working. I'm using `node v0.11.13` unstable. — Mritunjay, Jul 08 '14 at 14:16
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` — dbenny, Jul 08 '14 at 14:26
Also @Mritunjay I just tried it on 0.11.13 and couldn't get it to print — dbenny, Jul 08 '14 at 15:05
are you getting warning like `util.print: Use console.log instead`. Try with console.log once. — Mritunjay, Jul 09 '14 at 02:41
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. — dbenny, Jul 09 '14 at 15:01
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? — Boushley, Mar 18 '16 at 05:26
It actually works fine in latest node version. Probably, you'd just better update yours! — blade091, Apr 05 '16 at 12:15

score 0 · Answer 1 · edited Jul 12 '23 at 14:35

0

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.

edited Jul 12 '23 at 14:35

Peter Mortensen

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answered Nov 13 '16 at 11:55

Gireesh Punathil

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

1 Answers1

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