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I've having a problem with the Windows program WMIC.exe (used for querying Windows Management Instrumentation), from within NodeJS.

I can spawn a wmic.exe process, but it'll not output anything or accept any input. However, if I set the stdin to null it'll run correctly and I'll receive output.

var spawn = require('child_process').spawn;
var wmic = spawn('wmic', [], {stdio: ['ignore']});

wmic.stdout.on('data', function(data) {
  console.log('Data received:' + data);
});

wmic.on('close', function(code) {
  console.log('Spawned process ended with code: ' + code);
});

I want to use wmic.exe interactively, which it supports, keeping it open so I don't have to repeatedly start it for each WMI query. Others have had similar problems with wmic.exe, but they only wanted to capture output for a single query, so having a null stdin doesn't matter to them.

Update

If I do the following...

var spawn = require('child_process').spawn;
var wmic = spawn('wmic', []);

wmic.stdout.on('data', function(data) {
  console.log('Data received:' + data);
});

wmic.stderr.on('data', function(data) {
  console.log('Error! - ' + data);
});

wmic.on('close', function(code) {
  console.log('Spawned process ended with code: ' + code);
});

wmic.stdin.end('cpu get caption /format:csv\n');

Then it'll actually respond with a result, prompt back ready for the next query but next ends the process and obviously I can't use .write() after .end(). Changing it to a .write() instead, process doesn't respond at all and I don't even get the prompt from stdout "wmic:root\cli>" that I receive when using .end().

Or

wmic.stdin.push(null);
wmic.stdin.write('cpu/n'); // Ctrl-Z aka Windows EOF control code

If I use the above, instead of the .end() call, it'll also work. But it seems that if I don't keep throwing constant data at it, the process ends.

Or even

wmic.stdin.write('cpu');
wmic.stdin.write('\x1a');

This works too, but yet again, the wmic.exe process decides to exit after the cpu results are returned. :/

Almost

var wmic = spawn('wmic.exe', [], {stdio: [process.stdin, 'pipe', 'pipe']});

This works. The process starts up correctly so I receive the prompt from wmic.exe through wmic.stdout.on('data', fn) and it stays open. However, I'm unable to send input to it via code but can type directly into the mand prompt window. Anything I type is correctly executed and output via the node script. Again, weird that it works with this pipe but not any I set and useless to me as I want to send mands via code, not typing them.

I've having a problem with the Windows program WMIC.exe (used for querying Windows Management Instrumentation), from within NodeJS.

I can spawn a wmic.exe process, but it'll not output anything or accept any input. However, if I set the stdin to null it'll run correctly and I'll receive output.

var spawn = require('child_process').spawn;
var wmic = spawn('wmic', [], {stdio: ['ignore']});

wmic.stdout.on('data', function(data) {
  console.log('Data received:' + data);
});

wmic.on('close', function(code) {
  console.log('Spawned process ended with code: ' + code);
});

I want to use wmic.exe interactively, which it supports, keeping it open so I don't have to repeatedly start it for each WMI query. Others have had similar problems with wmic.exe, but they only wanted to capture output for a single query, so having a null stdin doesn't matter to them.

Update

If I do the following...

var spawn = require('child_process').spawn;
var wmic = spawn('wmic', []);

wmic.stdout.on('data', function(data) {
  console.log('Data received:' + data);
});

wmic.stderr.on('data', function(data) {
  console.log('Error! - ' + data);
});

wmic.on('close', function(code) {
  console.log('Spawned process ended with code: ' + code);
});

wmic.stdin.end('cpu get caption /format:csv\n');

Then it'll actually respond with a result, prompt back ready for the next query but next ends the process and obviously I can't use .write() after .end(). Changing it to a .write() instead, process doesn't respond at all and I don't even get the prompt from stdout "wmic:root\cli>" that I receive when using .end().

Or

wmic.stdin.push(null);
wmic.stdin.write('cpu/n'); // Ctrl-Z aka Windows EOF control code

If I use the above, instead of the .end() call, it'll also work. But it seems that if I don't keep throwing constant data at it, the process ends.

Or even

wmic.stdin.write('cpu');
wmic.stdin.write('\x1a');

This works too, but yet again, the wmic.exe process decides to exit after the cpu results are returned. :/

Almost

var wmic = spawn('wmic.exe', [], {stdio: [process.stdin, 'pipe', 'pipe']});

This works. The process starts up correctly so I receive the prompt from wmic.exe through wmic.stdout.on('data', fn) and it stays open. However, I'm unable to send input to it via code but can type directly into the mand prompt window. Anything I type is correctly executed and output via the node script. Again, weird that it works with this pipe but not any I set and useless to me as I want to send mands via code, not typing them.

• javascript
• windows
• node.js
• wmi
• wmic

Share Improve this question Follow edited Sep 2, 2013 at 15:11 Mister Dai asked Aug 29, 2013 at 14:41 Mister DaiMister Dai 81611 gold badge88 silver badges2020 bronze badges 2

• 1 This is quite a strange problem, I've done testing for about two hours and WMIC consistently treats EOF like a newline (which automatically closes streams). I also tested spawning cmd (powershell too) and running wmic.exe as a mand, and the result is the same. Will continue testing. – hexacyanide Commented Sep 2, 2013 at 20:50
• Agreed, I went down the route of spawning cmd and using that to fire up wmic.exe but same issue as always. I find it odd that process.stdin will get it going, definitely something odd going on. Thanks for taking a look ;) – Mister Dai Commented Sep 2, 2013 at 22:00

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3 Answers 3

Sorted by: Reset to default 3 +100

You want to use wmic interactively, but driven by input from an application rather than input typed at the console.

The normal way to achieve this for any application is to feed it input from a pipe. Conveniently, NodeJS appears to create a pipe by default if you don't specify any other stdio option.

The problem is that wmic behaves badly when it is receiving input from a pipe. It refuses to do anything until the input is plete; it then processes all the input and stops looking for any more. wmic thinks that the input is plete when the pipe is closed or it receives a CTRL-Z.

Hence you cannot issue one mand, read the output and then issue another mand. wmic doesn't write output until it thinks it has received all the mands, and then it won't accept any more.

You can illustrate this problem at the console. type con: | wmic puts keyboard input through a pipe so wmic goes into its broken mode. In the example below this is followed by a couple of mands. Note that wmic does nothing until CTRL-Z is entered.

C:\>type con: | wmic
cpu get name
useraccount list brief
^Z
wmic:root\cli>cpu get name
Name
Pentium(R) Dual-Core CPU       T4500  @ 2.30GHz

wmic:root\cli>
"/?" for help, QUIT to Exit.
wmic:root\cli>useraccount list brief
AccountType  Caption                    Domain     FullName         Name
512          frog\Administrator         frog                        Administrator

wmic:root\cli>
"/?" for help, QUIT to Exit.
wmic:root\cli>
C:\>

You might think (looking at this example) that wmic just has a large output buffer. However, if you turn on tracing (/TRACE:ON as the first mand) wmic produces reams of output, but nothing before the input is plete.

A solution is to not redirect the standard input, like your final example, so the input is ing from the keyboard. (N.B. when a Windows console application receives input from the keyboard it is not ing through a pipe. Hence your final example is not using a pipe, which is why is behaves differently.)

You can then supply input to wmic by simulating keystrokes. Answers to this question suggest a couple of methods: either using SendKeys or by sending WM_KEYUP messages. This is not a great solution.

For posterity...

This is now possible by using the ms-wmic package:

• https://www.npmjs./package/ms-wmic

This pletely bypasses having to directly interact with the CLI, and allows you to deal with WMIC through a preset API.

---

But the specific thing you tried to use WMIC for in your example above, is also handled by the windows-cpu package:

• https://www.npmjs./package/windows-cpu

That's how it worked for me:

var spawn = require('child_process').spawn;
var wmic = spawn('wmic', []);

wmic.stdout.on('data', function(data) {
  console.log('Data received: [[' + data + ']]');
});

wmic.stderr.on('data', function(data) {
  console.log('Error! - ' + data);
});

wmic.on('close', function(code) {
  console.log('Spawned process ended with code: ' + code);
});

wmic.stdin.end( 'cpu get caption /format:csv' );

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