I want to use child_process.spawn to execute a windows exe file and catch it's output.
When I use command line to run a thirdparty exe file (says A.exe), it will print some logs to the cmd window. Like this:
C:\> A.exe
some outputs...
some more outputs...
However, when I spawn it in node.js, using this
import childProcess from 'child_process';
const cp = childProcess.spawn('A.exe');
cp.stdout.on('data', data => console.log(`stdout: ${data}`));
cp.stderr.on('data', data => console.log(`stderr: ${data}`));
There is no outputs at all.
I think the outputs of A.exe is not to the stdout (so I can never get data by listening stdout), but I don't know how it print logs when running from command line.
Any help would be greatly appreciated.
On Unix-type operating systems (Unix, Linux, macOS) child_process.execFile() can be more efficient because it does not spawn a shell. On Windows, however, .bat and .cmd files are not executable on their own without a terminal, and therefore cannot be launched using child_process.execFile(). When running on Windows, .bat and .cmd files can be invoked using child_process.spawn() with the shell option set, with child_process.exec(), or by spawning cmd.exe and passing the .bat or .cmd file as an argument (which is what the shell option and child_process.exec() do). In any case, if the script filename contains spaces it needs to be quoted.
// On Windows Only ...
const { spawn } = require('child_process');
const bat = spawn('cmd.exe', ['/c', 'my.bat']);
bat.stdout.on('data', (data) => {
console.log(data.toString());
});
bat.stderr.on('data', (data) => {
console.log(data.toString());
});
bat.on('exit', (code) => {
console.log(`Child exited with code ${code}`);
});
Maybe give this approach a go:
var childProcess = require('child_process');
childProcess.exec('A.exe', function(error, stdout, stderr) {
if (error != null) {
console.log('error occurred: ' + error);
} else {
console.log('stdout: ' + stdout);
console.log('stderr: ' + stderr);
}
});
// OR
var cp = childProcess.spawn('A.exe');
cp.stdout.on('data', (data) => {
console.log('stdout: ' + data.toString());
});
cp.stderr.on('data', (data) => {
console.log('stderr: ' + data.toString());
});
I'm reading a pdf text from a s3 bucket using S3fs.readFile, and i would like to get the result, transform in string and immediately open a spawn child_process calling pdftotext, passing the string:
S3Fs.readFile('./my-pdf-in-s3-bucket', {encoding: 'binary'}, (error, result) => {
mychild = child_process.spawn('pdftotext', [
result.Body
]);
});
This is causing the spawn process break because the string is to long, and i don't want save the file in disk just to read it again.
Is it possible?
Thanks!
pdftotext should allow reading from stdin and writing to stdout (at least it worked for me with v0.41.0), so you could do this instead:
S3Fs.readFile('./my-pdf-in-s3-bucket', (err, result) => {
if (err) throw err; // Handle better
var cp = child_process.spawn('pdftotext', [ '-', '-' ]);
cp.stdout.pipe(process.stdout);
cp.on('close', (code, signal) => {
console.log(`pdftotext finished with status ${code}`);
});
cp.stdin.end(result);
});
Or possibly better yet, you might be able to stream the file to the child process instead of buffering its entire contents in memory first:
var cp = child_process.spawn('pdftotext', [ '-', '-' ]);
var rs = S3Fs.createReadStream('./my-pdf-in-s3-bucket');
rs.on('error', (err) => {
cp.kill();
});
cp.stdout.pipe(process.stdout);
cp.on('close', (code, signal) => {
console.log(`pdftotext finished with status ${code}`);
});
rs.pipe(cp.stdin);
I'm trying to run a command (aws configure) from a .js file called with node. I've been using child_process which allows me to execute a command or batch file. However when that file would normally prompt the user for something like a username and password, I am unable to have that interaction take place.
I tried using process.stdin with node in order to perhaps use node as the middle man of a user's input, but I am unable to link the two together.
If anyone has any ideas that would be great!
var spawn = require('child_process').spawn;
var mystdin = process.stdin;
var conf = spawn('aws configure', {input: mystdin, shell: true});
//When command returns
conf.stdout.on('data', function(data) {
console.log('stdout: ' + data);
//prompt for input
mystdin.resume();
mystdin.setEncoding('utf8');
var util = require('util');
mystdin.on('data', function (text) {
//INSERT ANSWER HERE
console.log('received data:', util.inspect(text));
if (text === 'quit\n') {
done();
}
});
function done() {
console.log('Now that process.stdin is paused, there is nothing more to do.');
process.exit();
}
});
conf.stderr.on('data', function(data) {
console.log('stdout: ' + data);
});
conf.on('close', function(code) {
console.log('child precess exited with code ' + code);
});
This is just the path I was currently going down. If anyone has a better suggestion let me know!
Thanks
Two ideas: first, I think the arguments are supposed to be in the second parameters. Second, maybe the aws command doesn't realize it is an interactive terminal or something. You may try with child_process like that, or if that doesn't work, with pty.js:
var pty = require('pty.js');
var term = pty.spawn('aws', ['configure'], {
name: 'xterm-color',
cols: 80,
rows: 30,
cwd: process.env.HOME,
env: process.env
});
term.on('data', function(data) {
console.log(data);
});
I want to get free -m (linux shell command) and store its result into a variable
by using source code below:
var spawn = require('child_process').spawn,
command = spawn('free', ['-m']);
command.stdout.pipe(process.stdout);
Is there any way to store process.stdout in a variable, please me some suggestions
This is fairly simple with child_process.exec:
var child = require("child_process");
var freeOut;
child.exec("free", ["-m"], function (error, stdout, stderr) {
if (error) {
console.error(error, stderr);
return;
}
//stdout and stderr are available here
freeOut = stdout;
process.stdout.write(stdout);
});
//Note. Do NOT use freeOut here. exec is async. Can only use in the callback
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.
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);
I'm still getting my feet wet with Node.js, but I have a few ideas. first, I believe you need to use execFile instead of spawn; execFile is for when you have the path to a script, whereas spawn is for executing a well-known command that Node.js can resolve against your system path.
1. Provide a callback to process the buffered output:
var child = require('child_process').execFile('path/to/script', [
'arg1', 'arg2', 'arg3',
], function(err, stdout, stderr) {
// Node.js will invoke this callback when process terminates.
console.log(stdout);
});
2. Add a listener to the child process' stdout stream (9thport.net)
var child = require('child_process').execFile('path/to/script', [
'arg1', 'arg2', 'arg3' ]);
// use event hooks to provide a callback to execute when data are available:
child.stdout.on('data', function(data) {
console.log(data.toString());
});
Further, there appear to be options whereby you can detach the spawned process from Node's controlling terminal, which would allow it to run asynchronously. I haven't tested this yet, but there are examples in the API docs that go something like this:
child = require('child_process').execFile('path/to/script', [
'arg1', 'arg2', 'arg3',
], {
// detachment and ignored stdin are the key here:
detached: true,
stdio: [ 'ignore', 1, 2 ]
});
// and unref() somehow disentangles the child's event loop from the parent's:
child.unref();
child.stdout.on('data', function(data) {
console.log(data.toString());
});
Here is the cleanest approach I've found:
require("child_process").spawn('bash', ['./script.sh'], {
cwd: process.cwd(),
detached: true,
stdio: "inherit"
});
I had a little trouble getting logging output from the "npm install" command when I spawned npm in a child process. The realtime logging of dependencies did not show in the parent console.
The simplest way to do what the original poster wants seems to be this (spawn npm on windows and log everything to parent console):
var args = ['install'];
var options = {
stdio: 'inherit' //feed all child process logging into parent process
};
var childProcess = spawn('npm.cmd', args, options);
childProcess.on('close', function(code) {
process.stdout.write('"npm install" finished with code ' + code + '\n');
});
PHP-like passthru
import { spawn } from 'child_process';
export default async function passthru(exe, args, options) {
return new Promise((resolve, reject) => {
const env = Object.create(process.env);
const child = spawn(exe, args, {
...options,
env: {
...env,
...options.env,
},
});
child.stdout.setEncoding('utf8');
child.stderr.setEncoding('utf8');
child.stdout.on('data', data => console.log(data));
child.stderr.on('data', data => console.log(data));
child.on('error', error => reject(error));
child.on('close', exitCode => {
console.log('Exit code:', exitCode);
resolve(exitCode);
});
});
}
Usage
const exitCode = await passthru('ls', ['-al'], { cwd: '/var/www/html' })
child:
setInterval(function() {
process.stdout.write("hi");
}, 1000); // or however else you want to run a timer
parent:
require('child_process').fork('./childfile.js');
// fork'd children use the parent's stdio
I found myself requiring this functionality often enough that I packaged it into a library called std-pour. It should let you execute a command and view the output in real time. To install simply:
npm install std-pour
Then it's simple enough to execute a command and see the output in realtime:
const { pour } = require('std-pour');
pour('ping', ['8.8.8.8', '-c', '4']).then(code => console.log(`Error Code: ${code}`));
It's promised based so you can chain multiple commands. It's even function signature-compatible with child_process.spawn so it should be a drop in replacement anywhere you're using it.
Adding a sample for exec as I too had needed live feedback and wasn't getting any until after the script finished. exec does return an EventEmitter, contrary to the many claims that only spawn works in such a way.
This supplements the comment I made to the accepted answer more thoroughly.
The interface for exec is similar to spawn:
// INCLUDES
import * as childProcess from 'child_process'; // ES6 Syntax
// DEFINES
let exec = childProcess.exec; // Use 'var' for more proper
// semantics, or 'const' it all
// if that's your thing; though 'let' is
// true-to-scope;
// Return an EventEmitter to work with, though
// you can also chain stdout too:
// (i.e. exec( ... ).stdout.on( ... ); )
let childProcess = exec
(
'./binary command -- --argument argumentValue',
( error, stdout, stderr ) =>
{ // When the process completes:
if( error )
{
console.log( `${error.name}: ${error.message}` );
console.log( `[STACK] ${error.stack}` );
}
console.log( stdout );
console.log( stderr );
callback(); // Gulp stuff
}
);
Now its as simple as registering an event handler for stdout:
childProcess.stdout.on( 'data', data => console.log( data ) );
And for stderr:
childProcess.stderr.on( 'data', data => console.log( `[ERROR]: ${data}` ) );
You can also pipe stdout to the main process' stdout:
childProcess.stdout.pipe( process.stdout );
Not too bad at all - HTH
I was interested into running a script that gets the input and outputs from my terminal, and that will close my process once the child script finishes.
import { spawn } from 'node:child_process'
import process from 'node:process'
const script = spawn('path/to/script', { stdio: 'inherit' })
script.on('close', process.exit)
I ran into a situation where none of the above worked when I was spawning a Python 3 script. I would get data from stdout, but only once the child terminated.
As it turns out, Python buffers stdout by default. It's possible to disable stdout buffering by including -u as a command line parameter to python3.