Node.js: Spawning an echo process with the ">" flag - node.js

I am looking to spawn an echo process to write some text to a "file".
*The fs package is off limits because the "file" is a communication pathway for a linux driver.
Below is my code to just see if I can get an echo process working with writing to a normal file however the spawn doesn't appear to like the > flag. Any ideas?
var spawn = require('child_process').spawn;
echo = spawn('echo', ["test", ">", __dirname+"/test.txt"]);
echo.on('error', function (err) {
console.log('ls error', err);
});
echo.stdout.on('data', function (data) {
console.log('stdout: ' + data);
});
echo.stderr.on('data', function (data) {
console.log('stderr: ' + data);
});
echo.on('close', function (code) {
console.log('child process exited with code ' + code);
});
Right now I just get the following output with no written file:
stdout: test > <*PATH*>/test.txt
child process exited with code 0

I ended up just creating a bash file (echo-test.sh) with the following contents:
echo "test" > <*PATH*>/test.txt
and executed in node like such:
echo = spawn('bash', [__dirname+"/echo-test.sh"]);

Related

Not able to take the MongoDB backup/dump using node js

When executing the written script, it exit with code 'one' while it should save the dump of the database
const cron = require('node-cron');
const { exit } = require('process');
const spawn = require('child_process').spawn;
let backupProcess = spawn('mongodump', [
'--host=localhost',
'--port=27017',
'--out=.',
'--db=testdb',
], {shell: true});
backupProcess.on('exit', (code, signal) => {
console.log('Function called');
if(code)
console.log('Backup process exited with code ', code);
else if (signal)
console.error('Backup process was killed with singal ', signal);
else
console.log('Successfully backedup the database')
});
while executing the same through VS code terminal, getting following error,
Function called
Backup process exited with code 1 while should save the dump at given location`your text`
Actual Result : Backup process exited with code 1
Expected result : Must dump the database

Node.js Child process can not catch windows exe file's output

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());
});

Node child.exec working but child.spawn not

I'm trying to work with Child Spawn (not working) instead of Exec (working). My Exec code provides me with console output, I see nothing if I run my child spawn code, how can I get console output using Child Spawn:
Here is my working exec code:
var exec = require('child_process').exec,
child;
child = exec('myProgram --version', {},
function (error, stdout, stderr) {
console.log('stdout: ' + stdout);
console.log('stderr: ' + stderr);
if (error !== null) {
console.log('exec error: ' + error);
}
});
child.stdout.on('data', function(data) {
console.log(data.toString());
});
child.stderr.on('data', function (data) {
console.log('stderr: ' + data);
});
Here is my non-working attempt at using spawn:
var spawn = require('child_process').spawn;
var spawnchild = spawn('myProgram', ['--version']);
spawnchild.stdout.on('data', function(data) {
console.log('stdout: ' + data);
});
spawnchild.stderr.on('data', function(data) {
console.log('stdout: ' + data);
});
If you add a 'close' event handler for spawnchild, you will see a non-zero exit code. The reason for this is that the first argument for spawn() differs from that of exec(). exec() takes the full command line string, whereas spawn() has just the program name/path for the first argument and the second argument is an array of command line arguments passed to that program.
So in your particular case, you'd use:
var spawnchild = spawn('myProgram', ['--version']);

Node Kue and Child Process - get error from spawned process

I try to spawn a child process performing cpu intensive calculations through a job queue with Kue. My code at the moment looks like this:
consumer.js
var kue = require('kue');
var util = require('util');
var spawn = require('child_process').spawn;
var jobs = kue.createQueue();
jobs.process('calc', 2, function(job, done){
var work = spawn('Rscript', ['opti2.R', job.data.file]);
work.stderr.on('data', function (data) {
job.log('stderr: ' + data);
});
work.stdout.on('data', function (data) {
job.log('stdout: ' + data);
});
work.on('exit', function (code, signal) {
console.log('child process exited with code ' + code + ' with singal ' + signal);
if(code != 0){
done(****How to get the stderr of the child process as an error here***);
} else {
done(Error());
}
});
});
The code somewhat do what i would like it to do, but is there a better way to report the job as failed (to Kue) and get the stderr from the spawned process?
You can use job.log method to send data directly to Kue.
I would also recommend you to switch from .spawn to .exec, because it returns stdout and stderr as strings in its final callback along with a good error, which suits your needs well:
var exec = require('child_process').exec;
jobs.process('calc', 2, function(job, done){
exec('Rscript opti2.R ' + job.data.file, function (error, stdout, stderr) {
if (stdout.length > 0) job.log('stdout: ' + stdout);
if (stderr.length > 0) job.log('stderr: ' + stderr);
done(error);
});
});
Though solution should work with .spawn as well: simply replace each console.log call in your code with job.log.
Though, you may want to bufferize your stderr in order to send it to Kue in one chunk:
jobs.process('calc', 2, function(job, done){
var work = spawn('Rscript', ['opti2.R', job.data.file]);
var stderr = '';
work.stderr.on('data', function (data) {
stderr += data;
});
work.stdout.on('data', function (data) {
job.log(data); // sending arriving `stdout` chunks as normal log events
});
work.on('close', function (code, signal) {
console.log('child process exited with code ' + code + ' with singal ' + signal);
if(code != 0){
done(stderr); // sending all collected stderr as an explanation
} else {
done();
}
});
});
I would also recommend using close event instead of exit, because it waits for child's stdio streams.
For more information see Event: 'exit' docs:
This event is emitted after the child process ends.
Note that the child process stdio streams might still be open.
and Event: 'close' docs:
This event is emitted when the stdio streams of a child process have
all terminated.

Exec : display stdout "live"

I have this simple script :
var exec = require('child_process').exec;
exec('coffee -cw my_file.coffee', function(error, stdout, stderr) {
console.log(stdout);
});
where I simply execute a command to compile a coffee-script file. But stdout never get displayed in the console, because the command never ends (because of the -w option of coffee).
If I execute the command directly from the console I get message like this :
18:05:59 - compiled my_file.coffee
My question is : is it possible to display these messages with the node.js exec ? If yes how ? !
Thanks
Don't use exec. Use spawn which is an EventEmmiter object. Then you can listen to stdout/stderr events (spawn.stdout.on('data',callback..)) as they happen.
From NodeJS documentation:
var spawn = require('child_process').spawn,
ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', function (data) {
console.log('stdout: ' + data.toString());
});
ls.stderr.on('data', function (data) {
console.log('stderr: ' + data.toString());
});
ls.on('exit', function (code) {
console.log('child process exited with code ' + code.toString());
});
exec buffers the output and usually returns it when the command has finished executing.
exec will also return a ChildProcess object that is an EventEmitter.
var exec = require('child_process').exec;
var coffeeProcess = exec('coffee -cw my_file.coffee');
coffeeProcess.stdout.on('data', function(data) {
console.log(data);
});
OR pipe the child process's stdout to the main stdout.
coffeeProcess.stdout.pipe(process.stdout);
OR inherit stdio using spawn
spawn('coffee -cw my_file.coffee', { stdio: 'inherit' });
There are already several answers however none of them mention the best (and easiest) way to do this, which is using spawn and the { stdio: 'inherit' } option. It seems to produce the most accurate output, for example when displaying the progress information from a git clone.
Simply do this:
var spawn = require('child_process').spawn;
spawn('coffee', ['-cw', 'my_file.coffee'], { stdio: 'inherit' });
Credit to #MorganTouvereyQuilling for pointing this out in this comment.
Inspired by Nathanael Smith's answer and Eric Freese's comment, it could be as simple as:
var exec = require('child_process').exec;
exec('coffee -cw my_file.coffee').stdout.pipe(process.stdout);
I'd just like to add that one small issue with outputting the buffer strings from a spawned process with console.log() is that it adds newlines, which can spread your spawned process output over additional lines. If you output stdout or stderr with process.stdout.write() instead of console.log(), then you'll get the console output from the spawned process 'as is'.
I saw that solution here:
Node.js: printing to console without a trailing newline?
Hope that helps someone using the solution above (which is a great one for live output, even if it is from the documentation).
I have found it helpful to add a custom exec script to my utilities that do this.
utilities.js
const { exec } = require('child_process')
module.exports.exec = (command) => {
const process = exec(command)
process.stdout.on('data', (data) => {
console.log('stdout: ' + data.toString())
})
process.stderr.on('data', (data) => {
console.log('stderr: ' + data.toString())
})
process.on('exit', (code) => {
console.log('child process exited with code ' + code.toString())
})
}
app.js
const { exec } = require('./utilities.js')
exec('coffee -cw my_file.coffee')
After reviewing all the other answers, I ended up with this:
function oldSchoolMakeBuild(cb) {
var makeProcess = exec('make -C ./oldSchoolMakeBuild',
function (error, stdout, stderr) {
stderr && console.error(stderr);
cb(error);
});
makeProcess.stdout.on('data', function(data) {
process.stdout.write('oldSchoolMakeBuild: '+ data);
});
}
Sometimes data will be multiple lines, so the oldSchoolMakeBuild header will appear once for multiple lines. But this didn't bother me enough to change it.
child_process.spawn returns an object with stdout and stderr streams.
You can tap on the stdout stream to read data that the child process sends back to Node. stdout being a stream has the "data", "end", and other events that streams have. spawn is best used to when you want the child process to return a large amount of data to Node - image processing, reading binary data etc.
so you can solve your problem using child_process.spawn as used below.
var spawn = require('child_process').spawn,
ls = spawn('coffee -cw my_file.coffee');
ls.stdout.on('data', function (data) {
console.log('stdout: ' + data.toString());
});
ls.stderr.on('data', function (data) {
console.log('stderr: ' + data.toString());
});
ls.on('exit', function (code) {
console.log('code ' + code.toString());
});
Here is an async helper function written in typescript that seems to do the trick for me. I guess this will not work for long-lived processes but still might be handy for someone?
import * as child_process from "child_process";
private async spawn(command: string, args: string[]): Promise<{code: number | null, result: string}> {
return new Promise((resolve, reject) => {
const spawn = child_process.spawn(command, args)
let result: string
spawn.stdout.on('data', (data: any) => {
if (result) {
reject(Error('Helper function does not work for long lived proccess'))
}
result = data.toString()
})
spawn.stderr.on('data', (error: any) => {
reject(Error(error.toString()))
})
spawn.on('exit', code => {
resolve({code, result})
})
})
}

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