Exec : display stdout "live"
node.jsCoffeescriptnode.js Problem Overview
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
node.js Solutions
Solution 1 - node.js
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.
Solution 2 - node.js
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' });
Solution 3 - node.js
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.
Solution 4 - node.js
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);
Solution 5 - node.js
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: https://stackoverflow.com/questions/6157497/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).
Solution 6 - node.js
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')
Solution 7 - node.js
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.
Solution 8 - node.js
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());
});
Solution 9 - node.js
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})
})
})
}