Running a shell command from Node.js without buffering output

Question

I'm trying to launch a shell command from Node.js, without redirecting that command's input and output -- just like shelling out to a command using a shell script, or using Ruby's system command. If the child process wants to write to STDOUT, I want that to go straight to the console (or get redirected, if my Node app's output was redirected).

Node doesn't seem to have any straightforward way to do this. It looks like the only way to run another process is with child_process, which always redirects the child process's input and output to pipes. I can write code to accept data from those pipes and write it to my process's STDOUT and STDERR, but if I do that, the APIs force me to sacrifice some flexibility.

I want two features:

• Shell syntax. I want to be able to pipe output between commands, or run Windows batch files.
• Unlimited output. If I'm shelling out to a compiler and it wants to generate megabytes of compiler warnings, I want them all to scroll across the screen (until the user gets sick of it and hits Ctrl+C).

It looks like Node wants to force me choose between those two features.

• If I want an unlimited amount of output, I can use child_process.spawn and then do child.stdout.on('data', function(data) { process.stdout.write(data); }); and the same thing for stderr, and it'll happily pipe data until the cows come home. Unfortunately, spawn doesn't support shell syntax.
• If I want shell syntax, I can use child_process.exec. But exec insists on buffering the child process's STDOUT and STDERR for me and giving them to me all at the end, and it limits the size of those buffers (configurable, 200K by default). I can still hook the on('data') events, if I want to see the output as it's generated, but exec will still add the data to its buffers too. When the amount of data exceeds the predefined buffer size, exec will terminate the child process.

(There's also child_process.execFile, which is the worst of both worlds from a flexibility standpoint: no shell syntax, but you still have to cap the amount of output you expect.)

Am I missing something? Is there any way to just shell out to a child process in Node, and not redirect its input and output? Something that supports shell syntax and doesn't crap out after a predefined amount of output, just like is available in shell scripts, Ruby, etc.?

Answer 1

You can inherit stdin/out/error streams via spawn argument so you don't need to pipe them manually:

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

Use shell for shell syntax - for bash it's -c parameter to read script from string:

var spawn = require('child_process').spawn;
var shellSyntaxCommand = 'ls -l | grep test | wc -c';
spawn('sh', ['-c', shellSyntaxCommand], { stdio: 'inherit' });

To summarise:

var spawn = require('child_process').spawn;
function shspawn(command) {
   spawn('sh', ['-c', command], { stdio: 'inherit' });
} 

shspawn('ls -l | grep test | wc -c');

Answer 2

You can replace exec by spawn and use the shell syntax simply with:

const {spawn} = require ('child_process');
const cmd = 'ls -l | grep test | wc -c';
const p = spawn (cmd, [], {shell: true});

p.stdout.on ('data', (data) => {
  console.log (data.toString ());
});

The magic is just {shell: true}.

Answer 3

I haven't used it, but I've seen this library: https://github.com/polotek/procstreams

It you'd do this. The .out() automatically pipes to the process's stdin/out.

var $p = require('procstreams');
$p('cat lines.txt').pipe('wc -l').out();

If doesn't support shell syntax, but that's pretty trivial I think.

var command_str = "cat lines.txt | wc -l";
var cmds = command_str.split(/\s?\|\s?/);
var cmd = $p(cmds.shift());
while(cmds.length) cmd = cmd.pipe(cmds.shift());
cmd
  .out()
  .on('exit', function() {
    // Do whatever
  });

Answer 4

There's an example in the node docs for the child_process module:

Example of detaching a long-running process and redirecting its output to a file:

 var fs = require('fs'),
     spawn = require('child_process').spawn,
     out = fs.openSync('./out.log', 'a'),
     err = fs.openSync('./out.log', 'a');

 var child = spawn('prg', [], {
   detached: true,
   stdio: [ 'ignore', out, err ]
 });

 child.unref();