node.js shell command execution

Question

I am still trying to grasp the finer points of how I can run a linux or windows shell command and capture output within node.js; ultimately, I want to do something like this...

//pseudocode
output = run_command(cmd, args)

The important piece is that output must be available to a globally scoped variable (or object). I tried the following function, but for some reason, I get undefined printed to the console...

function run_cmd(cmd, args, cb) {
  var spawn = require('child_process').spawn
  var child = spawn(cmd, args);
  var me = this;
  child.stdout.on('data', function(me, data) {
    cb(me, data);
  });
}
foo = new run_cmd('dir', ['/B'], function (me, data){me.stdout=data;});
console.log(foo.stdout);  // yields "undefined" <------

I'm having trouble understanding where the code breaks above... a very simple prototype of that model works...

function try_this(cmd, cb) {
  var me = this;
  cb(me, cmd)
}
bar = new try_this('guacamole', function (me, cmd){me.output=cmd;})
console.log(bar.output); // yields "guacamole" <----

Can someone help me understand why try_this() works, and run_cmd() does not? FWIW, I need to use child_process.spawn, because child_process.exec has a 200KB buffer limit.

Final Resolution

I'm accepting James White's answer, but this is the exact code that worked for me...

function cmd_exec(cmd, args, cb_stdout, cb_end) {
  var spawn = require('child_process').spawn,
    child = spawn(cmd, args),
    me = this;
  me.exit = 0;  // Send a cb to set 1 when cmd exits
  me.stdout = "";
  child.stdout.on('data', function (data) { cb_stdout(me, data) });
  child.stdout.on('end', function () { cb_end(me) });
}
foo = new cmd_exec('netstat', ['-rn'], 
  function (me, data) {me.stdout += data.toString();},
  function (me) {me.exit = 1;}
);
function log_console() {
  console.log(foo.stdout);
}
setTimeout(
  // wait 0.25 seconds and print the output
  log_console,
250);

Answer 1

A simplified version of the accepted answer (third point), just worked for me.

function run_cmd(cmd, args, callBack ) {
    var spawn = require('child_process').spawn;
    var child = spawn(cmd, args);
    var resp = "";

    child.stdout.on('data', function (buffer) { resp += buffer.toString() });
    child.stdout.on('end', function() { callBack (resp) });
} // ()

Usage:

run_cmd( "ls", ["-l"], function(text) { console.log (text) });

run_cmd( "hostname", [], function(text) { console.log (text) });

Answer 2

Synchronous one-liner:

require('child_process').execSync("echo 'hi'", function puts(error, stdout, stderr) { console.log(stdout) });

Answer 3

I used this more concisely :

var sys = require('sys')
var exec = require('child_process').exec;
function puts(error, stdout, stderr) { sys.puts(stdout) }
exec("ls -la", puts);

it works perfectly. :)

Answer 4

I had a similar problem and I ended up writing a node extension for this. You can check out the git repository. It's open source and free and all that good stuff !

https://github.com/aponxi/npm-execxi

ExecXI is a node extension written in C++ to execute shell commands one by one, outputting the command's output to the console in real-time. Optional chained, and unchained ways are present; meaning that you can choose to stop the script after a command fails (chained), or you can continue as if nothing has happened !

Usage instructions are in the ReadMe file. Feel free to make pull requests or submit issues!

I thought it was worth to mention it.

Answer 5

A promisified version of the most-awarded answer:

  runCmd: (cmd, args) => {
    return new Promise((resolve, reject) => {
      var spawn = require('child_process').spawn
      var child = spawn(cmd, args)
      var resp = ''
      child.stdout.on('data', function (buffer) { resp += buffer.toString() })
      child.stdout.on('end', function () { resolve(resp) })
    })
  }

To use:

 runCmd('ls').then(ret => console.log(ret))

Answer 6

Simplest way is to just use the ShellJS lib ...

$ npm install [-g] shelljs

EXEC Example:

require('shelljs/global');

// Sync call to exec()
var version = exec('node --version', {silent:true}).output;

// Async call to exec()
exec('netstat.exe -an', function(status, output) {
  console.log('Exit status:', status);
  console.log('Program output:', output);
});

ShellJs.org supports many common shell commands mapped as NodeJS functions including:

• cat
• cd
• chmod
• cp
• dirs
• echo
• exec
• exit
• find
• grep
• ln
• ls
• mkdir
• mv
• popd
• pushd
• pwd
• rm
• sed
• test
• which