Node - Child Processes

node_child_processes.markdown

#Node - Processes To launch an external shell command or executable file you can use the child_process. Apart from command line arguments and environment variables you cannot communicate with the process using child_process.exec. However you can use child_process.spawn to create a more integrated processes.

##Executing Child Processes

###To launch an external shell command

var child_process = require('child_process');
var exec = child_process.exec;

//exec(command, callback);
exec('ls', function(err,stout,stderr) {
  if (err) {
    console.log('Child process exited with error code', err.code);
    return
  }
  console.log(stdout);
});

###Passing options You can also pass options to the child process.

var child_process = require('child_process');
var exec = child_process.exec;
var options = {
  timeout: 10000,
  killSignal: 'SIGKILL'
}

//exec(command, [options], callback);
exec('ls', options, function(err,stout,stderr) {
  if (err) {
    console.log('Child process exited with error code', err.code);
    return
  }
  console.log(stdout);
});

####Available Process Options

• cwd - Current working directory. Force process to use current working directory.
• encoding - Expected encoding for the child output. The default is utf8. Supported encodings are ascii, utf8, ucs2 and base64.
• timeout - Timeout in milliseconds for the command execution. Default is 0 which wait indefinately
• killSignal - The signal to be sent to the child process if it times out or exceeds the output buffer. the default is SIGTERM. child_process.spawn allows more fine grained control.
• env - Environment variables to be passed to the child process. Default is null which means that the child process inherits all the parent environment variables.

####Available Signals

Name	Default Action	Description
SIGHUP	Terminate process	Terminal line hangup
SIGINT	Terminate process	Terminal line hangup
SIGQUIT	Create core image	Quit program
SIGILL	Create core image	Illegal instruction
SIGTRAP	Create core image	Trace trap
SIGABRT	Create core image	Abort program
SIGFPE	Create core image	Floating point exception
SIGKILL	Terminate process	Kill program
SIGBUS	Create core image	Bus error
SIGSEGV	Create core image	Segmentation violation
SIGSYS	Create core image	Nonexistant system call invoked
SIGPIPE	Terminate process	Software termination signal
SIGALRM	Terminate process	Real-time timer expired
SIGTERM	Terminate process	Software termination signal
SIGURG	Discard signal	Urgent condition present on socket
SIGSTOP	Stop process	Stop (cannot be caught or ignored)
SIGTSTP	Stop process	Stop signal generated from keyboard
SIGCONT	Discard signal	Continue after stop
SIGCHLD	Discard signal	Child status has changed
SIGTTIN	Stop process	Backround read attempted from control terminal
SIGTTOU	Stop process	Backround write attempted from control terminal
SIGIO	Discard process	I/O is possible on a descriptor
SIGXCPU	Terminate process	CPU time limit exceeded
SIGXFSZ	Terminate process	File size limit exceeded
SIGVTALRM	Terminate process	Virtual time alarm
SIGPROF	Terminate process	Profiling timer alarm
SIGWINCH	Discard process	Window size change
SIGINFO	Discard process	Status request from keyboard
SIGUSR1	Terminate process	User defined signal 1
SIGUSR2	Terminate process	User defined signal 2

###Passing Environment Variables You can pass new environment variables by copying the existing variables and adjusting as neccessary.

var env = process.env,
    varName,
    envCopy = {};
   
var exec = require('child_process').exec;

// Copy process.env into envCopy
for (varName in env) {
  envCopy[varName] = env[varName];
}

// Assign some custom variables
envCopy['CUSTOM ENV VAR'] = 'some value';
envCopy['CUSTOM ENV VAR 2'] = 'some other value';

// Execute some command with process.env and my custom variables
exec('ls -la', { env: envCopy }, function(err, stdout, stderr) {
  if (err) { throw err; }
  console.log('stdout:', stdout);
  console.log('stderr:', stderr);
});

####Passing Environment Variables - parent.js Here is an example of passing environment variables.

var exec = require('child_process').exec;

exec('node child.js', {env: {number: 123}}, function(err, stdout, stderr) {
  if (err) { throw err; }
  console.log('stdout:\n', stdout);
  console.log('stderr:\n', stderr);
});

####Reading Environment Variables - child.js Here is an example of reading environment variables which have been passed to the child.

var number = process.env.number;
console.log(typeof(number)); // → "string"

number = parseInt(number, 10);
console.log(typeof(number)); // → "number"

##Spawning Child Processes

var spawn = require('child_process').spawn;

var child = spawn()

###Creating a Child Process

Can child_process.exec execute 'su' command? I tried but it didn't work.

Hi @xingchch , any luck with this? I'm currently in the same situation where im using "su - B" to change the user from A to B from my node application.