Spawning child processes

1433442827.js

/**
 * As you have seen, you can use the child_process.exec() function to launch external processes.
 * You get your callback invoked when the process ends. This approach is very simple, but it has some drawbacks:
 * > except for the command-line arguments and the environment variables, using exec() does not allow you to 
 * communicate with the child process.
 * > the child process output is buffered. As a result, you cannot stream it, and it can consume memory.
 * 
 * Fortunately, Node’s child_process module allows for finer-grained control when launching, stopping, 
 * and generally interacting with child processes.
 * You may launch a new process, called the child process, from your application, which is called the parent process.
 * Once you’ve launched a new child process, Node establishes a two-way communication channel, 
 * which both processes can use to send and receive data strings to and from each other. 
 * The parent process can also exercise some control over the child, sending it signals and forcing it to terminate.
 * 
 * Creating the child process
*/

var spawn = require('child_process').spawn;
var child = spawn('tail', ['-f', '/var/log/system.log']);

/**
 * Here you spawn a child process to run a tail command, passing in arguments -f and /var/log/ system.log. 
 * This tail command will monitor the fi le in /var/log/system.log — if it exists — and output every new data
 * appended to it into the stdout stream. The spawn function call returns a ChildProcess object, which is a handler 
 * object that encapsulates access to the real process. Here you assign this new descriptor to the variable named child.
*/

// Listening for data from child process

/**
 * Any child process handler has a property named stdout that represents the child process standard output as a stream. 
 * You can then bind to the data event on that stream, which is the same as saying 
 * “for every data chunk you get from the child output, invoke this callback.” Here is an example of that happening:
*/

// print child output to the console 
child.stdout.on('data', function(data) {    
  console.log('tail output: ' + data); 
  }
);

/**
 * Sending Data to the child process
*/