node/doc/api/child_processes.markdown

Child Processes

Node provides a tri-directional popen(3) facility through the ChildProcess class.

It is possible to stream data through the child's stdin, stdout, and stderr in a fully non-blocking way.

To create a child process use require('child_process').spawn().

Child processes always have three streams associated with them. child.stdin, child.stdout, and child.stderr.

ChildProcess is an EventEmitter.

Event: 'exit'

function (code, signal) {}

This event is emitted after the child process ends. If the process terminated normally, code is the final exit code of the process, otherwise null. If the process terminated due to receipt of a signal, signal is the string name of the signal, otherwise null.

See waitpid(2).

child.stdin

A Writable Stream that represents the child process's stdin. Closing this stream via end() often causes the child process to terminate.

child.stdout

A Readable Stream that represents the child process's stdout.

child.stderr

A Readable Stream that represents the child process's stderr.

child.pid

The PID of the child process.

Example:

var spawn = require('child_process').spawn,
    grep  = spawn('grep', ['ssh']);

console.log('Spawned child pid: ' + grep.pid);
grep.stdin.end();

child_process.spawn(command, args=[], [options])

Launches a new process with the given command, with command line arguments in args. If omitted, args defaults to an empty Array.

The third argument is used to specify additional options, which defaults to:

{ cwd: undefined
, env: process.env,
, customFds: [-1, -1, -1]
}

cwd allows you to specify the working directory from which the process is spawned. Use env to specify environment variables that will be visible to the new process. With customFds it is possible to hook up the new process' [stdin, stout, stderr] to existing streams; -1 means that a new stream should be created.

Example of running ls -lh /usr, capturing stdout, stderr, and the exit code:

var util   = require('util'),
    spawn = require('child_process').spawn,
    ls    = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', function (data) {
  util.print('stdout: ' + data);
});

ls.stderr.on('data', function (data) {
  util.print('stderr: ' + data);
});

ls.on('exit', function (code) {
  console.log('child process exited with code ' + code);
});

Example: A very elaborate way to run 'ps ax | grep ssh'

var util   = require('util'),
    spawn = require('child_process').spawn,
    ps    = spawn('ps', ['ax']),
    grep  = spawn('grep', ['ssh']);

ps.stdout.on('data', function (data) {
  grep.stdin.write(data);
});

ps.stderr.on('data', function (data) {
  util.print('ps stderr: ' + data);
});

ps.on('exit', function (code) {
  if (code !== 0) {
    console.log('ps process exited with code ' + code);
  }
  grep.stdin.end();
});

grep.stdout.on('data', function (data) {
  util.print(data);
});

grep.stderr.on('data', function (data) {
  util.print('grep stderr: ' + data);
});

grep.on('exit', function (code) {
  if (code !== 0) {
    console.log('grep process exited with code ' + code);
  }
});

Example of checking for failed exec:

var spawn = require('child_process').spawn,
    child = spawn('bad_command');

child.stderr.on('data', function (data) {
  if (/^execvp\(\)/.test(data.asciiSlice(0,data.length))) {
    console.log('Failed to start child process.');
  }
});

See also: child_process.exec()

child_process.exec(command, [options], callback)

High-level way to execute a command as a child process, buffer the output, and return it all in a callback.

var util   = require('util'),
    exec  = require('child_process').exec,
    child;

child = exec('cat *.js bad_file | wc -l', 
  function (error, stdout, stderr) {
    util.print('stdout: ' + stdout);
    util.print('stderr: ' + stderr);
    if (error !== null) {
      console.log('exec error: ' + error);
    }
});

The callback gets the arguments (error, stdout, stderr). On success, error will be null. On error, error will be an instance of Error and err.code will be the exit code of the child process, and err.signal will be set to the signal that terminated the process.

There is a second optional argument to specify several options. The default options are

{ encoding: 'utf8'
, timeout: 0
, maxBuffer: 200*1024
, killSignal: 'SIGTERM'
, cwd: null
, env: null
}

If timeout is greater than 0, then it will kill the child process if it runs longer than timeout milliseconds. The child process is killed with killSignal (default: 'SIGTERM'). maxBuffer specifies the largest amount of data allowed on stdout or stderr - if this value is exceeded then the child process is killed.

child.kill(signal='SIGTERM')

Send a signal to the child process. If no argument is given, the process will be sent 'SIGTERM'. See signal(7) for a list of available signals.

var spawn = require('child_process').spawn,
    grep  = spawn('grep', ['ssh']);

grep.on('exit', function (code, signal) {
  console.log('child process terminated due to receipt of signal '+signal);
});

// send SIGHUP to process
grep.kill('SIGHUP');

Note that while the function is called kill, the signal delivered to the child process may not actually kill it. kill really just sends a signal to a process.

See kill(2)