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process

The process object is a global object and can be accessed from anywhere. It is an instance of EventEmitter.

Exit Codes

Node will normally exit with a 0 status code when no more async operations are pending. The following status codes are used in other cases:

  • 1 Uncaught Fatal Exception - There was an uncaught exception, and it was not handled by a domain or an uncaughtException event handler.
  • 2 - Unused (reserved by Bash for builtin misuse)
  • 3 Internal JavaScript Parse Error - The JavaScript source code internal in Node's bootstrapping process caused a parse error. This is extremely rare, and generally can only happen during development of Node itself.
  • 4 Internal JavaScript Evaluation Failure - The JavaScript source code internal in Node's bootstrapping process failed to return a function value when evaluated. This is extremely rare, and generally can only happen during development of Node itself.
  • 5 Fatal Error - There was a fatal unrecoverable error in V8. Typically a message will be printed to stderr with the prefix FATAL ERROR.
  • 6 Non-function Internal Exception Handler - There was an uncaught exception, but the internal fatal exception handler function was somehow set to a non-function, and could not be called.
  • 7 Internal Exception Handler Run-Time Failure - There was an uncaught exception, and the internal fatal exception handler function itself threw an error while attempting to handle it. This can happen, for example, if a process.on('uncaughtException') or domain.on('error') handler throws an error.
  • 8 - Unused. In previous versions of Node, exit code 8 sometimes indicated an uncaught exception.
  • 9 - Invalid Argument - Either an unknown option was specified, or an option requiring a value was provided without a value.
  • 10 Internal JavaScript Run-Time Failure - The JavaScript source code internal in Node's bootstrapping process threw an error when the bootstrapping function was called. This is extremely rare, and generally can only happen during development of Node itself.
  • 12 Invalid Debug Argument - The --debug and/or --debug-brk options were set, but an invalid port number was chosen.
  • >128 Signal Exits - If Node receives a fatal signal such as SIGKILL or SIGHUP, then its exit code will be 128 plus the value of the signal code. This is a standard Unix practice, since exit codes are defined to be 7-bit integers, and signal exits set the high-order bit, and then contain the value of the signal code.

Event: 'exit'

Emitted when the process is about to exit. There is no way to prevent the exiting of the event loop at this point, and once all exit listeners have finished running the process will exit. Therefore you must only perform synchronous operations in this handler. This is a good hook to perform checks on the module's state (like for unit tests). The callback takes one argument, the code the process is exiting with.

Example of listening for exit:

process.on('exit', function(code) {
  // do *NOT* do this
  setTimeout(function() {
    console.log('This will not run');
  }, 0);
  console.log('About to exit with code:', code);
});

Event: 'uncaughtException'

Emitted when an exception bubbles all the way back to the event loop. If a listener is added for this exception, the default action (which is to print a stack trace and exit) will not occur.

Example of listening for uncaughtException:

process.on('uncaughtException', function(err) {
  console.log('Caught exception: ' + err);
});

setTimeout(function() {
  console.log('This will still run.');
}, 500);

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');

Note that uncaughtException is a very crude mechanism for exception handling.

Don't use it, use domains instead. If you do use it, restart your application after every unhandled exception!

Do not use it as the node.js equivalent of On Error Resume Next. An unhandled exception means your application - and by extension node.js itself - is in an undefined state. Blindly resuming means anything could happen.

Think of resuming as pulling the power cord when you are upgrading your system. Nine out of ten times nothing happens - but the 10th time, your system is bust.

You have been warned.

Signal Events

Emitted when the processes receives a signal. See sigaction(2) for a list of standard POSIX signal names such as SIGINT, SIGHUP, etc.

Example of listening for SIGINT:

// Start reading from stdin so we don't exit.
process.stdin.resume();

process.on('SIGINT', function() {
  console.log('Got SIGINT.  Press Control-D to exit.');
});

An easy way to send the SIGINT signal is with Control-C in most terminal programs.

Note:

  • SIGUSR1 is reserved by node.js to start the debugger. It's possible to install a listener but that won't stop the debugger from starting.
  • SIGTERM and SIGINT have default handlers on non-Windows platforms that resets the terminal mode before exiting with code 128 + signal number. If one of these signals has a listener installed, its default behaviour will be removed (node will no longer exit).
  • SIGPIPE is ignored by default, it can have a listener installed.
  • SIGHUP is generated on Windows when the console window is closed, and on other platforms under various similar conditions, see signal(7). It can have a listener installed, however node will be unconditionally terminated by Windows about 10 seconds later. On non-Windows platforms, the default behaviour of SIGHUP is to terminate node, but once a listener has been installed its default behaviour will be removed.
  • SIGTERM is not supported on Windows, it can be listened on.
  • SIGINT from the terminal is supported on all platforms, and can usually be generated with CTRL+C (though this may be configurable). It is not generated when terminal raw mode is enabled.
  • SIGBREAK is delivered on Windows when CTRL+BREAK is pressed, on non-Windows platforms it can be listened on, but there is no way to send or generate it.
  • SIGWINCH is delivered when the console has been resized. On Windows, this will only happen on write to the console when the cursor is being moved, or when a readable tty is used in raw mode.
  • SIGKILL cannot have a listener installed, it will unconditionally terminate node on all platforms.
  • SIGSTOP cannot have a listener installed.

Note that Windows does not support sending Signals, but node offers some emulation with process.kill(), and child_process.kill():

  • Sending signal 0 can be used to search for the existence of a process
  • Sending SIGINT, SIGTERM, and SIGKILL cause the unconditional exit of the target process.

process.stdout

A Writable Stream to stdout.

Example: the definition of console.log

console.log = function(d) {
  process.stdout.write(d + '\n');
};

process.stderr and process.stdout are unlike other streams in Node in that writes to them are usually blocking. They are blocking in the case that they refer to regular files or TTY file descriptors. In the case they refer to pipes, they are non-blocking like other streams.

To check if Node is being run in a TTY context, read the isTTY property on process.stderr, process.stdout, or process.stdin:

$ node -p "Boolean(process.stdin.isTTY)"
true
$ echo "foo" | node -p "Boolean(process.stdin.isTTY)"
false

$ node -p "Boolean(process.stdout.isTTY)"
true
$ node -p "Boolean(process.stdout.isTTY)" | cat
false

See the tty docs for more information.

process.stderr

A writable stream to stderr.

process.stderr and process.stdout are unlike other streams in Node in that writes to them are usually blocking. They are blocking in the case that they refer to regular files or TTY file descriptors. In the case they refer to pipes, they are non-blocking like other streams.

process.stdin

A Readable Stream for stdin. The stdin stream is paused by default, so one must call process.stdin.resume() to read from it.

Example of opening standard input and listening for both events:

process.stdin.resume();
process.stdin.setEncoding('utf8');

process.stdin.on('data', function(chunk) {
  process.stdout.write('data: ' + chunk);
});

process.stdin.on('end', function() {
  process.stdout.write('end');
});

process.argv

An array containing the command line arguments. The first element will be 'node', the second element will be the name of the JavaScript file. The next elements will be any additional command line arguments.

// print process.argv
process.argv.forEach(function(val, index, array) {
  console.log(index + ': ' + val);
});

This will generate:

$ node process-2.js one two=three four
0: node
1: /Users/mjr/work/node/process-2.js
2: one
3: two=three
4: four

process.execPath

This is the absolute pathname of the executable that started the process.

Example:

/usr/local/bin/node

process.execArgv

This is the set of node-specific command line options from the executable that started the process. These options do not show up in process.argv, and do not include the node executable, the name of the script, or any options following the script name. These options are useful in order to spawn child processes with the same execution environment as the parent.

Example:

$ node --harmony script.js --version

results in process.execArgv:

['--harmony']

and process.argv:

['/usr/local/bin/node', 'script.js', '--version']

process.abort()

This causes node to emit an abort. This will cause node to exit and generate a core file.

process.chdir(directory)

Changes the current working directory of the process or throws an exception if that fails.

console.log('Starting directory: ' + process.cwd());
try {
  process.chdir('/tmp');
  console.log('New directory: ' + process.cwd());
}
catch (err) {
  console.log('chdir: ' + err);
}

process.cwd()

Returns the current working directory of the process.

console.log('Current directory: ' + process.cwd());

process.env

An object containing the user environment. See environ(7).

process.exit([code])

Ends the process with the specified code. If omitted, exit uses the 'success' code 0.

To exit with a 'failure' code:

process.exit(1);

The shell that executed node should see the exit code as 1.

process.exitCode

A number which will be the process exit code, when the process either exits gracefully, or is exited via process.exit() without specifying a code.

Specifying a code to process.exit(code) will override any previous setting of process.exitCode.

process.getgid()

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Gets the group identity of the process. (See getgid(2).) This is the numerical group id, not the group name.

if (process.getgid) {
  console.log('Current gid: ' + process.getgid());
}

process.setgid(id)

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Sets the group identity of the process. (See setgid(2).) This accepts either a numerical ID or a groupname string. If a groupname is specified, this method blocks while resolving it to a numerical ID.

if (process.getgid && process.setgid) {
  console.log('Current gid: ' + process.getgid());
  try {
    process.setgid(501);
    console.log('New gid: ' + process.getgid());
  }
  catch (err) {
    console.log('Failed to set gid: ' + err);
  }
}

process.getuid()

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Gets the user identity of the process. (See getuid(2).) This is the numerical userid, not the username.

if (process.getuid) {
  console.log('Current uid: ' + process.getuid());
}

process.setuid(id)

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Sets the user identity of the process. (See setuid(2).) This accepts either a numerical ID or a username string. If a username is specified, this method blocks while resolving it to a numerical ID.

if (process.getuid && process.setuid) {
  console.log('Current uid: ' + process.getuid());
  try {
    process.setuid(501);
    console.log('New uid: ' + process.getuid());
  }
  catch (err) {
    console.log('Failed to set uid: ' + err);
  }
}

process.getgroups()

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Returns an array with the supplementary group IDs. POSIX leaves it unspecified if the effective group ID is included but node.js ensures it always is.

process.setgroups(groups)

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Sets the supplementary group IDs. This is a privileged operation, meaning you need to be root or have the CAP_SETGID capability.

The list can contain group IDs, group names or both.

process.initgroups(user, extra_group)

Note: this function is only available on POSIX platforms (i.e. not Windows, Android)

Reads /etc/group and initializes the group access list, using all groups of which the user is a member. This is a privileged operation, meaning you need to be root or have the CAP_SETGID capability.

user is a user name or user ID. extra_group is a group name or group ID.

Some care needs to be taken when dropping privileges. Example:

console.log(process.getgroups());         // [ 0 ]
process.initgroups('bnoordhuis', 1000);   // switch user
console.log(process.getgroups());         // [ 27, 30, 46, 1000, 0 ]
process.setgid(1000);                     // drop root gid
console.log(process.getgroups());         // [ 27, 30, 46, 1000 ]

process.version

A compiled-in property that exposes NODE_VERSION.

console.log('Version: ' + process.version);

process.versions

A property exposing version strings of node and its dependencies.

console.log(process.versions);

Will print something like:

{ http_parser: '1.0',
  node: '0.10.4',
  v8: '3.14.5.8',
  ares: '1.9.0-DEV',
  uv: '0.10.3',
  zlib: '1.2.3',
  modules: '11',
  openssl: '1.0.1e' }

process.config

An Object containing the JavaScript representation of the configure options that were used to compile the current node executable. This is the same as the "config.gypi" file that was produced when running the ./configure script.

An example of the possible output looks like:

{ target_defaults:
   { cflags: [],
     default_configuration: 'Release',
     defines: [],
     include_dirs: [],
     libraries: [] },
  variables:
   { host_arch: 'x64',
     node_install_npm: 'true',
     node_prefix: '',
     node_shared_cares: 'false',
     node_shared_http_parser: 'false',
     node_shared_libuv: 'false',
     node_shared_v8: 'false',
     node_shared_zlib: 'false',
     node_use_dtrace: 'false',
     node_use_openssl: 'true',
     node_shared_openssl: 'false',
     strict_aliasing: 'true',
     target_arch: 'x64',
     v8_use_snapshot: 'true' } }

process.kill(pid, [signal])

Send a signal to a process. pid is the process id and signal is the string describing the signal to send. Signal names are strings like 'SIGINT' or 'SIGHUP'. If omitted, the signal will be 'SIGTERM'. See Signal Events and kill(2) for more information.

Will throw an error if target does not exist, and as a special case, a signal of 0 can be used to test for the existence of a process.

Note that just because the name of this function is process.kill, it is really just a signal sender, like the kill system call. The signal sent may do something other than kill the target process.

Example of sending a signal to yourself:

process.on('SIGHUP', function() {
  console.log('Got SIGHUP signal.');
});

setTimeout(function() {
  console.log('Exiting.');
  process.exit(0);
}, 100);

process.kill(process.pid, 'SIGHUP');

Note: When SIGUSR1 is received by Node.js it starts the debugger, see Signal Events.

process.pid

The PID of the process.

console.log('This process is pid ' + process.pid);

process.title

Getter/setter to set what is displayed in 'ps'.

When used as a setter, the maximum length is platform-specific and probably short.

On Linux and OS X, it's limited to the size of the binary name plus the length of the command line arguments because it overwrites the argv memory.

v0.8 allowed for longer process title strings by also overwriting the environ memory but that was potentially insecure/confusing in some (rather obscure) cases.

process.arch

What processor architecture you're running on: 'arm', 'ia32', or 'x64'.

console.log('This processor architecture is ' + process.arch);

process.platform

What platform you're running on: 'darwin', 'freebsd', 'linux', 'sunos' or 'win32'

console.log('This platform is ' + process.platform);

process.memoryUsage()

Returns an object describing the memory usage of the Node process measured in bytes.

var util = require('util');

console.log(util.inspect(process.memoryUsage()));

This will generate:

{ rss: 4935680,
  heapTotal: 1826816,
  heapUsed: 650472 }

heapTotal and heapUsed refer to V8's memory usage.

process.nextTick(callback)

  • callback {Function}

Once the current event loop turn runs to completion, call the callback function.

This is not a simple alias to setTimeout(fn, 0), it's much more efficient. It runs before any additional I/O events (including timers) fire in subsequent ticks of the event loop.

console.log('start');
process.nextTick(function() {
  console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback

This is important in developing APIs where you want to give the user the chance to assign event handlers after an object has been constructed, but before any I/O has occurred.

function MyThing(options) {
  this.setupOptions(options);

  process.nextTick(function() {
    this.startDoingStuff();
  }.bind(this));
}

var thing = new MyThing();
thing.getReadyForStuff();

// thing.startDoingStuff() gets called now, not before.

It is very important for APIs to be either 100% synchronous or 100% asynchronous. Consider this example:

// WARNING!  DO NOT USE!  BAD UNSAFE HAZARD!
function maybeSync(arg, cb) {
  if (arg) {
    cb();
    return;
  }

  fs.stat('file', cb);
}

This API is hazardous. If you do this:

maybeSync(true, function() {
  foo();
});
bar();

then it's not clear whether foo() or bar() will be called first.

This approach is much better:

function definitelyAsync(arg, cb) {
  if (arg) {
    process.nextTick(cb);
    return;
  }

  fs.stat('file', cb);
}

Note: the nextTick queue is completely drained on each pass of the event loop before additional I/O is processed. As a result, recursively setting nextTick callbacks will block any I/O from happening, just like a while(true); loop.

process.umask([mask])

Sets or reads the process's file mode creation mask. Child processes inherit the mask from the parent process. Returns the old mask if mask argument is given, otherwise returns the current mask.

var oldmask, newmask = 0644;

oldmask = process.umask(newmask);
console.log('Changed umask from: ' + oldmask.toString(8) +
            ' to ' + newmask.toString(8));

process.uptime()

Number of seconds Node has been running.

process.hrtime()

Returns the current high-resolution real time in a [seconds, nanoseconds] tuple Array. It is relative to an arbitrary time in the past. It is not related to the time of day and therefore not subject to clock drift. The primary use is for measuring performance between intervals.

You may pass in the result of a previous call to process.hrtime() to get a diff reading, useful for benchmarks and measuring intervals:

var time = process.hrtime();
// [ 1800216, 25 ]

setTimeout(function() {
  var diff = process.hrtime(time);
  // [ 1, 552 ]

  console.log('benchmark took %d nanoseconds', diff[0] * 1e9 + diff[1]);
  // benchmark took 1000000527 nanoseconds
}, 1000);

Async Listeners

Stability: 1 - Experimental

The AsyncListener API is the JavaScript interface for the AsyncWrap class which allows developers to be notified about key events in the lifetime of an asynchronous event. Node performs a lot of asynchronous events internally, and significant use of this API may have a significant performance impact on your application.

process.createAsyncListener(callbacksObj[, userData])

  • callbacksObj {Object} Contains optional callbacks that will fire at specific times in the life cycle of the asynchronous event.
  • userData {Value} a value that will be passed to all callbacks.

Returns a constructed AsyncListener object.

To begin capturing asynchronous events pass either the callbacksObj or and existing AsyncListener instance to process.addAsyncListener(). The same AsyncListener instance can only be added once to the active queue, and subsequent attempts to add the instance will be ignored.

To stop capturing pass the AsyncListener instance to process.removeAsyncListener(). This does not mean the AsyncListener previously added will stop triggering callbacks. Once attached to an asynchronous event it will persist with the lifetime of the asynchronous call stack.

Explanation of function parameters:

callbacksObj: An Object which may contain three optional fields:

  • create(userData): A Function called when an asynchronous event is instantiated. If a Value is returned then it will be attached to the event and overwrite any value that had been passed to process.createAsyncListener()'s userData argument. If an initial userData was passed when created, then create() will receive that as a function argument.

  • before(context, userData): A Function that is called immediately before the asynchronous callback is about to run. It will be passed both the context (i.e. this) of the calling function and the userData either returned from create() or passed during construction (if either occurred).

  • after(context, userData): A Function called immediately after the asynchronous event's callback has run. Note this will not be called if the callback throws and the error is not handled.

  • error(userData, error): A Function called if the event's callback threw. If this registered callback returns true then Node will assume the error has been properly handled and resume execution normally. When multiple error() callbacks have been registered only one of those callbacks needs to return true for AsyncListener to accept that the error has been handled, but all error() callbacks will always be run.

userData: A Value (i.e. anything) that will be, by default, attached to all new event instances. This will be overwritten if a Value is returned by create().

Here is an example of overwriting the userData:

process.createAsyncListener({
  create: function listener(value) {
    // value === true
    return false;
}, {
  before: function before(context, value) {
    // value === false
  }
}, true);

Note: The EventEmitter, while used to emit status of an asynchronous event, is not itself asynchronous. So create() will not fire when an event is added, and before/after will not fire when emitted callbacks are called.

process.addAsyncListener(callbacksObj[, userData])

process.addAsyncListener(asyncListener)

Returns a constructed AsyncListener object and immediately adds it to the listening queue to begin capturing asynchronous events.

Function parameters can either be the same as process.createAsyncListener(), or a constructed AsyncListener object.

Example usage for capturing errors:

var fs = require('fs');

var cntr = 0;
var key = process.addAsyncListener({
  create: function onCreate() {
    return { uid: cntr++ };
  },
  before: function onBefore(context, storage) {
    // Write directly to stdout or we'll enter a recursive loop
    fs.writeSync(1, 'uid: ' + storage.uid + ' is about to run\n');
  },
  after: function onAfter(context, storage) {
    fs.writeSync(1, 'uid: ' + storage.uid + ' is about to run\n');
  },
  error: function onError(storage, err) {
    // Handle known errors
    if (err.message === 'everything is fine') {
      fs.writeSync(1, 'handled error just threw:\n');
      fs.writeSync(1, err.stack + '\n');
      return true;
    }
  }
});

process.nextTick(function() {
  throw new Error('everything is fine');
});

// Output:
// uid: 0 is about to run
// handled error just threw:
// Error: really, it's ok
//     at /tmp/test2.js:27:9
//     at process._tickCallback (node.js:583:11)
//     at Function.Module.runMain (module.js:492:11)
//     at startup (node.js:123:16)
//     at node.js:1012:3

process.removeAsyncListener(asyncListener)

Removes the AsyncListener from the listening queue.

Removing the AsyncListener from the active queue does not mean the asyncListener callbacks will cease to fire on the events they've been registered. Subsequently, any asynchronous events fired during the execution of a callback will also have the same asyncListener callbacks attached for future execution. For example:

var fs = require('fs');

var key = process.createAsyncListener({
  create: function asyncListener() {
    // Write directly to stdout or we'll enter a recursive loop
    fs.writeSync(1, 'You summoned me?\n');
  }
});

// We want to begin capturing async events some time in the future.
setTimeout(function() {
  process.addAsyncListener(key);

  // Perform a few additional async events.
  setTimeout(function() {
    setImmediate(function() {
      process.nextTick(function() { });
    });
  });

  // Removing the listener doesn't mean to stop capturing events that
  // have already been added.
  process.removeAsyncListener(key);
}, 100);

// Output:
// You summoned me?
// You summoned me?
// You summoned me?
// You summoned me?

The fact that we logged 4 asynchronous events is an implementation detail of Node's Timers.

To stop capturing from a specific asynchronous event stack process.removeAsyncListener() must be called from within the call stack itself. For example:

var fs = require('fs');

var key = process.createAsyncListener({
  create: function asyncListener() {
    // Write directly to stdout or we'll enter a recursive loop
    fs.writeSync(1, 'You summoned me?\n');
  }
});

// We want to begin capturing async events some time in the future.
setTimeout(function() {
  process.addAsyncListener(key);

  // Perform a few additional async events.
  setImmediate(function() {
    // Stop capturing from this call stack.
    process.removeAsyncListener(key);

    process.nextTick(function() { });
  });
}, 100);

// Output:
// You summoned me?

The user must be explicit and always pass the AsyncListener they wish to remove. It is not possible to simply remove all listeners at once.

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