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Rich Trott 4dcf24cfab doc: refine process.kill() and exit explanations

Add corrections about when exit event fires and how .kill() works on
Windows.

PR-URL: https://github.com/nodejs/node/pull/2918
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

2015-09-17 15:03:04 -07:00

30 KiB

Raw Blame History

process

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

Exit Codes

Node.js 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.js's bootstrapping process caused a parse error. This is extremely rare, and generally can only happen during development of Node.js itself.
4 Internal JavaScript Evaluation Failure - The JavaScript source code internal in Node.js's bootstrapping process failed to return a function value when evaluated. This is extremely rare, and generally can only happen during development of Node.js 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.js, 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.js'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.js 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.js 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.

This event is only emitted when node exits explicitly by process.exit() or implicitly by the event loop draining.

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: 'message'

message {Object} a parsed JSON object or primitive value
sendHandle {Handle object} a net.Socket or net.Server object, or undefined.

Messages sent by ChildProcess.send() are obtained using the 'message' event on the child's process object.

Event: 'beforeExit'

This event is emitted when Node.js empties its event loop and has nothing else to schedule. Normally, Node.js exits when there is no work scheduled, but a listener for 'beforeExit' can make asynchronous calls, and cause Node.js to continue.

'beforeExit' is not emitted for conditions causing explicit termination, such as process.exit() or uncaught exceptions, and should not be used as an alternative to the 'exit' event unless the intention is to schedule more work.

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.

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.

uncaughtException should be used to perform synchronous cleanup before shutting down the process. It is not safe to resume normal operation after uncaughtException. If you do use it, restart your application after every unhandled exception!

You have been warned.

Event: 'unhandledRejection'

Emitted whenever a Promise is rejected and no error handler is attached to the promise within a turn of the event loop. When programming with promises exceptions are encapsulated as rejected promises. Such promises can be caught and handled using promise.catch(...) and rejections are propagated through a promise chain. This event is useful for detecting and keeping track of promises that were rejected whose rejections were not handled yet. This event is emitted with the following arguments:

reason the object with which the promise was rejected (usually an Error instance).
p the promise that was rejected.

Here is an example that logs every unhandled rejection to the console

process.on('unhandledRejection', function(reason, p) {
    console.log("Unhandled Rejection at: Promise ", p, " reason: ", reason);
    // application specific logging, throwing an error, or other logic here
});

For example, here is a rejection that will trigger the 'unhandledRejection' event:

somePromise.then(function(res) {
  return reportToUser(JSON.pasre(res)); // note the typo
}); // no `.catch` or `.then`

Event: 'rejectionHandled'

Emitted whenever a Promise was rejected and an error handler was attached to it (for example with .catch()) later than after an event loop turn. This event is emitted with the following arguments:

p the promise that was previously emitted in an 'unhandledRejection' event, but which has now gained a rejection handler.

There is no notion of a top level for a promise chain at which rejections can always be handled. Being inherently asynchronous in nature, a promise rejection can be be handled at a future point in time — possibly much later than the event loop turn it takes for the 'unhandledRejection' event to be emitted.

Another way of stating this is that, unlike in synchronous code where there is an ever-growing list of unhandled exceptions, with promises there is a growing-and-shrinking list of unhandled rejections. In synchronous code, the 'uncaughtException' event tells you when the list of unhandled exceptions grows. And in asynchronous code, the 'unhandledRejection' event tells you when the list of unhandled rejections grows, while the 'rejectionHandled' event tells you when the list of unhandled rejections shrinks.

For example using the rejection detection hooks in order to keep a list of all the rejected promises at a given time:

var unhandledRejections = [];
process.on('unhandledRejection', function(reason, p) {
    unhandledRejections.push(p);
});
process.on('rejectionHandled', function(p) {
    var index = unhandledRejections.indexOf(p);
    unhandledRejections.splice(index, 1);
});

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 behavior will be removed (Node.js 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.js will be unconditionally terminated by Windows about 10 seconds later. On non-Windows platforms, the default behavior of SIGHUP is to terminate Node.js, but once a listener has been installed its default behavior 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.js on all platforms.
SIGSTOP cannot have a listener installed.

Note that Windows does not support sending Signals, but Node.js offers some emulation with process.kill(), and child_process.kill(). Sending signal 0 can be used to test for the existence of a process. Sending SIGINT, SIGTERM, and SIGKILL cause the unconditional termination of the target process.

process.stdout

A Writable Stream to stdout (on fd 1).

For example, a console.log equivalent could look like this:

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

process.stderr and process.stdout are unlike other streams in Node.js in that they cannot be closed (end() will throw), they never emit the finish event and that writes are always blocking.

To check if Node.js 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 (on fd 2).

process.stderr and process.stdout are unlike other streams in Node.js in that they cannot be closed (end() will throw), they never emit the finish event and that writes 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 blocking in Linux/Unix.
- They are non-blocking like other streams in Windows.

process.stdin

A Readable Stream for stdin (on fd 0).

Example of opening standard input and listening for both events:

process.stdin.setEncoding('utf8');

process.stdin.on('readable', function() {
  var chunk = process.stdin.read();
  if (chunk !== null) {
    process.stdout.write('data: ' + chunk);
  }
});

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

As a Stream, process.stdin can also be used in "old" mode that is compatible with scripts written for node.js prior to v0.10. For more information see Stream compatibility.

In "old" Streams mode the stdin stream is paused by default, so one must call process.stdin.resume() to read from it. Note also that calling process.stdin.resume() itself would switch stream to "old" mode.

If you are starting a new project you should prefer a more recent "new" Streams mode over "old" one.

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.js-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.js 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.js to emit an abort. This will cause Node.js 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).

An example of this object looks like:

{ TERM: 'xterm-256color',
  SHELL: '/usr/local/bin/bash',
  USER: 'maciej',
  PATH: '~/.bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin',
  PWD: '/Users/maciej',
  EDITOR: 'vim',
  SHLVL: '1',
  HOME: '/Users/maciej',
  LOGNAME: 'maciej',
  _: '/usr/local/bin/node' }

You can write to this object, but changes won't be reflected outside of your process. That means that the following won't work:

$ node -e 'process.env.foo = "bar"' && echo $foo

But this will:

process.env.foo = 'bar';
console.log(process.env.foo);

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.js 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.