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:

  1. process.on('exit', function(code) {
  2. // do *NOT* do this
  3. setTimeout(function() {
  4. console.log('This will not run');
  5. }, 0);
  6. console.log('About to exit with code:', code);
  7. });

Event: ‘beforeExit’

This event is emitted when node empties it’s event loop and has nothing else to
schedule. Normally, node exits when there is no work scheduled, but a listener
for ‘beforeExit’ can make asynchronous calls, and cause node 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:

  1. process.on('uncaughtException', function(err) {
  2. console.log('Caught exception: ' + err);
  3. });
  4. setTimeout(function() {
  5. console.log('This will still run.');
  6. }, 500);
  7. // Intentionally cause an exception, but don't catch it.
  8. nonexistentFunc();
  9. 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:

  1. // Start reading from stdin so we don't exit.
  2. process.stdin.resume();
  3. process.on('SIGINT', function() {
  4. console.log('Got SIGINT. Press Control-D to exit.');
  5. });

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 (on fd 1).

Example: the definition of console.log

  1. console.log = function(d) {
  2. process.stdout.write(d + '\n');
  3. };

process.stderr and process.stdout are unlike other streams in Node 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.

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

  1. $ node -p "Boolean(process.stdin.isTTY)"
  2. true
  3. $ echo "foo" | node -p "Boolean(process.stdin.isTTY)"
  4. false
  5. $ node -p "Boolean(process.stdout.isTTY)"
  6. true
  7. $ node -p "Boolean(process.stdout.isTTY)" | cat
  8. 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 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:

  1. process.stdin.setEncoding('utf8');
  2. process.stdin.on('readable', function() {
  3. var chunk = process.stdin.read();
  4. if (chunk !== null) {
  5. process.stdout.write('data: ' + chunk);
  6. }
  7. });
  8. process.stdin.on('end', function() {
  9. process.stdout.write('end');
  10. });

As a Stream, process.stdin can also be used in “old” mode that is compatible
with scripts written for node prior 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.

  1. // print process.argv
  2. process.argv.forEach(function(val, index, array) {
  3. console.log(index + ': ' + val);
  4. });

This will generate:

  1. $ node process-2.js one two=three four
  2. 0: node
  3. 1: /Users/mjr/work/node/process-2.js
  4. 2: one
  5. 3: two=three
  6. 4: four

process.execPath

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

Example:

  1. /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:

  1. $ node --harmony script.js --version

results in process.execArgv:

  1. ['--harmony']

and process.argv:

  1. ['/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.

  1. console.log('Starting directory: ' + process.cwd());
  2. try {
  3. process.chdir('/tmp');
  4. console.log('New directory: ' + process.cwd());
  5. }
  6. catch (err) {
  7. console.log('chdir: ' + err);
  8. }

process.cwd()

Returns the current working directory of the process.

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

process.env

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

An example of this object looks like:

  1. { TERM: 'xterm-256color',
  2. SHELL: '/usr/local/bin/bash',
  3. USER: 'maciej',
  4. PATH: '~/.bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin',
  5. PWD: '/Users/maciej',
  6. EDITOR: 'vim',
  7. SHLVL: '1',
  8. HOME: '/Users/maciej',
  9. LOGNAME: 'maciej',
  10. _: '/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:

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

But this will:

  1. process.env.foo = 'bar';
  2. 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:

  1. 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.

  1. if (process.getgid) {
  2. console.log('Current gid: ' + process.getgid());
  3. }

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.

  1. if (process.getgid && process.setgid) {
  2. console.log('Current gid: ' + process.getgid());
  3. try {
  4. process.setgid(501);
  5. console.log('New gid: ' + process.getgid());
  6. }
  7. catch (err) {
  8. console.log('Failed to set gid: ' + err);
  9. }
  10. }

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.

  1. if (process.getuid) {
  2. console.log('Current uid: ' + process.getuid());
  3. }

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.

  1. if (process.getuid && process.setuid) {
  2. console.log('Current uid: ' + process.getuid());
  3. try {
  4. process.setuid(501);
  5. console.log('New uid: ' + process.getuid());
  6. }
  7. catch (err) {
  8. console.log('Failed to set uid: ' + err);
  9. }
  10. }

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:

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

process.version

A compiled-in property that exposes NODE_VERSION.

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

process.versions

A property exposing version strings of node and its dependencies.

  1. console.log(process.versions);

Will print something like:

  1. { http_parser: '1.0',
  2. node: '0.10.4',
  3. v8: '3.14.5.8',
  4. ares: '1.9.0-DEV',
  5. uv: '0.10.3',
  6. zlib: '1.2.3',
  7. modules: '11',
  8. 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:

  1. { target_defaults:
  2. { cflags: [],
  3. default_configuration: 'Release',
  4. defines: [],
  5. include_dirs: [],
  6. libraries: [] },
  7. variables:
  8. { host_arch: 'x64',
  9. node_install_npm: 'true',
  10. node_prefix: '',
  11. node_shared_cares: 'false',
  12. node_shared_http_parser: 'false',
  13. node_shared_libuv: 'false',
  14. node_shared_v8: 'false',
  15. node_shared_zlib: 'false',
  16. node_use_dtrace: 'false',
  17. node_use_openssl: 'true',
  18. node_shared_openssl: 'false',
  19. strict_aliasing: 'true',
  20. target_arch: 'x64',
  21. 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:

  1. process.on('SIGHUP', function() {
  2. console.log('Got SIGHUP signal.');
  3. });
  4. setTimeout(function() {
  5. console.log('Exiting.');
  6. process.exit(0);
  7. }, 100);
  8. 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.

  1. 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'.

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

process.platform

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

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

process.memoryUsage()

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

  1. var util = require('util');
  2. console.log(util.inspect(process.memoryUsage()));

This will generate:

  1. { rss: 4935680,
  2. heapTotal: 1826816,
  3. 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.

  1. console.log('start');
  2. process.nextTick(function() {
  3. console.log('nextTick callback');
  4. });
  5. console.log('scheduled');
  6. // Output:
  7. // start
  8. // scheduled
  9. // 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.

  1. function MyThing(options) {
  2. this.setupOptions(options);
  3. process.nextTick(function() {
  4. this.startDoingStuff();
  5. }.bind(this));
  6. }
  7. var thing = new MyThing();
  8. thing.getReadyForStuff();
  9. // thing.startDoingStuff() gets called now, not before.

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

  1. // WARNING! DO NOT USE! BAD UNSAFE HAZARD!
  2. function maybeSync(arg, cb) {
  3. if (arg) {
  4. cb();
  5. return;
  6. }
  7. fs.stat('file', cb);
  8. }

This API is hazardous. If you do this:

  1. maybeSync(true, function() {
  2. foo();
  3. });
  4. bar();

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

This approach is much better:

  1. function definitelyAsync(arg, cb) {
  2. if (arg) {
  3. process.nextTick(cb);
  4. return;
  5. }
  6. fs.stat('file', cb);
  7. }

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.

  1. var oldmask, newmask = 0022;
  2. oldmask = process.umask(newmask);
  3. console.log('Changed umask from: ' + oldmask.toString(8) +
  4. ' 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:

  1. var time = process.hrtime();
  2. // [ 1800216, 25 ]
  3. setTimeout(function() {
  4. var diff = process.hrtime(time);
  5. // [ 1, 552 ]
  6. console.log('benchmark took %d nanoseconds', diff[0] * 1e9 + diff[1]);
  7. // benchmark took 1000000527 nanoseconds
  8. }, 1000);

process.mainModule

Alternate way to retrieve
require.main.
The difference is that if the main module changes at runtime, require.main
might still refer to the original main module in modules that were required
before the change occurred. Generally it’s safe to assume that the two refer
to the same module.

As with require.main, it will be undefined if there was no entry script.