File System

Stability: 3 - Stable

File I/O is provided by simple wrappers around standard POSIX functions. To
use this module do require('fs'). All the methods have asynchronous and
synchronous forms.

The asynchronous form always take a completion callback as its last argument.
The arguments passed to the completion callback depend on the method, but the
first argument is always reserved for an exception. If the operation was
completed successfully, then the first argument will be null or undefined.

When using the synchronous form any exceptions are immediately thrown.
You can use try/catch to handle exceptions or allow them to bubble up.

Here is an example of the asynchronous version:

var fs = require('fs');
fs.unlink('/tmp/hello', function (err) {
  if (err) throw err;
  console.log('successfully deleted /tmp/hello');
});

Here is the synchronous version:

var fs = require('fs');
fs.unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');

With the asynchronous methods there is no guaranteed ordering. So the
following is prone to error:

fs.rename('/tmp/hello', '/tmp/world', function (err) {
  if (err) throw err;
  console.log('renamed complete');
});
fs.stat('/tmp/world', function (err, stats) {
  if (err) throw err;
  console.log('stats: ' + JSON.stringify(stats));
});

It could be that fs.stat is executed before fs.rename.
The correct way to do this is to chain the callbacks.

fs.rename('/tmp/hello', '/tmp/world', function (err) {
  if (err) throw err;
  fs.stat('/tmp/world', function (err, stats) {
    if (err) throw err;
    console.log('stats: ' + JSON.stringify(stats));
  });
});

In busy processes, the programmer is strongly encouraged to use the
asynchronous versions of these calls. The synchronous versions will block
the entire process until they complete—halting all connections.

Relative path to filename can be used, remember however that this path will be
relative to process.cwd().

Most fs functions let you omit the callback argument. If you do, a default
callback is used that rethrows errors. To get a trace to the original call
site, set the NODE_DEBUG environment variable:

$ cat script.js
function bad() {
  require('fs').readFile('/');
}
bad();
$ env NODE_DEBUG=fs node script.js
fs.js:66
        throw err;
              ^
Error: EISDIR, read
    at rethrow (fs.js:61:21)
    at maybeCallback (fs.js:79:42)
    at Object.fs.readFile (fs.js:153:18)
    at bad (/path/to/script.js:2:17)
    at Object.<anonymous> (/path/to/script.js:5:1)
    <etc.>

fs.rename(oldPath, newPath, callback)

Asynchronous rename(2). No arguments other than a possible exception are given
to the completion callback.

fs.renameSync(oldPath, newPath)

Synchronous rename(2). Returns undefined.

fs.ftruncate(fd, len, callback)

Asynchronous ftruncate(2). No arguments other than a possible exception are
given to the completion callback.

fs.ftruncateSync(fd, len)

Synchronous ftruncate(2). Returns undefined.

fs.truncate(path, len, callback)

Asynchronous truncate(2). No arguments other than a possible exception are
given to the completion callback. A file descriptor can also be passed as the
first argument. In this case, fs.ftruncate() is called.

fs.truncateSync(path, len)

Synchronous truncate(2). Returns undefined.

fs.chown(path, uid, gid, callback)

Asynchronous chown(2). No arguments other than a possible exception are given
to the completion callback.

fs.chownSync(path, uid, gid)

Synchronous chown(2). Returns undefined.

fs.fchown(fd, uid, gid, callback)

Asynchronous fchown(2). No arguments other than a possible exception are given
to the completion callback.

fs.fchownSync(fd, uid, gid)

Synchronous fchown(2). Returns undefined.

fs.lchown(path, uid, gid, callback)

Asynchronous lchown(2). No arguments other than a possible exception are given
to the completion callback.

fs.lchownSync(path, uid, gid)

Synchronous lchown(2). Returns undefined.

fs.chmod(path, mode, callback)

Asynchronous chmod(2). No arguments other than a possible exception are given
to the completion callback.

fs.chmodSync(path, mode)

Synchronous chmod(2). Returns undefined.

fs.fchmod(fd, mode, callback)

Asynchronous fchmod(2). No arguments other than a possible exception
are given to the completion callback.

fs.fchmodSync(fd, mode)

Synchronous fchmod(2). Returns undefined.

fs.lchmod(path, mode, callback)

Asynchronous lchmod(2). No arguments other than a possible exception
are given to the completion callback.

Only available on Mac OS X.

fs.lchmodSync(path, mode)

Synchronous lchmod(2). Returns undefined.

fs.stat(path, callback)

Asynchronous stat(2). The callback gets two arguments (err, stats) where
stats is a fs.Stats object. See the fs.Stats
section below for more information.

fs.lstat(path, callback)

Asynchronous lstat(2). The callback gets two arguments (err, stats) where
stats is a fs.Stats object. lstat() is identical to stat(), except that if
path is a symbolic link, then the link itself is stat-ed, not the file that it
refers to.

fs.fstat(fd, callback)

Asynchronous fstat(2). The callback gets two arguments (err, stats) where
stats is a fs.Stats object. fstat() is identical to stat(), except that
the file to be stat-ed is specified by the file descriptor fd.

fs.statSync(path)

Synchronous stat(2). Returns an instance of fs.Stats.

fs.lstatSync(path)

Synchronous lstat(2). Returns an instance of fs.Stats.

fs.fstatSync(fd)

Synchronous fstat(2). Returns an instance of fs.Stats.

fs.link(srcpath, dstpath, callback)

Asynchronous link(2). No arguments other than a possible exception are given to
the completion callback.

fs.linkSync(srcpath, dstpath)

Synchronous link(2). Returns undefined.

fs.symlink(srcpath, dstpath[, type], callback)

Asynchronous symlink(2). No arguments other than a possible exception are given
to the completion callback.
The type argument can be set to 'dir', 'file', or 'junction' (default
is 'file') and is only available on Windows (ignored on other platforms).
Note that Windows junction points require the destination path to be absolute. When using
'junction', the destination argument will automatically be normalized to absolute path.

fs.symlinkSync(srcpath, dstpath[, type])

Synchronous symlink(2). Returns undefined.

fs.readlink(path, callback)

Asynchronous readlink(2). The callback gets two arguments (err, linkString).

fs.readlinkSync(path)

Synchronous readlink(2). Returns the symbolic link’s string value.

fs.realpath(path[, cache], callback)

Asynchronous realpath(2). The callback gets two arguments (err, resolvedPath). May use process.cwd to resolve relative paths. cache is an
object literal of mapped paths that can be used to force a specific path
resolution or avoid additional fs.stat calls for known real paths.

Example:

var cache = {'/etc':'/private/etc'};
fs.realpath('/etc/passwd', cache, function (err, resolvedPath) {
  if (err) throw err;
  console.log(resolvedPath);
});

fs.realpathSync(path[, cache])

Synchronous realpath(2). Returns the resolved path.

fs.unlink(path, callback)

Asynchronous unlink(2). No arguments other than a possible exception are given
to the completion callback.

fs.unlinkSync(path)

Synchronous unlink(2). Returns undefined.

fs.rmdir(path, callback)

Asynchronous rmdir(2). No arguments other than a possible exception are given
to the completion callback.

fs.rmdirSync(path)

Synchronous rmdir(2). Returns undefined.

fs.mkdir(path[, mode], callback)

Asynchronous mkdir(2). No arguments other than a possible exception are given
to the completion callback. mode defaults to 0777.

fs.mkdirSync(path[, mode])

Synchronous mkdir(2). Returns undefined.

fs.readdir(path, callback)

Asynchronous readdir(3). Reads the contents of a directory.
The callback gets two arguments (err, files) where files is an array of
the names of the files in the directory excluding '.' and '..'.

fs.readdirSync(path)

Synchronous readdir(3). Returns an array of filenames excluding '.' and
'..'.

fs.close(fd, callback)

Asynchronous close(2). No arguments other than a possible exception are given
to the completion callback.

fs.closeSync(fd)

Synchronous close(2). Returns undefined.

fs.open(path, flags[, mode], callback)

Asynchronous file open. See open(2). flags can be:

• 'r' - Open file for reading.
  An exception occurs if the file does not exist.

• 'r+' - Open file for reading and writing.
  An exception occurs if the file does not exist.

• 'rs' - Open file for reading in synchronous mode. Instructs the operating
  system to bypass the local file system cache.

  This is primarily useful for opening files on NFS mounts as it allows you to
  skip the potentially stale local cache. It has a very real impact on I/O
  performance so don’t use this flag unless you need it.

  Note that this doesn’t turn fs.open() into a synchronous blocking call.
  If that’s what you want then you should be using fs.openSync()

• 'rs+' - Open file for reading and writing, telling the OS to open it
  synchronously. See notes for 'rs' about using this with caution.

• 'w' - Open file for writing.
  The file is created (if it does not exist) or truncated (if it exists).

• 'wx' - Like 'w' but fails if path exists.

• 'w+' - Open file for reading and writing.
  The file is created (if it does not exist) or truncated (if it exists).

• 'wx+' - Like 'w+' but fails if path exists.

• 'a' - Open file for appending.
  The file is created if it does not exist.

• 'ax' - Like 'a' but fails if path exists.

• 'a+' - Open file for reading and appending.
  The file is created if it does not exist.

• 'ax+' - Like 'a+' but fails if path exists.

mode sets the file mode (permission and sticky bits), but only if the file was
created. It defaults to 0666, readable and writeable.

The callback gets two arguments (err, fd).

The exclusive flag 'x' (O_EXCL flag in open(2)) ensures that path is newly
created. On POSIX systems, path is considered to exist even if it is a symlink
to a non-existent file. The exclusive flag may or may not work with network file
systems.

On Linux, positional writes don’t work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.

fs.openSync(path, flags[, mode])

Synchronous version of fs.open(). Returns an integer representing the file
descriptor.

fs.utimes(path, atime, mtime, callback)

Change file timestamps of the file referenced by the supplied path.

fs.utimesSync(path, atime, mtime)

Synchronous version of fs.utimes(). Returns undefined.

fs.futimes(fd, atime, mtime, callback)

Change the file timestamps of a file referenced by the supplied file
descriptor.

fs.futimesSync(fd, atime, mtime)

Synchronous version of fs.futimes(). Returns undefined.

fs.fsync(fd, callback)

Asynchronous fsync(2). No arguments other than a possible exception are given
to the completion callback.

fs.fsyncSync(fd)

Synchronous fsync(2). Returns undefined.

fs.write(fd, buffer, offset, length[, position], callback)

Write buffer to the file specified by fd.

offset and length determine the part of the buffer to be written.

position refers to the offset from the beginning of the file where this data
should be written. If typeof position !== 'number', the data will be written
at the current position. See pwrite(2).

The callback will be given three arguments (err, written, buffer) where
written specifies how many bytes were written from buffer.

Note that it is unsafe to use fs.write multiple times on the same file
without waiting for the callback. For this scenario,
fs.createWriteStream is strongly recommended.

On Linux, positional writes don’t work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.

fs.write(fd, data[, position[, encoding]], callback)

Write data to the file specified by fd. If data is not a Buffer instance
then the value will be coerced to a string.

position refers to the offset from the beginning of the file where this data
should be written. If typeof position !== 'number' the data will be written at
the current position. See pwrite(2).

encoding is the expected string encoding.

The callback will receive the arguments (err, written, string) where written
specifies how many bytes the passed string required to be written. Note that
bytes written is not the same as string characters. See
Buffer.byteLength.

Unlike when writing buffer, the entire string must be written. No substring
may be specified. This is because the byte offset of the resulting data may not
be the same as the string offset.

Note that it is unsafe to use fs.write multiple times on the same file
without waiting for the callback. For this scenario,
fs.createWriteStream is strongly recommended.

On Linux, positional writes don’t work when the file is opened in append mode.
The kernel ignores the position argument and always appends the data to
the end of the file.

fs.writeSync(fd, buffer, offset, length[, position])

fs.writeSync(fd, data[, position[, encoding]])

Synchronous versions of fs.write(). Returns the number of bytes written.

fs.read(fd, buffer, offset, length, position, callback)

Read data from the file specified by fd.

buffer is the buffer that the data will be written to.

offset is the offset in the buffer to start writing at.

length is an integer specifying the number of bytes to read.

position is an integer specifying where to begin reading from in the file.
If position is null, data will be read from the current file position.

The callback is given the three arguments, (err, bytesRead, buffer).

fs.readSync(fd, buffer, offset, length, position)

Synchronous version of fs.read. Returns the number of bytesRead.

fs.readFile(filename[, options], callback)

• filename {String}
• options {Object}
  • encoding {String | Null} default = null
  • flag {String} default = 'r'
• callback {Function}

Asynchronously reads the entire contents of a file. Example:

fs.readFile('/etc/passwd', function (err, data) {
  if (err) throw err;
  console.log(data);
});

The callback is passed two arguments (err, data), where data is the
contents of the file.

If no encoding is specified, then the raw buffer is returned.

fs.readFileSync(filename[, options])

Synchronous version of fs.readFile. Returns the contents of the filename.

If the encoding option is specified then this function returns a
string. Otherwise it returns a buffer.

fs.writeFile(filename, data[, options], callback)

• filename {String}
• data {String | Buffer}
• options {Object}
  • encoding {String | Null} default = 'utf8'
  • mode {Number} default = 438 (aka 0666 in Octal)
  • flag {String} default = 'w'
• callback {Function}

Asynchronously writes data to a file, replacing the file if it already exists.
data can be a string or a buffer.

The encoding option is ignored if data is a buffer. It defaults
to 'utf8'.

Example:

fs.writeFile('message.txt', 'Hello Node', function (err) {
  if (err) throw err;
  console.log('It\'s saved!');
});

fs.writeFileSync(filename, data[, options])

The synchronous version of fs.writeFile. Returns undefined.

fs.appendFile(filename, data[, options], callback)

• filename {String}
• data {String | Buffer}
• options {Object}
  • encoding {String | Null} default = 'utf8'
  • mode {Number} default = 438 (aka 0666 in Octal)
  • flag {String} default = 'a'
• callback {Function}

Asynchronously append data to a file, creating the file if it not yet exists.
data can be a string or a buffer.

Example:

fs.appendFile('message.txt', 'data to append', function (err) {
  if (err) throw err;
  console.log('The "data to append" was appended to file!');
});

fs.appendFileSync(filename, data[, options])

The synchronous version of fs.appendFile. Returns undefined.

fs.watchFile(filename[, options], listener)

Stability: 2 - Unstable.  Use fs.watch instead, if possible.

Watch for changes on filename. The callback listener will be called each
time the file is accessed.

The second argument is optional. The options if provided should be an object
containing two members a boolean, persistent, and interval. persistent
indicates whether the process should continue to run as long as files are
being watched. interval indicates how often the target should be polled,
in milliseconds. The default is { persistent: true, interval: 5007 }.

The listener gets two arguments the current stat object and the previous
stat object:

fs.watchFile('message.text', function (curr, prev) {
  console.log('the current mtime is: ' + curr.mtime);
  console.log('the previous mtime was: ' + prev.mtime);
});

These stat objects are instances of fs.Stat.

If you want to be notified when the file was modified, not just accessed
you need to compare curr.mtime and prev.mtime.

fs.unwatchFile(filename[, listener])

Stability: 2 - Unstable.  Use fs.watch instead, if possible.

Stop watching for changes on filename. If listener is specified, only that
particular listener is removed. Otherwise, all listeners are removed and you
have effectively stopped watching filename.

Calling fs.unwatchFile() with a filename that is not being watched is a
no-op, not an error.

fs.watch(filename[, options][, listener])

Stability: 2 - Unstable.

Watch for changes on filename, where filename is either a file or a
directory. The returned object is a fs.FSWatcher.

The second argument is optional. The options if provided should be an object.
The supported boolean members are persistent and recursive. persistent
indicates whether the process should continue to run as long as files are being
watched. recursive indicates whether all subdirectories should be watched, or
only the current directory. This applies when a directory is specified, and only
on supported platforms (See Caveats below).

The default is { persistent: true, recursive: false }.

The listener callback gets two arguments (event, filename). event is either
‘rename’ or ‘change’, and filename is the name of the file which triggered
the event.

Caveats

The fs.watch API is not 100% consistent across platforms, and is
unavailable in some situations.

The recursive option is currently supported on OS X. Only FSEvents supports this
type of file watching so it is unlikely any additional platforms will be added
soon.

Availability

This feature depends on the underlying operating system providing a way
to be notified of filesystem changes.

• On Linux systems, this uses inotify.
• On BSD systems, this uses kqueue.
• On OS X, this uses kqueue for files and ‘FSEvents’ for directories.
• On SunOS systems (including Solaris and SmartOS), this uses event ports.
• On Windows systems, this feature depends on ReadDirectoryChangesW.

If the underlying functionality is not available for some reason, then
fs.watch will not be able to function. For example, watching files or
directories on network file systems (NFS, SMB, etc.) often doesn’t work
reliably or at all.

You can still use fs.watchFile, which uses stat polling, but it is slower and
less reliable.

Filename Argument

Providing filename argument in the callback is not supported
on every platform (currently it’s only supported on Linux and Windows). Even
on supported platforms filename is not always guaranteed to be provided.
Therefore, don’t assume that filename argument is always provided in the
callback, and have some fallback logic if it is null.

fs.watch('somedir', function (event, filename) {
  console.log('event is: ' + event);
  if (filename) {
    console.log('filename provided: ' + filename);
  } else {
    console.log('filename not provided');
  }
});

fs.exists(path, callback)

Test whether or not the given path exists by checking with the file system.
Then call the callback argument with either true or false. Example:

fs.exists('/etc/passwd', function (exists) {
  util.debug(exists ? "it's there" : "no passwd!");
});

fs.exists() is an anachronism and exists only for historical reasons.
There should almost never be a reason to use it in your own code.

In particular, checking if a file exists before opening it is an anti-pattern
that leaves you vulnerable to race conditions: another process may remove the
file between the calls to fs.exists() and fs.open(). Just open the file
and handle the error when it’s not there.

fs.exists() will be deprecated.

fs.existsSync(path)

Synchronous version of fs.exists(). Returns true if the file exists,
false otherwise.

fs.existsSync() will be deprecated.

fs.access(path[, mode], callback)

Tests a user’s permissions for the file specified by path. mode is an
optional integer that specifies the accessibility checks to be performed. The
following constants define the possible values of mode. It is possible to
create a mask consisting of the bitwise OR of two or more values.

• fs.F_OK - File is visible to the calling process. This is useful for
  determining if a file exists, but says nothing about rwx permissions.
  Default if no mode is specified.
• fs.R_OK - File can be read by the calling process.
• fs.W_OK - File can be written by the calling process.
• fs.X_OK - File can be executed by the calling process. This has no effect
  on Windows (will behave like fs.F_OK).

The final argument, callback, is a callback function that is invoked with
a possible error argument. If any of the accessibility checks fail, the error
argument will be populated. The following example checks if the file
/etc/passwd can be read and written by the current process.

fs.access('/etc/passwd', fs.R_OK | fs.W_OK, function(err) {
  util.debug(err ? 'no access!' : 'can read/write');
});

fs.accessSync(path[, mode])

Synchronous version of fs.access. This throws if any accessibility checks
fail, and does nothing otherwise.

Class: fs.Stats

Objects returned from fs.stat(), fs.lstat() and fs.fstat() and their
synchronous counterparts are of this type.

• stats.isFile()
• stats.isDirectory()
• stats.isBlockDevice()
• stats.isCharacterDevice()
• stats.isSymbolicLink() (only valid with fs.lstat())
• stats.isFIFO()
• stats.isSocket()

For a regular file util.inspect(stats) would return a string very
similar to this:

{ dev: 2114,
  ino: 48064969,
  mode: 33188,
  nlink: 1,
  uid: 85,
  gid: 100,
  rdev: 0,
  size: 527,
  blksize: 4096,
  blocks: 8,
  atime: Mon, 10 Oct 2011 23:24:11 GMT,
  mtime: Mon, 10 Oct 2011 23:24:11 GMT,
  ctime: Mon, 10 Oct 2011 23:24:11 GMT,
  birthtime: Mon, 10 Oct 2011 23:24:11 GMT }

Please note that atime, mtime, birthtime, and ctime are
instances of Date object and to compare the values of
these objects you should use appropriate methods. For most general
uses getTime() will return the number of
milliseconds elapsed since 1 January 1970 00:00:00 UTC and this
integer should be sufficient for any comparison, however there are
additional methods which can be used for displaying fuzzy information.
More details can be found in the MDN JavaScript Reference
page.

Stat Time Values

The times in the stat object have the following semantics:

• atime “Access Time” - Time when file data last accessed. Changed
  by the mknod(2), utimes(2), and read(2) system calls.
• mtime “Modified Time” - Time when file data last modified.
  Changed by the mknod(2), utimes(2), and write(2) system calls.
• ctime “Change Time” - Time when file status was last changed
  (inode data modification). Changed by the chmod(2), chown(2),
  link(2), mknod(2), rename(2), unlink(2), utimes(2),
  read(2), and write(2) system calls.
• birthtime “Birth Time” - Time of file creation. Set once when the
  file is created. On filesystems where birthtime is not available,
  this field may instead hold either the ctime or
  1970-01-01T00:00Z (ie, unix epoch timestamp 0). On Darwin and
  other FreeBSD variants, also set if the atime is explicitly set to
  an earlier value than the current birthtime using the utimes(2)
  system call.

Prior to Node v0.12, the ctime held the birthtime on Windows
systems. Note that as of v0.12, ctime is not “creation time”, and
on Unix systems, it never was.

fs.createReadStream(path[, options])

Returns a new ReadStream object (See Readable Stream).

options is an object with the following defaults:

{ flags: 'r',
  encoding: null,
  fd: null,
  mode: 0666,
  autoClose: true
}

options can include start and end values to read a range of bytes from
the file instead of the entire file. Both start and end are inclusive and
start at 0. The encoding can be 'utf8', 'ascii', or 'base64'.

If fd is specified, ReadStream will ignore the path argument and will use
the specified file descriptor. This means that no open event will be emitted.

If autoClose is false, then the file descriptor won’t be closed, even if
there’s an error. It is your responsibility to close it and make sure
there’s no file descriptor leak. If autoClose is set to true (default
behavior), on error or end the file descriptor will be closed
automatically.

mode sets the file mode (permission and sticky bits), but only if the
file was created.

An example to read the last 10 bytes of a file which is 100 bytes long:

fs.createReadStream('sample.txt', {start: 90, end: 99});

Class: fs.ReadStream

ReadStream is a Readable Stream.

Event: ‘open’

• fd {Integer} file descriptor used by the ReadStream.

Emitted when the ReadStream’s file is opened.

fs.createWriteStream(path[, options])

Returns a new WriteStream object (See Writable Stream).

options is an object with the following defaults:

{ flags: 'w',
  defaultEncoding: 'utf8',
  fd: null,
  mode: 0666 }

options may also include a start option to allow writing data at
some position past the beginning of the file. Modifying a file rather
than replacing it may require a flags mode of r+ rather than the
default mode w.

Like ReadStream above, if fd is specified, WriteStream will ignore the
path argument and will use the specified file descriptor. This means that no
open event will be emitted.

Class: fs.WriteStream

WriteStream is a Writable Stream.

Event: ‘open’

• fd {Integer} file descriptor used by the WriteStream.

Emitted when the WriteStream’s file is opened.

file.bytesWritten

The number of bytes written so far. Does not include data that is still queued
for writing.

Class: fs.FSWatcher

Objects returned from fs.watch() are of this type.

watcher.close()

Stop watching for changes on the given fs.FSWatcher.

Event: ‘change’

• event {String} The type of fs change
• filename {String} The filename that changed (if relevant/available)

Emitted when something changes in a watched directory or file.
See more details in fs.watch.

Event: ‘error’

• error {Error object}

Emitted when an error occurs.