File System

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 takes 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.

The relative path to a 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 RUFF_DEBUG environment variable:

$ cat script.js
function bad() {
  require('fs').readFile('/');
}
bad();

$ env RUFF_DEBUG=fs node script.js
fs.js:66
        throw err;
              ^
Error: EISDIR, read
    rethrow (fs.js:61)
    maybeCallback (fs.js:79)
    Object.fs.readFile (fs.js:153)
    bad (/path/to/script.js:2)
    Object.<anonymous> (/path/to/script.js:5)
    <etc.>

fs.rename(oldPath, newPath, callback)

Ruff available: v1.6.0

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

fs.renameSync(oldPath, newPath)

Ruff available: v1.6.0

Synchronous rename(2). Returns undefined.

fs.ftruncate(fd, len, callback)

Ruff available: v1.6.0

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

fs.ftruncateSync(fd, len)

Ruff available: v1.6.0

Synchronous ftruncate(2). Returns undefined.

fs.truncate(path, len, callback)

Ruff available: v1.6.0

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)

Ruff available: v1.6.0

Synchronous truncate(2). Returns undefined.

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

Ruff available: v1.6.0

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

fs.chownSync(path, uid, gid)

Ruff available: v1.6.0

Synchronous chown(2). Returns undefined.

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

Ruff available: v1.6.0

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

fs.fchownSync(fd, uid, gid)

Ruff available: v1.6.0

Synchronous fchown(2). Returns undefined.

fs.chmod(path, mode, callback)

Ruff available: v1.6.0

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

fs.chmodSync(path, mode)

Ruff available: v1.6.0

Synchronous chmod(2). Returns undefined.

fs.fchmod(fd, mode, callback)

Ruff available: v1.6.0

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

fs.fchmodSync(fd, mode)

Ruff available: v1.6.0

Synchronous fchmod(2). Returns undefined.

fs.stat(path, callback)

Ruff available: v1.6.0

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)

Ruff available: v1.6.0

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)

Ruff available: v1.6.0

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)

Ruff available: v1.6.0

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

fs.lstatSync(path)

Ruff available: v1.6.0

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

fs.fstatSync(fd)

Ruff available: v1.6.0

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

fs.link(srcpath, dstpath, callback)

Ruff available: v1.6.0

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

fs.linkSync(srcpath, dstpath)

Ruff available: v1.6.0

Synchronous link(2). Returns undefined.

fs.symlink(destination, path[, type], callback)

Ruff available: v1.6.0

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(destination, path[, type])

Ruff available: v1.6.0

Synchronous symlink(2). Returns undefined.

fs.readlink(path, callback)

Ruff available: v1.6.0

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

fs.readlinkSync(path)

Ruff available: v1.6.0

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

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

Ruff available: v1.6.0

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])

Ruff available: v1.6.0

Synchronous realpath(2). Returns the resolved path.

fs.unlink(path, callback)

Ruff available: v1.6.0

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

fs.unlinkSync(path)

Ruff available: v1.6.0

Synchronous unlink(2). Returns undefined.

fs.rmdir(path, callback)

Ruff available: v1.6.0

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

fs.rmdirSync(path)

Ruff available: v1.6.0

Synchronous rmdir(2). Returns undefined.

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

Ruff available: v1.6.0

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

fs.mkdirSync(path[, mode])

Ruff available: v1.6.0

Synchronous mkdir(2). Returns undefined.

fs.close(fd, callback)

Ruff available: v1.6.0

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

fs.closeSync(fd)

Ruff available: v1.6.0

Synchronous close(2). Returns undefined.

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

Ruff available: v1.6.0

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])

Ruff available: v1.6.0

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

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

Ruff available: v1.6.0

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

Note: the arguments atime and mtime of the following related functions does
follow the below rules:

  • If the value is a numberable string like “123456789”, the value would get
    converted to corresponding number.
  • If the value is NaN or Infinity, the value would get converted to
    Date.now().

fs.utimesSync(path, atime, mtime)

Ruff available: v1.6.0

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

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

Ruff available: v1.6.0

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

fs.futimesSync(fd, atime, mtime)

Ruff available: v1.6.0

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

fs.fsync(fd, callback)

Ruff available: v1.6.0

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

fs.fsyncSync(fd)

Ruff available: v1.6.0

Synchronous fsync(2). Returns undefined.

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

Ruff available: v1.6.0

Write buffer to the file specified by fd.

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 0. 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], callback)

Ruff available: v1.6.0

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

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 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])

Ruff available: v1.6.0

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

Ruff available: v1.6.0

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

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

Ruff available: v1.6.0

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, position)

Ruff available: v1.6.0

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

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

Ruff available: v1.6.0
Ruff Lite available: v0.7.1

  • filename {String}
  • options {Object | String}
    • 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.

If options is a string, then it specifies the encoding. Example:

fs.readFile('/etc/passwd', 'utf8', callback);

fs.readFileSync(filename[, options])

Ruff available: v1.6.0
Ruff Lite available: v0.7.1

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)

Ruff available: v1.6.0

  • filename {String}
  • data {String | Buffer}
  • options {Object | String}
    • encoding {String | Null} default = 'utf8'
    • mode {Number} default = 0666
    • 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.js', function (err) {
  if (err) throw err;
  console.log('It\'s saved!');
});

If options is a string, then it specifies the encoding. Example:

fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback);

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

Ruff available: v1.6.0

The synchronous version of fs.writeFile. Returns undefined.

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

Ruff available: v1.6.0

  • filename {String}
  • data {String | Buffer}
  • options {Object | String}
    • encoding {String | Null} default = 'utf8'
    • mode {Number} default = 0666
    • flag {String} default = 'a'
  • callback {Function}

Asynchronously append data to a file, creating the file if it does not yet exist.
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!');
});

If options is a string, then it specifies the encoding. Example:

fs.appendFile('message.txt', 'data to append', 'utf8', callback);

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

Ruff available: v1.6.0

The synchronous version of fs.appendFile. Returns undefined.

fs.exists(path, callback)

Ruff available: v1.6.0

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) {
  console.log(exists ? "it's there" : 'no passwd!');
});

fs.exists() should not be used to check if a file exists before calling
fs.open(). Doing so introduces a race condition since other processes may
change the file’s state between the two calls. Instead, user code should
call fs.open() directly and handle the error raised if the file is
non-existent.

fs.existsSync(path)

Ruff available: v1.6.0

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

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

Ruff available: v1.6.0

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.

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', 'rw', function (err) {
  console.log(err ? 'no access!' : 'can read/write');
});

fs.accessSync(path[, mode])

Ruff available: v1.6.0

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:

{ mode: 33188,
  uid: 501,
  gid: 20,
  size: 0,
  atime: 2015-11-24 13:56:20.000+08:00,
  mtime: 2015-11-24 11:48:37.000+08:00,
  ctime: 2015-11-24 11:48:37.000+08:00,
  type: 'file' }

Please note that atime, mtime, 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.

fs.createReadStream(path[, options])

Ruff available: v1.6.0

Returns a new ReadStream object (See Readable Stream).

Be aware that, unlike the default value set for highWaterMark on a
readable stream (16 kb), the stream returned by this method has a
default value of 64 kb for the same parameter.

options is an object or string 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});

If options is a string, then it specifies the encoding.

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])

Ruff available: v1.6.0

Returns a new WriteStream object (See Writable Stream).

options is an object or string 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. The defaultEncoding can be 'utf8', 'ascii', binary,
or 'base64'.

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.

If options is a string, then it specifies the encoding.

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.