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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).

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).

fs.truncate(path, len, callback)

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

fs.truncateSync(path, len)

Synchronous truncate(2).

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).

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).

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).

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).

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).

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).

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).

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).

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).

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).

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).

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).

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().

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

fs.utimesSync(path, atime, mtime)

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

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

fs.futimesSync(fd, atime, mtime)

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

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).

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.

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.

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.existsSync(path)

Synchronous version of fs.exists.

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 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 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.

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',
  encoding: 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.

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.

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