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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().

fs.rename(path1, path2, [callback])

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

fs.renameSync(path1, path2)

Synchronous rename(2).

fs.truncate(fd, len, [callback])

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

fs.truncateSync(fd, len)

Synchronous ftruncate(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.

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(linkdata, path, [type], [callback])

Asynchronous symlink(2). No arguments other than a possible exception are given to the completion callback. type argument can be either 'dir' or 'file' (default is 'file'). It is only used on Windows (ignored on other platforms).

fs.symlinkSync(linkdata, path, [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, [callback])

Asynchronous realpath(2). The callback gets two arguments (err, resolvedPath). May use process.cwd to resolve relative paths.

fs.realpathSync(path)

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.

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

  • 'wx' - Like 'w' but opens the file in exclusive mode.

  • '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 opens the file in exclusive mode.

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

  • 'ax' - Like 'a' but opens the file in exclusive mode.

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

  • 'ax+' - Like 'a+' but opens the file in exclusive mode.

mode defaults to 0666. The callback gets two arguments (err, fd).

Exclusive mode (O_EXCL) ensures that path is newly created. fs.open() fails if a file by that name already exists. On POSIX systems, symlinks are not followed. Exclusive mode may or may not work with network file systems.

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

Synchronous open(2).

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 position is null, 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.

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

Synchronous version of buffer-based fs.write(). Returns the number of bytes written.

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

Synchronous version of string-based fs.write(). encoding defaults to 'utf8'. 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 offset within the buffer where writing will start.

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 buffer-based fs.read. Returns the number of bytesRead.

fs.readSync(fd, length, position, encoding)

Synchronous version of string-based fs.read. Returns the number of bytesRead.

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

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, [encoding])

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

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

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

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string or a buffer. The encoding argument 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, [encoding])

The synchronous version of fs.writeFile.

fs.appendFile(filename, data, encoding='utf8', [callback])

Asynchronously append data to a file, creating the file if it not yet exists. data can be a string or a buffer. The encoding argument is ignored if data is 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, encoding='utf8')

The synchronous version of fs.appendFile.

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

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

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. (On Linux systems with inotify, interval is ignored.) The default is { persistent: true, interval: 0 }.

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)

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

Stop watching for changes on filename.

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

Stability: 2 - Unstable.  Not available on all platforms.

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 containing a boolean member persistent, which indicates whether the process should continue to run as long as files are being watched. The default is { persistent: true }.

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.

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 (including OS X), this uses kqueue.
  • 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. 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(p, [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.existsSync(p)

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 }

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 additional methods which can be used for displaying fuzzy information. More details can be found in the MDN JavaScript Reference page.

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,
  bufferSize: 64 * 1024
}

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.

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.

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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