- Home
- Download
- About
- npm Registry
- Docs
- Blog
- Community
- Logos
- Jobs

@nodejs

# 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 ignores errors, but prints a deprecation warning.

IMPORTANT: Omitting the callback is deprecated. v0.12 will throw the errors as exceptions.

## 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 `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.

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)#

Synchronous version 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 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. 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 }

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,
      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 responsiblity 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.