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:
const fs = require('fs'); fs.unlink('/tmp/hello', (err) => { if (err) throw err; console.log('successfully deleted /tmp/hello'); });
Here is the synchronous version:
const 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', (err) => { if (err) throw err; console.log('renamed complete'); }); fs.stat('/tmp/world', (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', (err) => { if (err) throw err; fs.stat('/tmp/world', (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 NODE_DEBUG
environment variable:
$ cat script.js function bad() { require('fs').readFile('/'); } bad(); $ env NODE_DEBUG=fs node script.js fs.js:88 throw backtrace; ^ Error: EISDIR: illegal operation on a directory, read <stack trace.>
Buffer API
fs
functions support passing and receiving paths as both strings and Buffers. The latter is intended to make it possible to work with filesystems that allow for non-UTF-8 filenames. For most typical uses, working with paths as Buffers will be unnecessary, as the string API converts to and from UTF-8 automatically.
Note that on certain file systems (such as NTFS and HFS+) filenames will always be encoded as UTF-8. On such file systems, passing non-UTF-8 encoded Buffers to fs
functions will not work as expected.
Class: fs.FSWatcher
Objects returned from fs.watch()
are of this type.
The listener
callback provided to fs.watch()
receives the returned FSWatcher's change
events.
The object itself emits these events:
Event: 'change'
-
eventType
<string> The type of fs change -
filename
<string> | <Buffer> The filename that changed (if relevant/available)
Emitted when something changes in a watched directory or file. See more details in fs.watch()
.
The filename
argument may not be provided depending on operating system support. If filename
is provided, it will be provided as a Buffer
if fs.watch()
is called with its encoding
option set to 'buffer'
, otherwise filename
will be a string.
// Example when handled through fs.watch listener fs.watch('./tmp', {encoding: 'buffer'}, (eventType, filename) => { if (filename) console.log(filename); // Prints: <Buffer ...> });
Event: 'error'
-
error
<Error>
Emitted when an error occurs.
watcher.close()
Stop watching for changes on the given fs.FSWatcher
.
Class: fs.ReadStream
ReadStream
is a Readable Stream.
Event: 'open'
-
fd
<Integer> Integer file descriptor used by the ReadStream.
Emitted when the ReadStream's file is opened.
Event: 'close'
-
fd
<integer> Integer file descriptor used by the ReadStream.
Emitted when the ReadStream
's underlying file descriptor has been closed.
readStream.bytesRead
The number of bytes read so far.
readStream.path
The path to the file the stream is reading from as specified in the first argument to fs.createReadStream()
. If path
is passed as a string, then readStream.path
will be a string. If path
is passed as a Buffer
, then readStream.path
will be a Buffer
.
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 withfs.lstat()
) stats.isFIFO()
stats.isSocket()
For a regular file util.inspect(stats)
would return a string very similar to this:
Stats { 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 thectime
or1970-01-01T00:00Z
(ie, unix epoch timestamp0
). Note that this value may be greater thanatime
ormtime
in this case. On Darwin and other FreeBSD variants, also set if theatime
is explicitly set to an earlier value than the currentbirthtime
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.
Class: fs.WriteStream
WriteStream
is a Writable Stream.
Event: 'open'
-
fd
<Integer> Integer file descriptor used by the WriteStream.
Emitted when the WriteStream's file is opened.
Event: 'close'
-
fd
<integer> Integer file descriptor used by the WriteStream.
Emitted when the WriteStream
's underlying file descriptor has been closed.
writeStream.bytesWritten
The number of bytes written so far. Does not include data that is still queued for writing.
writeStream.path
The path to the file the stream is writing to as specified in the first argument to fs.createWriteStream()
. If path
is passed as a string, then writeStream.path
will be a string. If path
is passed as a Buffer
, then writeStream.path
will be a Buffer
.
fs.access(path[, mode], callback)
-
path
<string> | <Buffer> -
mode
<integer> -
callback
<Function>-
err
<Error>
-
Tests a user's permissions for the file or directory specified by path
. The mode
argument 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.constants.F_OK
-path
is visible to the calling process. This is useful for determining if a file exists, but says nothing aboutrwx
permissions. Default if nomode
is specified. -
fs.constants.R_OK
-path
can be read by the calling process. -
fs.constants.W_OK
-path
can be written by the calling process. -
fs.constants.X_OK
-path
can be executed by the calling process. This has no effect on Windows (will behave likefs.constants.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.constants.R_OK | fs.constants.W_OK, (err) => { console.log(err ? 'no access!' : 'can read/write'); });
Using fs.access()
to check for the accessibility of a file before calling fs.open()
, fs.readFile()
or fs.writeFile()
is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file is not accessible.
For example:
write (NOT RECOMMENDED)
fs.access('myfile', (err) => { if (!err) { console.error('myfile already exists'); return; } fs.open('myfile', 'wx', (err, fd) => { if (err) throw err; writeMyData(fd); }); });
write (RECOMMENDED)
fs.open('myfile', 'wx', (err, fd) => { if (err) { if (err.code === 'EEXIST') { console.error('myfile already exists'); return; } throw err; } writeMyData(fd); });
read (NOT RECOMMENDED)
fs.access('myfile', (err) => { if (err) { if (err.code === 'ENOENT') { console.error('myfile does not exist'); return; } throw err; } fs.open('myfile', 'r', (err, fd) => { if (err) throw err; readMyData(fd); }); });
read (RECOMMENDED)
fs.open('myfile', 'r', (err, fd) => { if (err) { if (err.code === 'ENOENT') { console.error('myfile does not exist'); return; } throw err; } readMyData(fd); });
The "not recommended" examples above check for accessibility and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.
In general, check for the accessibility of a file only if the file won’t be used directly, for example when its accessibility is a signal from another process.
fs.accessSync(path[, mode])
Synchronous version of fs.access()
. This throws if any accessibility checks fail, and does nothing otherwise.
fs.appendFile(file, data[, options], callback)
-
file
<string> | <Buffer> | <number> filename or file descriptor -
data
<string> | <Buffer> -
options
<Object> | <string> -
callback
<Function>-
err
<Error>
-
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', (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);
Any specified file descriptor has to have been opened for appending.
Note: If a file descriptor is specified as the file
, it will not be closed automatically.
fs.appendFileSync(file, data[, options])
-
file
<string> | <Buffer> | <number> filename or file descriptor -
data
<string> | <Buffer> -
options
<Object> | <string>
The synchronous version of fs.appendFile()
. Returns undefined
.
fs.chmod(path, mode, callback)
-
path
<string> | <Buffer> -
mode
<integer> -
callback
<Function>-
err
<Error>
-
Asynchronously changes the permissions of a file. No arguments other than a possible exception are given to the completion callback.
See also: chmod(2)
fs.chmodSync(path, mode)
Synchronously changes the permissions of a file. Returns undefined
. This is the synchronous version of fs.chmod()
.
See also: chmod(2)
fs.chown(path, uid, gid, callback)
Asynchronously changes owner and group of a file. No arguments other than a possible exception are given to the completion callback.
See also: chown(2)
fs.chownSync(path, uid, gid)
Synchronously changes owner and group of a file. Returns undefined
. This is the synchronous version of fs.chown()
.
See also: chown(2)
fs.close(fd, callback)
-
fd
<integer> -
callback
<Function>-
err
<Error>
-
Asynchronous close(2). No arguments other than a possible exception are given to the completion callback.
fs.closeSync(fd)
-
fd
<integer>
Synchronous close(2). Returns undefined
.
fs.constants
Returns an object containing commonly used constants for file system operations. The specific constants currently defined are described in FS Constants.
fs.createReadStream(path[, options])
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:
const defaults = { flags: 'r', encoding: null, fd: null, mode: 0o666, autoClose: true, highWaterMark: 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 counting at 0. If fd
is specified and start
is omitted or undefined
, fs.createReadStream()
reads sequentially from the current file position. The encoding
can be any one of those accepted by Buffer
.
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. Note that fd
should be blocking; non-blocking fd
s should be passed to net.Socket
.
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.
fs.createWriteStream(path[, options])
Returns a new WriteStream
object. (See Writable Stream).
options
is an object or string with the following defaults:
const defaults = { flags: 'w', defaultEncoding: 'utf8', fd: null, mode: 0o666, autoClose: true };
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 any one of those accepted by Buffer
.
If autoClose
is set to true (default behavior) on error
or end
the file descriptor will be closed automatically. 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.
Like ReadStream
, 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. Note that fd
should be blocking; non-blocking fd
s should be passed to net.Socket
.
If options
is a string, then it specifies the encoding.
fs.exists(path, callback)
-
path
<string> | <Buffer> -
callback
<Function>-
exists
<Boolean>
-
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', (exists) => { console.log(exists ? 'it\'s there' : 'no passwd!'); });
Note that the parameter to this callback is not consistent with other Node.js callbacks. Normally, the first parameter to a Node.js callback is an err
parameter, optionally followed by other parameters. The fs.exists()
callback has only one boolean parameter. This is one reason fs.access()
is recommended instead of fs.exists()
.
Using fs.exists()
to check for the existence of a file before calling fs.open()
, fs.readFile()
or fs.writeFile()
is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file does not exist.
For example:
write (NOT RECOMMENDED)
fs.exists('myfile', (exists) => { if (exists) { console.error('myfile already exists'); } else { fs.open('myfile', 'wx', (err, fd) => { if (err) throw err; writeMyData(fd); }); } });
write (RECOMMENDED)
fs.open('myfile', 'wx', (err, fd) => { if (err) { if (err.code === 'EEXIST') { console.error('myfile already exists'); return; } throw err; } writeMyData(fd); });
read (NOT RECOMMENDED)
fs.exists('myfile', (exists) => { if (exists) { fs.open('myfile', 'r', (err, fd) => { readMyData(fd); }); } else { console.error('myfile does not exist'); } });
read (RECOMMENDED)
fs.open('myfile', 'r', (err, fd) => { if (err) { if (err.code === 'ENOENT') { console.error('myfile does not exist'); return; } throw err; } readMyData(fd); });
The "not recommended" examples above check for existence and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.
In general, check for the existence of a file only if the file won’t be used directly, for example when its existence is a signal from another process.
fs.existsSync(path)
Synchronous version of fs.exists()
. Returns true
if the path exists, false
otherwise.
Note that fs.exists()
is deprecated, but fs.existsSync()
is not. (The callback
parameter to fs.exists()
accepts parameters that are inconsistent with other Node.js callbacks. fs.existsSync()
does not use a callback.)
fs.fchmod(fd, mode, callback)
-
fd
<integer> -
mode
<integer> -
callback
<Function>-
err
<Error>
-
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.fchown(fd, uid, gid, callback)
-
fd
<integer> -
uid
<integer> -
gid
<integer> -
callback
<Function>-
err
<Error>
-
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.fdatasync(fd, callback)
-
fd
<integer> -
callback
<Function>-
err
<Error>
-
Asynchronous fdatasync(2). No arguments other than a possible exception are given to the completion callback.
fs.fdatasyncSync(fd)
-
fd
<integer>
Synchronous fdatasync(2). Returns undefined
.
fs.fstat(fd, callback)
-
fd
<integer> -
callback
<Function>-
err
<Error> -
stats
<fs.Stats>
-
Asynchronous fstat(2). The callback gets two arguments (err, stats)
where stats
is an fs.Stats
object. fstat()
is identical to stat()
, except that the file to be stat-ed is specified by the file descriptor fd
.
fs.fstatSync(fd)
-
fd
<integer>
Synchronous fstat(2). Returns an instance of fs.Stats
.
fs.fsync(fd, callback)
-
fd
<integer> -
callback
<Function>-
err
<Error>
-
Asynchronous fsync(2). No arguments other than a possible exception are given to the completion callback.
fs.fsyncSync(fd)
-
fd
<integer>
Synchronous fsync(2). Returns undefined
.
fs.ftruncate(fd, len, callback)
-
fd
<integer> -
len
<integer> default =0
-
callback
<Function>-
err
<Error>
-
Asynchronous ftruncate(2). No arguments other than a possible exception are given to the completion callback.
If the file referred to by the file descriptor was larger than len
bytes, only the first len
bytes will be retained in the file.
For example, the following program retains only the first four bytes of the file
console.log(fs.readFileSync('temp.txt', 'utf8')); // Prints: Node.js // get the file descriptor of the file to be truncated const fd = fs.openSync('temp.txt', 'r+'); // truncate the file to first four bytes fs.ftruncate(fd, 4, (err) => { assert.ifError(err); console.log(fs.readFileSync('temp.txt', 'utf8')); }); // Prints: Node
If the file previously was shorter than len
bytes, it is extended, and the extended part is filled with null bytes ('\0'). For example,
console.log(fs.readFileSync('temp.txt', 'utf8')); // Prints: Node.js // get the file descriptor of the file to be truncated const fd = fs.openSync('temp.txt', 'r+'); // truncate the file to 10 bytes, whereas the actual size is 7 bytes fs.ftruncate(fd, 10, (err) => { assert.ifError(err); console.log(fs.readFileSync('temp.txt')); }); // Prints: <Buffer 4e 6f 64 65 2e 6a 73 00 00 00> // ('Node.js\0\0\0' in UTF8)
The last three bytes are null bytes ('\0'), to compensate the over-truncation.
fs.ftruncateSync(fd, len)
Synchronous ftruncate(2). Returns undefined
.
fs.futimes(fd, atime, mtime, callback)
-
fd
<integer> -
atime
<integer> -
mtime
<integer> -
callback
<Function>-
err
<Error>
-
Change the file timestamps of a file referenced by the supplied file descriptor.
Note: This function does not work on AIX versions before 7.1, it will return the error UV_ENOSYS
.
fs.futimesSync(fd, atime, mtime)
Synchronous version of fs.futimes()
. Returns undefined
.
fs.lchmod(path, mode, callback)
-
path
<string> | <Buffer> -
mode
<integer> -
callback
<Function>-
err
<Error>
-
Asynchronous lchmod(2). No arguments other than a possible exception are given to the completion callback.
Only available on macOS.
fs.lchmodSync(path, mode)
Synchronous lchmod(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.link(existingPath, newPath, callback)
Asynchronous link(2). No arguments other than a possible exception are given to the completion callback.
fs.linkSync(existingPath, newPath)
Synchronous link(2). Returns undefined
.
fs.lstat(path, callback)
-
path
<string> | <Buffer> -
callback
<Function>-
err
<Error> -
stats
<fs.Stats>
-
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.lstatSync(path)
Synchronous lstat(2). Returns an instance of fs.Stats
.
fs.mkdir(path[, mode], callback)
-
path
<string> | <Buffer> -
mode
<integer> -
callback
<Function>-
err
<Error>
-
Asynchronously creates a directory. No arguments other than a possible exception are given to the completion callback. mode
defaults to 0o777
.
See also: mkdir(2)
fs.mkdirSync(path[, mode])
Synchronously creates a directory. Returns undefined
. This is the synchronous version of fs.mkdir()
.
See also: mkdir(2)
fs.mkdtemp(prefix[, options], callback)
-
prefix
<string> -
options
<string> | <Object>-
encoding
<string> default ='utf8'
-
-
callback
<Function>
Creates a unique temporary directory.
Generates six random characters to be appended behind a required prefix
to create a unique temporary directory.
The created folder path is passed as a string to the callback's second parameter.
The optional options
argument can be a string specifying an encoding, or an object with an encoding
property specifying the character encoding to use.
Example:
fs.mkdtemp(path.join(os.tmpdir(), 'foo-'), (err, folder) => { if (err) throw err; console.log(folder); // Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2 });
Note: The fs.mkdtemp()
method will append the six randomly selected characters directly to the prefix
string. For instance, given a directory /tmp
, if the intention is to create a temporary directory within /tmp
, the prefix
must end with a trailing platform-specific path separator (require('path').sep
).
// The parent directory for the new temporary directory const tmpDir = os.tmpdir(); // This method is *INCORRECT*: fs.mkdtemp(tmpDir, (err, folder) => { if (err) throw err; console.log(folder); // Will print something similar to `/tmpabc123`. // Note that a new temporary directory is created // at the file system root rather than *within* // the /tmp directory. }); // This method is *CORRECT*: const { sep } = require('path'); fs.mkdtemp(`${tmpDir}${sep}`, (err, folder) => { if (err) throw err; console.log(folder); // Will print something similar to `/tmp/abc123`. // A new temporary directory is created within // the /tmp directory. });
fs.mkdtempSync(prefix[, options])
The synchronous version of fs.mkdtemp()
. Returns the created folder path.
The optional options
argument can be a string specifying an encoding, or an object with an encoding
property specifying the character encoding to use.
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 and writing 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 usingfs.openSync()
-
'w'
- Open file for writing. The file is created (if it does not exist) or truncated (if it exists). -
'wx'
- Like'w'
but fails ifpath
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 ifpath
exists. -
'a'
- Open file for appending. The file is created if it does not exist. -
'ax'
- Like'a'
but fails ifpath
exists. -
'a+'
- Open file for reading and appending. The file is created if it does not exist. -
'ax+'
- Like'a+'
but fails ifpath
exists.
mode
sets the file mode (permission and sticky bits), but only if the file was created. It defaults to 0o666
(readable and writable).
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.
flags
can also be a number as documented by open(2); commonly used constants are available from fs.constants
. On Windows, flags are translated to their equivalent ones where applicable, e.g. O_WRONLY
to FILE_GENERIC_WRITE
, or O_EXCL|O_CREAT
to CREATE_NEW
, as accepted by CreateFileW.
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.
Note: The behavior of fs.open()
is platform-specific for some flags. As such, opening a directory on macOS and Linux with the 'a+'
flag - see example below - will return an error. In contrast, on Windows and FreeBSD, a file descriptor will be returned.
// macOS and Linux fs.open('<directory>', 'a+', (err, fd) => { // => [Error: EISDIR: illegal operation on a directory, open <directory>] }); // Windows and FreeBSD fs.open('<directory>', 'a+', (err, fd) => { // => null, <fd> });
Some characters (< > : " / \ | ? *
) are reserved under Windows as documented by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page.
Functions based on fs.open()
exhibit this behavior as well. eg. fs.writeFile()
, fs.readFile()
, etc.
fs.openSync(path, flags[, mode])
Synchronous version of fs.open()
. Returns an integer representing the file descriptor.
fs.read(fd, buffer, offset, length, position, callback)
-
fd
<integer> -
buffer
<string> | <Buffer> -
offset
<integer> -
length
<integer> -
position
<integer> -
callback
<Function>
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 argument specifying where to begin reading from in the file. If position
is null
, data will be read from the current file position, and the file position will be updated. If position
is an integer, the file position will remain unchanged.
The callback is given the three arguments, (err, bytesRead, buffer)
.
fs.readdir(path[, options], callback)
-
path
<string> | <Buffer> -
options
<string> | <Object>-
encoding
<string> default ='utf8'
-
-
callback
<Function>-
err
<Error> -
files
<string[]> | <Buffer[]>
-
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 '..'
.
The optional options
argument can be a string specifying an encoding, or an object with an encoding
property specifying the character encoding to use for the filenames passed to the callback. If the encoding
is set to 'buffer'
, the filenames returned will be passed as Buffer
objects.
fs.readdirSync(path[, options])
Synchronous readdir(3). Returns an array of filenames excluding '.'
and '..'
.
The optional options
argument can be a string specifying an encoding, or an object with an encoding
property specifying the character encoding to use for the filenames passed to the callback. If the encoding
is set to 'buffer'
, the filenames returned will be passed as Buffer
objects.
fs.readFile(file[, options], callback)
-
file
<string> | <Buffer> | <integer> filename or file descriptor -
options
<Object> | <string> -
callback
<Function>
Asynchronously reads the entire contents of a file. Example:
fs.readFile('/etc/passwd', (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);
Note: When the path is a directory, the behavior of fs.readFile()
and [fs.readFileSync()
][] is platform-specific. On macOS, Linux, and Windows, an error will be returned. On FreeBSD, a representation of the directory's contents will be returned.
// macOS, Linux and Windows fs.readFile('<directory>', (err, data) => { // => [Error: EISDIR: illegal operation on a directory, read <directory>] }); // FreeBSD fs.readFile('<directory>', (err, data) => { // => null, <data> });
Any specified file descriptor has to support reading.
Note: If a file descriptor is specified as the path
, it will not be closed automatically.
fs.readFileSync(file[, options])
Synchronous version of fs.readFile
. Returns the contents of the file
.
If the encoding
option is specified then this function returns a string. Otherwise it returns a buffer.
Note: Similar to [fs.readFile()
][], when the path is a directory, the behavior of fs.readFileSync()
is platform-specific.
// macOS, Linux and Windows fs.readFileSync('<directory>'); // => [Error: EISDIR: illegal operation on a directory, read <directory>] // FreeBSD fs.readFileSync('<directory>'); // => null, <data>
fs.readlink(path[, options], callback)
-
path
<string> | <Buffer> -
options
<string> | <Object>-
encoding
<string> default ='utf8'
-
-
callback
<Function>
Asynchronous readlink(2). The callback gets two arguments (err,
linkString)
.
The optional options
argument can be a string specifying an encoding, or an object with an encoding
property specifying the character encoding to use for the link path passed to the callback. If the encoding
is set to 'buffer'
, the link path returned will be passed as a Buffer
object.
fs.readlinkSync(path[, options])
Synchronous readlink(2). Returns the symbolic link's string value.
The optional options
argument can be a string specifying an encoding, or an object with an encoding
property specifying the character encoding to use for the link path passed to the callback. If the encoding
is set to 'buffer'
, the link path returned will be passed as a Buffer
object.
fs.readSync(fd, buffer, offset, length, position)
Synchronous version of fs.read()
. Returns the number of bytesRead
.
fs.realpath(path[, options], callback)
-
path
<string> | <Buffer> -
options
<string> | <Object>-
encoding
<string> default ='utf8'
-
-
callback
<Function>
Asynchronous realpath(3). The callback
gets two arguments (err,
resolvedPath)
. May use process.cwd
to resolve relative paths.
Only paths that can be converted to UTF8 strings are supported.
The optional options
argument can be a string specifying an encoding, or an object with an encoding
property specifying the character encoding to use for the path passed to the callback. If the encoding
is set to 'buffer'
, the path returned will be passed as a Buffer
object.
fs.realpathSync(path[, options])
Synchronous realpath(3). Returns the resolved path.
Only paths that can be converted to UTF8 strings are supported.
The optional options
argument can be a string specifying an encoding, or an object with an encoding
property specifying the character encoding to use for the returned value. If the encoding
is set to 'buffer'
, the path returned will be passed as a Buffer
object.
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.rmdir(path, callback)
-
path
<string> | <Buffer> -
callback
<Function>-
err
<Error>
-
Asynchronous rmdir(2). No arguments other than a possible exception are given to the completion callback.
Note: Using fs.rmdir()
on a file (not a directory) results in an ENOENT
error on Windows and an ENOTDIR
error on POSIX.
fs.rmdirSync(path)
Synchronous rmdir(2). Returns undefined
.
Note: Using fs.rmdirSync()
on a file (not a directory) results in an ENOENT
error on Windows and an ENOTDIR
error on POSIX.
fs.stat(path, callback)
-
path
<string> | <Buffer> -
callback
<Function>-
err
<Error> -
stats
<fs.Stats>
-
Asynchronous stat(2). The callback gets two arguments (err, stats)
where stats
is an fs.Stats
object.
In case of an error, the err.code
will be one of Common System Errors.
Using fs.stat()
to check for the existence of a file before calling fs.open()
, fs.readFile()
or fs.writeFile()
is not recommended. Instead, user code should open/read/write the file directly and handle the error raised if the file is not available.
To check if a file exists without manipulating it afterwards, fs.access()
is recommended.
fs.statSync(path)
Synchronous stat(2). Returns an instance of fs.Stats
.
fs.symlink(target, path[, 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 target
argument will automatically be normalized to absolute path.
Here is an example below:
fs.symlink('./foo', './new-port', callback);
It creates a symbolic link named "new-port" that points to "foo".
fs.symlinkSync(target, path[, type])
Synchronous symlink(2). Returns undefined
.
fs.truncate(path, len, callback)
-
path
<string> | <Buffer> -
len
<integer> default =0
-
callback
<Function>-
err
<Error>
-
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
. A file descriptor can also be passed as the first argument. In this case, fs.ftruncateSync()
is called.
fs.unlink(path, callback)
-
path
<string> | <Buffer> -
callback
<Function>-
err
<Error>
-
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.unwatchFile(filename[, listener])
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.
Note: fs.watch()
is more efficient than fs.watchFile()
and fs.unwatchFile()
. fs.watch()
should be used instead of fs.watchFile()
and fs.unwatchFile()
when possible.
fs.utimes(path, atime, mtime, callback)
Change file timestamps of the file referenced by the supplied path.
Note: the arguments atime
and mtime
of the following related functions follow these rules:
- The value should be a Unix timestamp in seconds. For example,
Date.now()
returns milliseconds, so it should be divided by 1000 before passing it in. - If the value is a numeric string like
'123456789'
, the value will get converted to the corresponding number. - If the value is
NaN
orInfinity
, the value will get converted toDate.now() / 1000
.
fs.utimesSync(path, atime, mtime)
Synchronous version of fs.utimes()
. Returns undefined
.
fs.watch(filename[, options][, listener])
-
filename
<string> | <Buffer> -
options
<string> | <Object>-
persistent
<boolean> Indicates whether the process should continue to run as long as files are being watched. default =true
-
recursive
<boolean> Indicates whether all subdirectories should be watched, or only the current directory. The applies when a directory is specified, and only on supported platforms (See Caveats). default =false
-
encoding
<string> Specifies the character encoding to be used for the filename passed to the listener. default ='utf8'
-
-
listener
<Function>
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. If options
is provided as a string, it specifies the encoding
. Otherwise options
should be passed as an object.
The listener callback gets two arguments (eventType, filename)
. eventType
is either 'rename'
or 'change'
, and filename
is the name of the file which triggered the event.
Note that on most platforms, 'rename'
is emitted whenever a filename appears or disappears in the directory.
Also note the listener callback is attached to the 'change'
event fired by fs.FSWatcher
, but it is not the same thing as the 'change'
value of eventType
.
Caveats
The fs.watch
API is not 100% consistent across platforms, and is unavailable in some situations.
The recursive option is only supported on macOS and Windows.
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 macOS, this uses
kqueue
for files andFSEvents
for directories. - On SunOS systems (including Solaris and SmartOS), this uses
event ports
. - On Windows systems, this feature depends on
ReadDirectoryChangesW
. - On Aix systems, this feature depends on
AHAFS
, which must be enabled.
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 can be unreliable, and in some cases impossible, on network file systems (NFS, SMB, etc), or host file systems when using virtualization software such as Vagrant, Docker, etc.
You can still use fs.watchFile
, which uses stat polling, but it is slower and less reliable.
Inodes
On Linux and macOS systems, fs.watch()
resolves the path to an inode and watches the inode. If the watched path is deleted and recreated, it is assigned a new inode. The watch will emit an event for the delete but will continue watching the original inode. Events for the new inode will not be emitted. This is expected behavior.
On AIX, save and close of a file being watched causes two notifications - one for adding new content, and one for truncation. Moreover, save and close operations on some platforms cause inode changes that force watch operations to become invalid and ineffective. AIX retains inode for the lifetime of a file, that way though this is different from Linux / OS X, this improves the usability of file watching. This is expected behavior.
Filename Argument
Providing filename
argument in the callback is 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', (eventType, filename) => { console.log(`event type is: ${eventType}`); if (filename) { console.log(`filename provided: ${filename}`); } else { console.log('filename not provided'); } });
fs.watchFile(filename[, options], listener)
-
filename
<string> | <Buffer> -
options
<Object> -
listener
<Function>-
current
<fs.Stats> -
previous
<fs.Stats>
-
Watch for changes on filename
. The callback listener
will be called each time the file is accessed.
The options
argument may be omitted. If provided, it should be an object. The options
object may contain a boolean named persistent
that indicates whether the process should continue to run as long as files are being watched. The options
object may specify an interval
property indicating 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', (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
.
Note: when an fs.watchFile
operation results in an ENOENT
error, it will invoke the listener once, with all the fields zeroed (or, for dates, the Unix Epoch). In Windows, blksize
and blocks
fields will be undefined
, instead of zero. If the file is created later on, the listener will be called again, with the latest stat objects. This is a change in functionality since v0.10.
Note: fs.watch()
is more efficient than fs.watchFile
and fs.unwatchFile
. fs.watch
should be used instead of fs.watchFile
and fs.unwatchFile
when possible.
Note: When a file being watched by fs.watchFile()
disappears and reappears, then the previousStat
reported in the second callback event (the file's reappearance) will be the same as the previousStat
of the first callback event (its disappearance).
This happens when:
- the file is deleted, followed by a restore
- the file is renamed twice - the second time back to its original name
fs.write(fd, buffer[, offset[, length[, position]]], callback)
-
fd
<integer> -
buffer
<Buffer> -
offset
<integer> -
length
<integer> -
position
<integer> -
callback
<Function>-
err
<Error> -
bytesWritten
<integer> -
buffer
<Buffer> | <Uint8Array>
-
Write buffer
to the file specified by fd
.
offset
determines the part of the buffer to be written, and length
is an integer specifying the number of bytes to write.
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, string[, position[, encoding]], callback)
-
fd
<integer> -
string
<string> -
position
<integer> -
encoding
<string> -
callback
<Function>
Write string
to the file specified by fd
. If string
is not a string, then the value will be coerced to one.
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.writeFile(file, data[, options], callback)
-
file
<string> | <Buffer> | <integer> filename or file descriptor -
data
<string> | <Buffer> -
options
<Object> | <string> -
callback
<Function>-
err
<Error>
-
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', (err) => { if (err) throw err; console.log('The file has been saved!'); });
If options
is a string, then it specifies the encoding. Example:
fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback);
Any specified file descriptor has to support writing.
Note that it is unsafe to use fs.writeFile
multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream
is strongly recommended.
Note: If a file descriptor is specified as the file
, it will not be closed automatically.
fs.writeFileSync(file, data[, options])
-
file
<string> | <Buffer> | <integer> filename or file descriptor -
data
<string> | <Buffer> -
options
<Object> | <string>
The synchronous version of fs.writeFile()
. Returns undefined
.
fs.writeSync(fd, buffer[, offset[, length[, position]]])
fs.writeSync(fd, string[, position[, encoding]])
Synchronous versions of fs.write()
. Returns the number of bytes written.
FS Constants
The following constants are exported by fs.constants
. Note: Not every constant will be available on every operating system.
File Access Constants
The following constants are meant for use with fs.access()
.
Constant | Description |
---|---|
F_OK | Flag indicating that the file is visible to the calling process. |
R_OK | Flag indicating that the file can be read by the calling process. |
W_OK | Flag indicating that the file can be written by the calling process. |
X_OK | Flag indicating that the file can be executed by the calling process. |
File Open Constants
The following constants are meant for use with fs.open()
.
Constant | Description |
---|---|
O_RDONLY | Flag indicating to open a file for read-only access. |
O_WRONLY | Flag indicating to open a file for write-only access. |
O_RDWR | Flag indicating to open a file for read-write access. |
O_CREAT | Flag indicating to create the file if it does not already exist. |
O_EXCL | Flag indicating that opening a file should fail if the O_CREAT flag is set and the file already exists. |
O_NOCTTY | Flag indicating that if path identifies a terminal device, opening the path shall not cause that terminal to become the controlling terminal for the process (if the process does not already have one). |
O_TRUNC | Flag indicating that if the file exists and is a regular file, and the file is opened successfully for write access, its length shall be truncated to zero. |
O_APPEND | Flag indicating that data will be appended to the end of the file. |
O_DIRECTORY | Flag indicating that the open should fail if the path is not a directory. |
O_NOATIME | Flag indicating reading accesses to the file system will no longer result in an update to the atime information associated with the file. This flag is available on Linux operating systems only. |
O_NOFOLLOW | Flag indicating that the open should fail if the path is a symbolic link. |
O_SYNC | Flag indicating that the file is opened for synchronous I/O. |
O_SYMLINK | Flag indicating to open the symbolic link itself rather than the resource it is pointing to. |
O_DIRECT | When set, an attempt will be made to minimize caching effects of file I/O. |
O_NONBLOCK | Flag indicating to open the file in nonblocking mode when possible. |
File Type Constants
The following constants are meant for use with the fs.Stats
object's mode
property for determining a file's type.
Constant | Description |
---|---|
S_IFMT | Bit mask used to extract the file type code. |
S_IFREG | File type constant for a regular file. |
S_IFDIR | File type constant for a directory. |
S_IFCHR | File type constant for a character-oriented device file. |
S_IFBLK | File type constant for a block-oriented device file. |
S_IFIFO | File type constant for a FIFO/pipe. |
S_IFLNK | File type constant for a symbolic link. |
S_IFSOCK | File type constant for a socket. |
File Mode Constants
The following constants are meant for use with the fs.Stats
object's mode
property for determining the access permissions for a file.
Constant | Description |
---|---|
S_IRWXU | File mode indicating readable, writable and executable by owner. |
S_IRUSR | File mode indicating readable by owner. |
S_IWUSR | File mode indicating writable by owner. |
S_IXUSR | File mode indicating executable by owner. |
S_IRWXG | File mode indicating readable, writable and executable by group. |
S_IRGRP | File mode indicating readable by group. |
S_IWGRP | File mode indicating writable by group. |
S_IXGRP | File mode indicating executable by group. |
S_IRWXO | File mode indicating readable, writable and executable by others. |
S_IROTH | File mode indicating readable by others. |
S_IWOTH | File mode indicating writable by others. |
S_IXOTH | File mode indicating executable by others. |
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Licensed under the MIT License.
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https://nodejs.org/dist/latest-v6.x/docs/api/fs.html