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Node.js v4.4.0-rc.4 Documentation
Table of Contents
- File System
- Class: fs.FSWatcher
- Class: fs.ReadStream
- Class: fs.Stats
- Class: fs.WriteStream
- fs.access(path[, mode], callback)
- fs.accessSync(path[, mode])
- fs.appendFile(file, data[, options], callback)
- fs.appendFileSync(file, data[, options])
- fs.chmod(path, mode, callback)
- fs.chmodSync(path, mode)
- fs.chown(path, uid, gid, callback)
- fs.chownSync(path, uid, gid)
- fs.close(fd, callback)
- fs.closeSync(fd)
- fs.createReadStream(path[, options])
- fs.createWriteStream(path[, options])
- fs.exists(path, callback)
- fs.existsSync(path)
- fs.fchmod(fd, mode, callback)
- fs.fchmodSync(fd, mode)
- fs.fchown(fd, uid, gid, callback)
- fs.fchownSync(fd, uid, gid)
- fs.fstat(fd, callback)
- fs.fstatSync(fd)
- fs.fsync(fd, callback)
- fs.fsyncSync(fd)
- fs.ftruncate(fd, len, callback)
- fs.ftruncateSync(fd, len)
- fs.futimes(fd, atime, mtime, callback)
- fs.futimesSync(fd, atime, mtime)
- fs.lchmod(path, mode, callback)
- fs.lchmodSync(path, mode)
- fs.lchown(path, uid, gid, callback)
- fs.lchownSync(path, uid, gid)
- fs.link(srcpath, dstpath, callback)
- fs.linkSync(srcpath, dstpath)
- fs.lstat(path, callback)
- fs.lstatSync(path)
- fs.mkdir(path[, mode], callback)
- fs.mkdirSync(path[, mode])
- fs.open(path, flags[, mode], callback)
- fs.openSync(path, flags[, mode])
- fs.read(fd, buffer, offset, length, position, callback)
- fs.readdir(path, callback)
- fs.readdirSync(path)
- fs.readFile(file[, options], callback)
- fs.readFileSync(file[, options])
- fs.readlink(path, callback)
- fs.readlinkSync(path)
- fs.realpath(path[, cache], callback)
- fs.readSync(fd, buffer, offset, length, position)
- fs.realpathSync(path[, cache])
- fs.rename(oldPath, newPath, callback)
- fs.renameSync(oldPath, newPath)
- fs.rmdir(path, callback)
- fs.rmdirSync(path)
- fs.stat(path, callback)
- fs.statSync(path)
- fs.symlink(target, path[, type], callback)
- fs.symlinkSync(target, path[, type])
- fs.truncate(path, len, callback)
- fs.truncateSync(path, len)
- fs.unlink(path, callback)
- fs.unlinkSync(path)
- fs.unwatchFile(filename[, listener])
- fs.utimes(path, atime, mtime, callback)
- fs.utimesSync(path, atime, mtime)
- fs.watch(filename[, options][, listener])
- fs.watchFile(filename[, options], listener)
- fs.write(fd, buffer, offset, length[, position], callback)
- fs.write(fd, data[, position[, encoding]], callback)
- fs.writeFile(file, data[, options], callback)
- fs.writeFileSync(file, data[, options])
- fs.writeSync(fd, buffer, offset, length[, position])
- fs.writeSync(fd, data[, position[, encoding]])
File System#
Stability: 2 - 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 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: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.>
Class: fs.FSWatcher#
Objects returned from fs.watch()
are of this type.
Event: 'change'#
Emitted when something changes in a watched directory or file.
See more details in fs.watch()
.
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
<Number> Integer file descriptor used by the ReadStream.
Emitted when the ReadStream's file is opened.
readStream.path#
The path to the file the stream is reading from.
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:
{
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 themknod(2)
,utimes(2)
, andread(2)
system calls.mtime
"Modified Time" - Time when file data last modified. Changed by themknod(2)
,utimes(2)
, andwrite(2)
system calls.ctime
"Change Time" - Time when file status was last changed (inode data modification). Changed by thechmod(2)
,chown(2)
,link(2)
,mknod(2)
,rename(2)
,unlink(2)
,utimes(2)
,read(2)
, andwrite(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
). On Darwin and other FreeBSD variants, also set if theatime
is explicitly set to an earlier value than the currentbirthtime
using theutimes(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
<Number> Integer file descriptor used by the WriteStream.
Emitted when the WriteStream's file is opened.
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.
fs.access(path[, mode], callback)#
Tests a user's permissions for the file specified by path
. mode
is an
optional integer that specifies the accessibility checks to be performed. The
following constants define the possible values of mode
. It is possible to
create a mask consisting of the bitwise OR of two or more values.
fs.F_OK
- File 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.R_OK
- File can be read by the calling process.fs.W_OK
- File can be written by the calling process.fs.X_OK
- File can be executed by the calling process. This has no effect on Windows (will behave likefs.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.R_OK | fs.W_OK, (err) => {
console.log(err ? 'no access!' : 'can read/write');
});
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)#
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: Specified file descriptors will not be closed automatically.
fs.appendFileSync(file, data[, options])#
The synchronous version of fs.appendFile()
. Returns undefined
.
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). Returns undefined
.
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). Returns undefined
.
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). Returns undefined
.
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:
{
flags: 'r',
encoding: null,
fd: null,
mode: 0o666,
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 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:
{
flags: 'w',
defaultEncoding: 'utf8',
fd: null,
mode: 0o666
}
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
.
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)#
Stability: 0 - Deprecated: Usefs.stat()
orfs.access()
instead.
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!');
});
fs.exists()
should not be used to check if a file exists before calling
fs.open()
. Doing so introduces a race condition since other processes may
change the file's state between the two calls. Instead, user code should
call fs.open()
directly and handle the error raised if the file is
non-existent.
fs.existsSync(path)#
Stability: 0 - Deprecated: Usefs.statSync()
orfs.accessSync()
instead.
Synchronous version of fs.exists()
.
Returns true
if the file exists, false
otherwise.
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). Returns undefined
.
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). Returns undefined
.
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.fstatSync(fd)#
Synchronous fstat(2). Returns an instance of fs.Stats
.
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). Returns undefined
.
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). Returns undefined
.
fs.futimes(fd, atime, mtime, callback)#
Change the file timestamps of a file referenced by the supplied file descriptor.
fs.futimesSync(fd, atime, mtime)#
Synchronous version of fs.futimes()
. Returns undefined
.
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). 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(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). Returns undefined
.
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.lstatSync(path)#
Synchronous lstat(2). Returns an instance of fs.Stats
.
fs.mkdir(path[, mode], callback)#
Asynchronous mkdir(2). No arguments other than a possible exception are given
to the completion callback. mode
defaults to 0o777
.
fs.mkdirSync(path[, mode])#
Synchronous mkdir(2). Returns undefined
.
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 usingfs.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 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 0666
, 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 require('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.
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)#
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.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.readFile(file[, options], callback)#
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);
Any specified file descriptor has to support reading.
Note: Specified file descriptors 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.
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, (err, resolvedPath) => {
if (err) throw err;
console.log(resolvedPath);
});
fs.readSync(fd, buffer, offset, length, position)#
Synchronous version of fs.read()
. Returns the number of bytesRead
.
fs.realpathSync(path[, cache])#
Synchronous realpath(2). Returns the resolved path. 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.
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)#
Asynchronous rmdir(2). No arguments other than a possible exception are given to the completion callback.
fs.rmdirSync(path)#
Synchronous rmdir(2). Returns undefined
.
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 for more
information.
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');
It would create a symlic link named with "new-port" that points to "foo".
fs.symlinkSync(target, path[, type])#
Synchronous symlink(2). Returns undefined
.
fs.truncate(path, len, callback)#
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
.
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). 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 does
follow the below rules:
- If the value is a numberable string like
'123456789'
, the value would get converted to corresponding number. - If the value is
NaN
orInfinity
, the value would get converted toDate.now()
.
fs.utimesSync(path, atime, mtime)#
Synchronous version of fs.utimes()
. Returns undefined
.
fs.watch(filename[, options][, listener])#
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).
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 only supported on OS X 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 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 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', (event, filename) => {
console.log(`event is: ${event}`);
if (filename) {
console.log(`filename provided: ${filename}`);
} else {
console.log('filename not provided');
}
});
fs.watchFile(filename[, options], listener)#
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.
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.writeFile(file, data[, options], callback)#
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('It\'s 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: Specified file descriptors will not be closed automatically.
fs.writeFileSync(file, data[, options])#
The synchronous version of fs.writeFile()
. Returns undefined
.
fs.writeSync(fd, buffer, offset, length[, position])#
fs.writeSync(fd, data[, position[, encoding]])#
Synchronous versions of fs.write()
. Returns the number of bytes written.