Node implements File I/O using simple wrappers around standard POSIX
functions. The Node File System (fs) module can be imported using the
following syntax −
var fs = require("fs")
Synchronous vs Asynchronous
Every method in the fs module has synchronous as well as asynchronous
forms. Asynchronous methods take the last parameter as the completion
function callback and the first parameter of the callback function as
error. It is better to use an asynchronous method instead of a
synchronous method, as the former never blocks a program during its
execution, whereas the second one does.
Example
Create a text file named input.txt with the following content −
Tutorials Point is giving self learning content
to teach the world in simple and easy way!!!!!
Let us create a js file named main.js with the following code −
Synchronous read: Tutorials Point is giving self learning content
to teach the world in simple and easy way!!!!!
Program Ended
Asynchronous read: Tutorials Point is giving self learning content
to teach the world in simple and easy way!!!!!
The following sections in this chapter provide a set of good examples on major File I/O methods.
Open a File
Syntax
Following is the syntax of the method to open a file in asynchronous mode −
fs.open(path, flags[, mode], callback)
Parameters
Here is the description of the parameters used −
path − This is the string having file name including path.
flags − Flags indicate the behavior of the file to be opened. All possible values have been mentioned below.
mode − It sets the file mode (permission and sticky bits),
but only if the file was created. It defaults to 0666, readable and
writeable.
callback − This is the callback function which gets two arguments (err, fd).
Flags
Flags for read/write operations are −
Flag
Description
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.
rs+
Open file for reading and writing, asking 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 the 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 the path exists.
a+
Open file for reading and appending. The file is created if it does not exist.
ax+
Like 'a+' but fails if the the path exists.
Example
Let us create a js file named main.js having the following code to open a file input.txt for reading and writing.
var fs =require("fs");// Asynchronous - Opening File
console.log("Going to open file!");
fs.open('input.txt','r+',function(err, fd){if(err){return console.error(err);}
console.log("File opened successfully!");});
Now run the main.js to see the result −
$ node main.js
Verify the Output.
Going to open file!
File opened successfully!
Get File Information
Syntax
Following is the syntax of the method to get the information about a file −
fs.stat(path, callback)
Parameters
Here is the description of the parameters used −
path − This is the string having file name including path.
callback − This is the callback function which gets two arguments (err, stats) where stats is an object of fs.Stats type which is printed below in the example.
Apart from the important attributes which are printed below in the example, there are several useful methods available in fs.Stats class which can be used to check file type. These methods are given in the following table.
Method
Description
stats.isFile()
Returns true if file type of a simple file.
stats.isDirectory()
Returns true if file type of a directory.
stats.isBlockDevice()
Returns true if file type of a block device.
stats.isCharacterDevice()
Returns true if file type of a character device.
stats.isSymbolicLink()
Returns true if file type of a symbolic link.
stats.isFIFO()
Returns true if file type of a FIFO.
stats.isSocket()
Returns true if file type of asocket.
Example
Let us create a js file named main.js with the following code −
var fs =require("fs");
console.log("Going to get file info!");
fs.stat('input.txt',function(err, stats){if(err){return console.error(err);}
console.log(stats);
console.log("Got file info successfully!");// Check file type
console.log("isFile ? "+ stats.isFile());
console.log("isDirectory ? "+ stats.isDirectory());});
Now run the main.js to see the result −
$ node main.js
Verify the Output.
Going to get file info!
{
dev: 1792,
mode: 33188,
nlink: 1,
uid: 48,
gid: 48,
rdev: 0,
blksize: 4096,
ino: 4318127,
size: 97,
blocks: 8,
atime: Sun Mar 22 2015 13:40:00 GMT-0500 (CDT),
mtime: Sun Mar 22 2015 13:40:57 GMT-0500 (CDT),
ctime: Sun Mar 22 2015 13:40:57 GMT-0500 (CDT)
}
Got file info successfully!
isFile ? true
isDirectory ? false
Writing a File
Syntax
Following is the syntax of one of the methods to write into a file −
fs.writeFile(filename, data[, options], callback)
This method will over-write the file if the file already exists. If
you want to write into an existing file then you should use another
method available.
Parameters
Here is the description of the parameters used −
path − This is the string having the file name including path.
data − This is the String or Buffer to be written into the file.
options − The third parameter is an object which will hold
{encoding, mode, flag}. By default. encoding is utf8, mode is octal
value 0666. and flag is 'w'
callback − This is the callback function which gets a single parameter err that returns an error in case of any writing error.
Example
Let us create a js file named main.js having the following code −
var fs =require("fs");
console.log("Going to write into existing file");
fs.writeFile('input.txt','Simply Easy Learning!',function(err){if(err){return console.error(err);}
console.log("Data written successfully!");
console.log("Let's read newly written data");
fs.readFile('input.txt',function(err, data){if(err){return console.error(err);}
console.log("Asynchronous read: "+ data.toString());});});
Now run the main.js to see the result −
$ node main.js
Verify the Output.
Going to write into existing file
Data written successfully!
Let's read newly written data
Asynchronous read: Simply Easy Learning!
Reading a File
Syntax
Following is the syntax of one of the methods to read from a file −
This method will use file descriptor to read the file. If you want to
read the file directly using the file name, then you should use another
method available.
Parameters
Here is the description of the parameters used −
fd − This is the file descriptor returned by fs.open().
buffer − This is the buffer that the data will be written to.
offset − This is the offset in the buffer to start writing at.
length − This is an integer specifying the number of bytes to read.
position − This 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.
callback − This is the callback function which gets the three arguments, (err, bytesRead, buffer).
Example
Let us create a js file named main.js with the following code −
var fs =require("fs");var buf =newBuffer(1024);
console.log("Going to open an existing file");
fs.open('input.txt','r+',function(err, fd){if(err){return console.error(err);}
console.log("File opened successfully!");
console.log("Going to read the file");
fs.read(fd, buf,0, buf.length,0,function(err, bytes){if(err){
console.log(err);}
console.log(bytes +" bytes read");// Print only read bytes to avoid junk.if(bytes >0){
console.log(buf.slice(0, bytes).toString());}});});
Now run the main.js to see the result −
$ node main.js
Verify the Output.
Going to open an existing file
File opened successfully!
Going to read the file
97 bytes read
Tutorials Point is giving self learning content
to teach the world in simple and easy way!!!!!
Closing a File
Syntax
Following is the syntax to close an opened file −
fs.close(fd, callback)
Parameters
Here is the description of the parameters used −
fd − This is the file descriptor returned by file fs.open() method.
callback − This is the callback function No arguments other than a possible exception are given to the completion callback.
Example
Let us create a js file named main.js having the following code −
var fs =require("fs");var buf =newBuffer(1024);
console.log("Going to open an existing file");
fs.open('input.txt','r+',function(err, fd){if(err){return console.error(err);}
console.log("File opened successfully!");
console.log("Going to read the file");
fs.read(fd, buf,0, buf.length,0,function(err, bytes){if(err){
console.log(err);}// Print only read bytes to avoid junk.if(bytes >0){
console.log(buf.slice(0, bytes).toString());}// Close the opened file.
fs.close(fd,function(err){if(err){
console.log(err);}
console.log("File closed successfully.");});});});
Now run the main.js to see the result −
$ node main.js
Verify the Output.
Going to open an existing file
File opened successfully!
Going to read the file
Tutorials Point is giving self learning content
to teach the world in simple and easy way!!!!!
File closed successfully.
Truncate a File
Syntax
Following is the syntax of the method to truncate an opened file −
fs.ftruncate(fd, len, callback)
Parameters
Here is the description of the parameters used −
fd − This is the file descriptor returned by fs.open().
len − This is the length of the file after which the file will be truncated.
callback − This is the callback function No arguments other than a possible exception are given to the completion callback.
Example
Let us create a js file named main.js having the following code −
var fs =require("fs");var buf =newBuffer(1024);
console.log("Going to open an existing file");
fs.open('input.txt','r+',function(err, fd){if(err){return console.error(err);}
console.log("File opened successfully!");
console.log("Going to truncate the file after 10 bytes");// Truncate the opened file.
fs.ftruncate(fd,10,function(err){if(err){
console.log(err);}
console.log("File truncated successfully.");
console.log("Going to read the same file");
fs.read(fd, buf,0, buf.length,0,function(err, bytes){if(err){
console.log(err);}// Print only read bytes to avoid junk.if(bytes >0){
console.log(buf.slice(0, bytes).toString());}// Close the opened file.
fs.close(fd,function(err){if(err){
console.log(err);}
console.log("File closed successfully.");});});});});
Now run the main.js to see the result −
$ node main.js
Verify the Output.
Going to open an existing file
File opened successfully!
Going to truncate the file after 10 bytes
File truncated successfully.
Going to read the same file
Tutorials
File closed successfully.
Delete a File
Syntax
Following is the syntax of the method to delete a file −
fs.unlink(path, callback)
Parameters
Here is the description of the parameters used −
path − This is the file name including path.
callback − This is the callback function No arguments other than a possible exception are given to the completion callback.
Example
Let us create a js file named main.js having the following code −
var fs =require("fs");
console.log("Going to delete an existing file");
fs.unlink('input.txt',function(err){if(err){return console.error(err);}
console.log("File deleted successfully!");});
Now run the main.js to see the result −
$ node main.js
Verify the Output.
Going to delete an existing file
File deleted successfully!
Create a Directory
Syntax
Following is the syntax of the method to create a directory −
fs.mkdir(path[, mode], callback)
Parameters
Here is the description of the parameters used −
path − This is the directory name including path.
mode − This is the directory permission to be set. Defaults to 0777.
callback − This is the callback function No arguments other than a possible exception are given to the completion callback.
Example
Let us create a js file named main.js having the following code −
var fs =require("fs");
console.log("Going to create directory /tmp/test");
fs.mkdir('/tmp/test',function(err){if(err){return console.error(err);}
console.log("Directory created successfully!");});
Now run the main.js to see the result −
$ node main.js
Verify the Output.
Going to create directory /tmp/test
Directory created successfully!
Read a Directory
Syntax
Following is the syntax of the method to read a directory −
fs.readdir(path, callback)
Parameters
Here is the description of the parameters used −
path − This is the directory name including path.
callback − This is the callback function which gets two
arguments (err, files) where files is an array of the names of the files
in the directory excluding '.' and '..'.
Example
Let us create a js file named main.js having the following code −
var fs =require("fs");
console.log("Going to read directory /tmp");
fs.readdir("/tmp/",function(err, files){if(err){return console.error(err);}
files.forEach(function(file){
console.log( file );});});
Now run the main.js to see the result −
$ node main.js
Verify the Output.
Going to read directory /tmp
ccmzx99o.out
ccyCSbkF.out
employee.ser
hsperfdata_apache
test
test.txt
Remove a Directory
Syntax
Following is the syntax of the method to remove a directory −
fs.rmdir(path, callback)
Parameters
Here is the description of the parameters used −
path − This is the directory name including path.
callback − This is the callback function No argume
nts other than a possible exception are given to the completion callback.
Example
Let us create a js file named main.js having the following code −
var fs =require("fs");
console.log("Going to delete directory /tmp/test");
fs.rmdir("/tmp/test",function(err){if(err){return console.error(err);}
console.log("Going to read directory /tmp");
fs.readdir("/tmp/",function(err, files){if(err){return console.error(err);}
files.forEach(function(file){
console.log( file );});});});
Now run the main.js to see the result −
$ node main.js
Verify the Output.
Going to read directory /tmp
ccmzx99o.out
ccyCSbkF.out
employee.ser
hsperfdata_apache
test.txt
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