Node.js fs.fchmod() Method

The fs.fchmod() method is used to change the permissions of a given file descriptor. These permissions can be specified as a parameter using string constants or octal numbers that corresponding to their respective file modes.

Note: The Windows platform only supports the changing of the write permission. It does not support the distinction between the permissions of users, groups, or others.

Syntax:

fs.fchmod( fd, mode, callback )

Parameters: This method accept three parameters as mentioned above and described below:

• fd: It is an integer value which denotes the file descriptor of the file of which the permission has to be changed.
• mode: It is string constant or octal constant that denotes the permission to be granted. The logical OR operator can be used to separate multiple permissions.
• callback: It is a function that would be called when the method is executed.
  • err: It is an error that would be thrown if the method fails.

Below examples illustrate the fs.fchmod() method in Node.js:

Example 1: This example shows the usage of string constants and OR operator to give the file permissions.

// Node.js program to demonstrate the
// fs.fchmod() method

// Import the filesystem module
const fs = require('fs');

// Getting the file descriptor
const fd = fs.openSync('example_file.txt', 'r');

// Allowing only read permission
console.log("Giving only read permission to the user");
fs.fchmod(fd, fs.constants.S_IRUSR, (err) => {
  if (err) throw err;

  // Check the file mode
  console.log("Current File Mode:",
        fs.statSync("example_file.txt").mode);

  // Reading the file
  console.log("File Contents:",
        fs.readFileSync("example_file.txt", 'utf8'));

  // Trying to write to file
  try {
    console.log("Trying to write to file");
    fs.writeFileSync('example_file.txt', "Hello");
  }
  catch (e) {
    console.log("Error Found, Code:", e.code);
  }

  // Allowing both read and write permission
  console.log("\nGiving both read and write "
                + "permission to the user");

  fs.fchmod(fd, fs.constants.S_IRUSR |
          fs.constants.S_IWUSR, (err) => {
    if (err) throw err;

    // Check the file mode
    console.log("Current File Mode:",
        fs.statSync("example_file.txt").mode);

    console.log("Trying to write to file");
    fs.writeFileSync('example_file.txt',
        "This file has been written over.");

    console.log("File Contents:",
        fs.readFileSync("example_file.txt", 'utf8'));
  });
});

Output:

Giving only read permission to the user
Current File Mode: 33024
File Contents: This file has been written over.
Trying to write to file
Error Found, Code: EACCES

Giving both read and write permission to the user
Current File Mode: 33152
Trying to write to file
File Contents: This file has been written over.

Example 2: This example shows the usage of octal constants to give the file permissions.

// Node.js program to demonstrate the
// fs.fchmod() method

// Import the filesystem module
const fs = require('fs');

// Getting the file descriptor
const fd = fs.openSync('example_file.txt', 'r');

// Allowing only read permission
console.log("Giving only read permission to everyone");
fs.fchmod(fd, 0o444, (err) => {
  if (err) throw err;

  // Check the file mode
  console.log("Current File Mode:",
        fs.statSync("example_file.txt").mode);

  // Reading the file
  console.log("File Contents:",
        fs.readFileSync("example_file.txt", 'utf8'));

  // Trying to write to file
  try {
    console.log("Trying to write to file");
    fs.writeFileSync('example_file.txt', "Hello");
  }
  catch (e) {
    console.log("Error Found, Code:", e.code);
  }

  // Allowing both read and write permission
  console.log("\nGiving both read and write "
                  + "permission to everyone");

  fs.fchmod(fd, 0o666, (err) => {
    if (err) throw err;

    // Check the file mode
    console.log("Current File Mode:",
        fs.statSync("example_file.txt").mode);

    console.log("Trying to write to file");
    fs.writeFileSync('example_file.txt',
        "This file has been written over.");

    console.log("File Contents:",
        fs.readFileSync("example_file.txt", 'utf8'));
  });
});

Output:

Giving only read permission to everyone
Current File Mode: 33060
File Contents: This file has been written over.
Trying to write to file
Error Found, Code: EACCES

Giving both read and write permission to everyone
Current File Mode: 33206
Trying to write to file
File Contents: This file has been written over.

Reference: https://nodejs.org/api/fs.html#fs_fs_fchmod_fd_mode_callback