How are permissions for a file or folder changed in Linux?

In Linux, changing permissions for a file or folder is done using the chmod command or the chmod() system call.

Permissions are represented by three groups: owner, group, and others, each of which can have read (r), write (w), and execute (x) rights.

Example in Node.js using the fs module:

const fs = require('fs');

// Set file permissions to rw-r--r-- (644)
fs.chmod('example.txt', 0o644, (err) => {
  if (err) throw err;
  console.log('Permissions changed');
});

Here, 0o644 is an octal representation of the permissions, where:

• 6 (110) — read and write for the owner
• 4 (100) — read for the group
• 4 (100) — read for others

Thus, changing permissions involves setting access bits that determine who can perform what operations on a file or folder.