Using chmod for File Permissions in Linux

In Linux, file permissions are crucial for maintaining the security and integrity of your system. The chmod command allows you to modify these permissions. This guide will walk you through the basics of using chmod to manage file permissions.

❗ Modifying file permissions wields great power in Linux. Missteps can lead to destructive commands executing without hesitation.

Understanding File Permissions

Linux uses a three-part permission system: read (r), write (w), and execute (x). These permissions are assigned to three entities: the owner of the file, the group associated with the file, and others (everyone else).

  • r (read): Allows reading or viewing the file.
  • w (write): Allows modifying or deleting the file.
  • x (execute): Allows executing the file if it’s a script or binary.

Each file has a 9-character permission string, representing read, write, and execute permissions for the owner, group, and others. For example, rw-r--r-- means the owner has read and write permissions, while the group and others have only read permissions.

Using chmod Command

The chmod command follows the syntax:

chmod [options] mode file
  • options: Additional parameters for the chmod command.
  • mode: The permissions to be set, expressed in the octal or symbolic notation
  • file: The file or directory whose permissions you want to modify.

Octal Notation

In octal notation, each permission has a numeric value:

  • 4 for read (r).
  • 2 for write (w).
  • 1 for execute (x).

To set permissions, add the values for the desired permissions. For example:

  • chmod 644 file.txt gives read and write permissions to the owner and read-only permissions to the group and others.

Symbolic Notation

In symbolic notation, you use letters and symbols to represent permissions. The syntax is:

chmod [who] operator permission file
  • who: Represents the user type (u for owner, g for group, o for others, a for all).
  • operator: Specifies whether to add (+), remove (-), or set (=) permissions.
  • permission: The permission to be added, removed, or set.

For example:

  • chmod u+x file.txt adds execute permission for the owner.


  1. Give read, write, and execute permissions to the owner, and read-only permissions to the group and others:
chmod 755
  1. Add write permission for the group:
chmod g+w file.txt
  1. Set read-only permissions for others:
chmod o=r file.txt

This guide covers the basics of using chmod in Linux. Experiment with different permissions to meet your specific needs.