Mastering File Permissions in Linux: A Beginner’s Guide

When working with Linux, understanding and managing file permissions is essential for maintaining security and controlling access. This guide will break down file permissions, explain how to view them, and show you how to change them with simple commands.

Understanding Linux File Permissions

In Linux, every file and directory has an associated set of permissions that define who can read, write, or execute them. There are three types of users in Linux:

1. Owner: The user who owns the file or directory.
2. Group: A set of users with similar permissions.
3. Others: All other users on the system.

File Permission Structure

File permissions in Linux are typically represented by a series of symbols. For example:

ls -ltr

You will see the result like this

drwxr-xr-x 3 shanikr shanikr 4096 Feb  7  2024 django_projects
_rwxr-xr-x 2 shanikr shanikr 4096 Feb 15  2024 docker-compose.yaml
drwxr-xr-x 2 shanikr shanikr 4096 Feb 15  2024 Documents
_rwx------ 3 shanikr shanikr 4096 Feb 18  2024 robots.txt
drwxr-xr-x 2 shanikr shanikr 4096 Jul 29 00:31 deploy_flask

Each section in this string represents permissions for a different type of user:

1. The first character (-) indicates the file type (- for files, d for directories).
2. The next three characters (rwx) indicate permissions for the owner.
3. The following three (r-x) are permissions for the group.
4. The last three (r--) represent permissions for others.

Here's a breakdown of permission types:

• r (read) – Allows viewing the contents of the file.
• w (write) – Allows modifying the file.
• x (execute) – Allows running the file as a program.

Viewing File Permissions

To check the permissions of a file or directory, use the ls -l command:

ls -l filename

This command displays detailed information about the file, including its permissions.

Changing Permissions with chmod

The chmod command is used to change permissions. You can specify permissions in two ways:

1. Symbolic Mode

In symbolic mode, you specify the type of users (u for owner, g for group, and o for others) and the permission changes. For example:

Add execute permission to the owner:

chmod u+x filename

Remove write permission from the group:

chmod g-w filename

Give all users read permission:

chmod a+r filename

Give recursively permission to a folder

chmod -R g+w folder

2. Octal Mode

In octal mode, permissions are represented by numbers. Each permission type has a numeric value:

• Read (r) = 4
• Write (w) = 2
• Execute (x) = 1

To set permissions, add up the values. For example, to give the owner full permissions and only read permission to group and others:

chmod 744 filename

This breakdown is:

• 7 (4+2+1) for owner – read, write, and execute
• 4 for group – read-only
• 4 for others – read-only

Here are some common chmod octal values:

• 777 – Full permissions for everyone.
• 755 – Full permissions for owner, read and execute for group and others.
• 644 – Read and write for owner, read-only for group and others.

Changing Ownership with chown

The chown command changes the ownership of files and directories. Use it to assign a new owner or group:

Change only owner of a file

chown newowner filename

Change owner and group of a file

chown newowner:newgroup file

Change only group of a file

chown :newgroup file

Change recursively ownership  of a folder

chown -R newowner folder

Changing Group ownership with chgrp

Change Group Ownership of a File

chgrp newgroup file

Change Group Ownership of a Directory

chgrp newgroup directory

Change Group Ownership Recursively

chgrp -R newgroup directory

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