🔐 Linux File Permissions Explained – chmod, chown, and More

Introduction

Linux File Permissions: In Linux, file security is managed through permissions. Every file and directory has access rules that define who can read, write, or execute them.

In this guide, we’ll cover:
✅ How to check file permissions
✅ Understanding permission types
✅ Using chmod, chown, and chgrp to control access

By the end, you’ll have a solid grasp of Linux file security.

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📌 Understanding Linux File Permissions

In Linux, every file and directory has three types of permissions:

Permission	Symbol	Meaning	Example
Read	`r`	View file contents	Read a text file
Write	`w`	Modify file contents	Edit a file
Execute	`x`	Run as a program/script	Execute a shell script

Permissions apply to three user groups:

Group	Symbol	Meaning
Owner	`u`	The file’s creator
Group	`g`	Users within the assigned group
Others	`o`	All other users

🔍 Checking File Permissions

To check file permissions, use:

ls -l

Example output:

-rw-r--r--  1 user  user  1024 Feb 5 10:30 file.txt

Here’s what it means:

rw- → Owner (read & write)
r-- → Group (read-only)
r-- → Others (read-only)

📌 Changing File Permissions with chmod

The chmod command modifies permissions using symbolic (+r, -w, etc.) or numeric (755, 644) notation.

1️⃣ Using Symbolic Mode

✅ Add execute permission for the owner:

chmod u+x script.sh

✅ Remove write permission for others:

chmod o-w file.txt

✅ Grant full access to the group:

chmod g+rwx folder/

2️⃣ Using Numeric Mode (Octal Values)

Permissions are represented as numbers:

Permission	Binary	Octal
`r--`	`100`	4
`rw-`	`110`	6
`rwx`	`111`	7

Example:
✅ Set file permissions to 755 (owner full access, others read & execute):

chmod 755 script.sh

✅ Set file permissions to 644 (owner read/write, others read-only):

chmod 644 document.txt

📌 Changing File Ownership with chown

The chown command changes file ownership.

✅ Change owner of a file:

chown newuser file.txt

✅ Change owner and group:

chown newuser:newgroup file.txt

✅ Apply changes to all files in a directory:

chown -R newuser:newgroup /path/to/directory/

📌 Changing Group Ownership with chgrp

The chgrp command changes group ownership.

✅ Assign a file to a new group:

chgrp developers project.doc

✅ Apply group changes recursively:

chgrp -R team /shared_folder/

📝 Practical Examples of File Permissions

Scenario 1: Making a Script Executable

If you create a shell script (myscript.sh), it won’t run unless it has execute permission.
Fix it using:

chmod +x myscript.sh
./myscript.sh  # Now it runs!

Scenario 2: Restricting Access to a File

If you have a private document:

chmod 600 private.txt

🔒 Now only the owner can read/write it!

Scenario 3: Giving Group Write Access

To let team members edit a shared file:

chmod 664 report.doc
chown :developers report.doc

🎯 Final Thoughts

Mastering Linux file permissions is essential for system security and multi-user environments. Now you know how to use chmod, chown, and chgrp to control access!

💡 Next Blog: Essential Linux Commands Every User Should Know