Linux File Permissions & chmod Command Cheat Sheet

Linux file permissions are key to controlling access to files and directories. It allows you to set who can read, write, or execute a file or directory. This guide will help you understand how to view and change these permissions.

The system divides users into:

• u – user (owner)

• g – group

• o – others (everyone else)

• a – all (user + group + others)

---

📂 1. Viewing File Permissions

ls -l

Sample Output:

-rwxr-xr-- 1 user group 1234 Jul 5  file.sh

Breakdown:

• - → file (or d for directory)

• rwx → owner can read, write, execute

• r-x → group can read and execute

• r-- → others can only read

---

🔧 2. Understanding Permission Types

---

✏️ 3. Modifying Permissions with chmod

➕ Add Permission

chmod u+x file.sh     # Add execute for user
chmod g+w file.txt    # Add write for group
chmod o+r file.log    # Add read for others
chmod +x file.sh      # Add execute for everyone

➖ Remove Permission

chmod u-x file.sh     # Remove execute from user
chmod g-w file.txt    # Remove write from group
chmod o-r file.log    # Remove read from others
chmod -x file.sh      # Remove execute from everyone

➡️ Set Specific Permissions

chmod u=rwx file.sh   # Set user permissions to read, write, execute
chmod go=rw file.txt  # Set group and others to read, write only
chmod a=r file.txt    # Everyone gets read-only

---

🔢 4. Numeric (Octal) Permissions with chmod

Sum them up for each level:

🔧 Examples:

chmod 755 script.sh   # rwx for user, rx for group and others
chmod 700 secret.txt  # rwx only for user (private)
chmod 644 file.txt    # rw for user, read-only for others

---

🧑‍🤝‍🧑 5. Changing Ownership with chown

chown user file.txt             # Change owner
chown user:group file.txt       # Change owner and group

Example:

chown ec2-user:ec2-user app.py

---

🔄 6. Changing Group with chgrp

chgrp groupname file.txt

---

📁 7. Apply Permissions Recursively

chmod -R 755 myfolder/    # Apply to all files & subfolders
chown -R user:group myfolder/

---

📌 Quick Reference: Symbolic vs Octal

---

📌Quick Reference: Symbolic chmod Commands (No Octal)

These don’t use numbers but letters to modify permissions.

---

👮 Extra: Checking Permissions of a File

stat file.txt

Output includes:

• Access rights in symbolic and numeric

• Owner and group

• Modification time

---

Conclusion

Mastering Linux file permissions is essential for effectively managing access to files and directories. By understanding and utilizing the chmod, chown, and chgrp commands, you can control who can read, write, or execute files, ensuring both security and proper access management. Whether you prefer symbolic or numeric (octal) methods, this guide provides a comprehensive overview to help you confidently modify permissions. Proper permission settings are crucial for maintaining system integrity and protecting sensitive data.