🔒 Linux File Permissions, ACLs & Special Permissions! 🚀

📌 1. Understanding File Permissions

Every file and directory in Linux has three sets of permissions for three types of users:

👤 Owner – The user who created the file.
👥 Group – A set of users who share access.
🌍 Others – Anyone else with system access.

Each file permission is made up of three types of actions:

r (read) 👀 – Can view the contents of a file.
w (write) ✍️ – Can modify the file or add/remove content.
x (execute) 🚀 – Can run the file if it’s a script or program.

📌 Example of File Permissions

Run this command to see file permissions:

ls -ltr

Example output:

-rwxr--r-- 1 user group 1234 Feb 9 12:00 myfile.txt

Let's break this down:

-rwxr--r-- → The first section shows permissions.
- rwx → Owner can read, write, and execute.
- r-- → Group can only read.
- r-- → Others can only read.
user → The owner of the file.
group → The group that owns the file.
1234 → File size in bytes.
Feb 9 12:00 → Last modified date and time.
myfile.txt → File name.

🎯 2. Changing File Permissions

🛠 Change File Permissions

You can modify who can read, write, or execute a file using chmod.

🔹 Add or Remove Specific Permissions

tchmod u+x file.txt  # Give execute permission to owner
chmod g-w file.txt  # Remove write permission from group
chmod o-r file.txt  # Remove read permission from others

u → Owner
g → Group
o → Others
+ → Add permission
- → Remove permission

🔹 Change Permissions Using Numbers

Each permission type has a number:

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

To set permissions for owner, group, and others, sum up the values:

chmod 755 file.txt

Breakdown of 755:

7 → Owner = rwx (4+2+1 = 7)
5 → Group = r-x (4+0+1 = 5)
5 → Others = r-x (4+0+1 = 5)

🛠 Change Ownership of a File

When a file is created, it automatically belongs to the user who created it. You can change this using:

🔹 Change File Owner

chown new_owner file.txt

Example: If you want to change the owner of report.txt to alice:
```
  chown alice report.txt
```

🔹 Change Group Ownership

chgrp new_group file.txt

Example: If you want to change the group of data.csv to developers:
```
  chgrp developers data.csv
```

🏆 3. Access Control Lists (ACLs) – Advanced Permissions

Linux Access Control Lists (ACLs) allow more flexible permission management. Instead of just Owner, Group, Others, you can assign specific permissions to individual users.

🛠 View ACL Permissions

getfacl file.txt

This shows detailed permissions including special ACL settings.

🛠 Set ACL for a Specific User

setfacl -m u:username:rwx file.txt

Example: Give full access to bob for notes.txt:
```
  setfacl -m u:bob:rwx notes.txt
```

🛠 Remove ACL for a User

setfacl -x u:username file.txt

Example: Remove bob from ACL of notes.txt:
```
  setfacl -x u:bob notes.txt
```

⚡ 4. Special Permissions: Sticky Bit, SUID, SGID

🎩 Sticky Bit (👢 Protecting shared folders!)

Used in shared directories like /tmp.
Prevents users from deleting files they don’t own.

🔹 Set Sticky Bit

chmod +t mydir

🔹 Check if Sticky Bit is set

ls -ld mydir

🔹 Example output: drwxrwxrwt (notice the t at the end)

🎭 SUID (Superpower Mode! 🦸)

Allows a file to run as the owner, not the user executing it.
Used for commands like passwd (which needs root privileges).

🔹 Set SUID on a script

chmod u+s script.sh

🔹 Check if SUID is set

ls -l script.sh

🔹 Example output: -rwsr-xr-x (notice the s in place of x)

🤝 SGID (Group Magic! 🧙)

Ensures new files inside a directory inherit the group.
Useful for collaborative project folders.

🔹 Set SGID on a directory

chmod g+s mydir

🔹 Check if SGID is set

ls -ld mydir

🔹 Example output: drwxrwsr-x (notice the s in the group section)

🛠 5. Automating Permissions with Scripts

📌 Change Permissions for Multiple Files

#!/bin/bash
echo "Enter permission (e.g., 755): "
read perm
chmod $perm *

Prompts user for a permission code and applies it to all files in the directory.

📌 Set ACL for a User on a File

#!/bin/bash
echo "Enter username: "
read user
setfacl -m u:$user:rwx myfile.txt

Prompts for a username and grants them full access to myfile.txt.

🛡️ 6. Backup & Restore Permissions

📌 Backup Permissions

getfacl -R mydir > permissions_backup.txt

Saves all file permissions in mydir to permissions_backup.txt.

📌 Restore Permissions

setfacl --restore=permissions_backup.txt

Restores previously saved permissions.