1. Linux Permission.md

🔐 Linux Permissions – Complete Guide

Permissions in Linux control what actions users can perform on files and directories. They define whether a user can read, write, or execute files and modify directory contents.

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🔹 1. Understanding Linux File Permissions

Permissions in Linux are represented in a 10-character field.

📌 Example: -rw-r--r--

Position	Meaning	Description
-	File Type	- (Regular File), d (Directory), l (Symbolic Link)
rw-	Owner (User) Permissions	Read (r), Write (w), Execute (x)
r--	Group Permissions	Read (r), Write (w), Execute (x)
r--	Others Permissions	Read (r), Write (w), Execute (x)

🔍 Breakdown of Characters:

• Owner (User): rw- → Read & Write access (❌ No execute permission).
• Group: r-- → Read-only access (❌ No write or execute).
• Others: r-- → Read-only access (❌ No write or execute).

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🔹 2. Viewing File Permissions 👀

To check file permissions, use the ls -l command:

ls -l

📌 Example Output:

-rw-r--r--  1 user group 4096 Jan 07 10:00 file.txt

Field	Meaning
-rw-r--r--	File permissions
1	Number of links
user	File owner
group	File group
4096	File size (bytes)
Jan 07 10:00	Last modification date
file.txt	File name

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🔹 3. Changing File Permissions 🛠️

✅ Method 1: Symbolic Mode

Syntax:

chmod <user_class><operation><permissions> file

User Class	Meaning
u	Owner (User)
g	Group
o	Others
a	All (Owner, Group, Others)

Operation	Action
+	Add permission
-	Remove permission
=	Set exact permissions

Permissions	Description
r	Read
w	Write
x	Execute

📌 Examples:

1️⃣ Add execute permission for the owner:

chmod u+x file.txt

2️⃣ Remove write permission for others:

chmod o-w file.txt

3️⃣ Set read and write for the group:

chmod g=rw file.txt

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✅ Method 2: Numeric Mode (Octal Format) 🔢

Permission	Numeric Value
Read (r)	4
Write (w)	2
Execute (x)	1

🔹 Each permission group (Owner, Group, Others) is represented as a 3-digit number.

Permission	Owner (User)	Group	Others
chmod 644	Read & Write (6)	Read-only (4)	Read-only (4)
chmod 755	Full (7)	Read & Execute (5)	Read & Execute (5)
chmod 777	Full (7)	Full (7)	Full (7)

📌 Examples:

1️⃣ Set read and write for the owner, read-only for others:

chmod 644 file.txt

2️⃣ Set full permissions for the owner, read/execute for others:

chmod 755 file.txt

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🔹 4. Changing File Ownership 🏷️ (chown Command)

To change file owner and group, use the chown command.

📌 Syntax:

chown new_owner:new_group file

📌 Examples:

1️⃣ Change the owner to john:

chown john file.txt

2️⃣ Change the owner to john and group to developers:

chown john:developers file.txt

3️⃣ Change all files in a directory recursively:

chown -R john:developers /myfolder

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🔹 5. Special Permissions 🔥

1️⃣ SetUID (s) – Execute as File Owner 👤

If the SetUID (set user ID) bit is set, the file executes with the owner’s permissions, not the user’s.

🔹 Example: Allow normal users to execute ping with root privileges:

ls -l /bin/ping

-rwsr-xr-x 1 root root 44160 Feb 9 12:34 /bin/ping

🔹 Set SetUID on a script:

chmod u+s script.sh

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2️⃣ SetGID (s) – Execute as Group 👥

If the SetGID (set group ID) bit is set, the file executes with the group’s permissions.

🔹 Example: Allow all users in developers to share files:

chmod g+s /shared_folder

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3️⃣ Sticky Bit (t) – Protect Files in Shared Folders 📂

If the sticky bit is set on a directory, only the file’s owner can delete their files.

🔹 Example: Set a sticky bit on /tmp to prevent file deletion by other users:

chmod +t /tmp

🔹 Verification:

ls -ld /tmp

drwxrwxrwt 10 root root 4096 Feb 9 12:34 /tmp

✅ The last t indicates the sticky bit is set.

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🔹 6. Troubleshooting Permissions & Errors 🚨

1️⃣ Permission Denied?

ls -l script.sh

If you see -rw-r--r--, add execute permissions:

chmod +x script.sh

2️⃣ Cannot Delete Files in /tmp?
Check if the sticky bit is set:

ls -ld /tmp

3️⃣ Find all files owned by a user (john)

find / -user john

4️⃣ Find all files with SetUID (s) permissions

find / -perm -4000

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📊 Summary Table of Commands

Command	Description
chmod 755 file.txt	Set read/write/execute for owner, read/execute for group/others
chown john file.txt	Change file owner to john
chown john:dev file.txt	Change owner to john and group to dev
chmod u+s script.sh	Enable SetUID (execute as owner)
chmod g+s folder/	Enable SetGID (execute as group)
chmod +t /shared	Enable Sticky Bit (prevent deletion by others)

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🎯 Final Thoughts

• Permissions are crucial for system security 🔐.
• Always follow the principle of least privilege.
• Use special permissions (SetUID, SetGID, Sticky Bit) when necessary.

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