Linux Advanced

🔐 What is Linux Secure Shell (SSH)?

SSH (Secure Shell) is a safe way to remotely connect to another Linux system over a network. 🌐
It encrypts everything — your commands, passwords, and data — making it private and secure.

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✅ Why SSH is Useful:

• 💻 Remote Login: Access your Linux server from anywhere

• 🔒 Secure: All data is encrypted

• 🧑‍💻 Control Servers: Run commands, transfer files, manage services

🔐 How SSH Works:

• Uses public & private keys or passwords for authentication

• Runs on port 22 by default

• Public Key 📤: Shared with the server

• Private Key 🔒: Kept secret by the user (on your computer)

✅ Why Key’s are Important:

• 🔒 High Security: Much safer than using passwords

• 🚫 Prevents Hacking: No one can connect without the matching private key

• 🤖 Automation: Perfect for scripts and tools that need secure, auto-login

• 🙅 Passwordless Login: Just run ssh user@ip — no typing passwords!

🧪 Example:

1. You generate the key pair:

   ssh-keygen

2. Copy the public key to the server:

   ssh-copy-id username@server-ip

3. Then connect securely:

   ssh usernameserveur iptv

👤 Users & 👪 Groups in Linux

🔹 User = A person who can log in (e.g., john, root)
🔹 Group = A collection of users with shared access

Linux uses users and groups to manage file and system permissions 🔐

🛠️ Basic Commands:

• Create user: sudo adduser john

• Create group: sudo groupadd devs

• Add user to group: sudo usermod -aG devs john

• Check your user: whoami

• List user’s groups: groups john

🔐 File access is controlled by:

• User

• Group

• Others

✅ This keeps the system secure and organized!

🔐 How to Connect to a Server via SSH

1. Get your server details:

   • IP address (e.g., 192.168.1.10)

   • Username (e.g., ubuntu, ec2-user)

2. Use the SSH command:ssh username@server-ip

   ✅ Example: ssh ubuntu@192.168.1.10

3. If using a private key (like for AWS EC2):ssh -i path/to/key.pem username@server-ip

   ✅ Example:ssh -i mykey.pem ubuntu@54.123.45.67

4. Accept the connection (first time only)

5. 🔐 You’re in! Now you can run commands on the remote server.

🔗 Connect EC2 to EC2 Using SSH

✅ 1. Make sure both EC2 instances:

• Are in the same VPC or reachable via public IP

• Have Security Group rules that allow SSH (port 22) from each other's IP

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🛠️ 2. On Source EC2 (Instance A):

• Have the private key (.pem) of the destination EC2 (Instance B)

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💻 3. SSH from Instance A to Instance B:

ssh -i "destination-key.pem" ec2-user@<instance-a-ip>

✅ Example:ssh -i "mykey.pem" user@<instance-a-ip>

• Use private IP if both EC2s are in the same VPC (faster and safer)

• Don’t forget to chmod 400 key.pem before using it!

🔐 File Permissions in Linux

Every file and folder in Linux has permissions to control who can read, write, or run it.

👥 Three Types of Users:

📄 Three Types of Permissions:

🔍 Example: Check Permissions

ls -l

You’ll see output like: -rwxr-xr-- 1 user group 1234 Jul 21 myscript.sh

Breakdown of -rwxr-xr--:

🔧 chmod Command Modify Permissions

Use the chmod command:

chmod 755 myfile

This gives:

• Owner: rwx (7)

• Group: r-x (5)

• Others: r-x (5)

linux File Permissions

🧪 Example: Make file readable & writable by owner, readable by others

chmod 644 filename

🧪 Example: Give full access to owner, read+execute to group/others

chmod 755 filename

groupadd in Linux?

groupadd is a Linux command used to create a new group 🆕👥

Groups are used to manage permissions for multiple users together.

Command: sudo groupadd groupname

• sudo: Run as superuser (you need admin rights to create a group)

• groupadd: Command to add a new group

• groupname: The name of the group you want to create

Add a user to the group

sudo usermod -aG devteam username

✅ This allows username to become a member of devteam

🔹 sudo chown — Change File Owner

The chown command lets you change the owner of a file or folder.

🧩 Syntax: sudo chown new_owner filename

✅ Example: sudo chown john report.txt

🎯 Now user john owns the file report.txt

🔸 sudo chgrp — Change File Group

The chgrp command lets you change only the group of a file or folder.

🧩 Syntax: sudo chgrp groupname filename

✅ Example: sudo chgrp devteam report.txt

🎯 Now the file belongs to the group devteam

🛠️ Why use these?

🔐 For controlling who can access or modify files
👨‍👩‍👧 Helps in multi-user environments like servers

🔍 grep – Search for text

Used to find matching words/lines in a file.

grep "apple" fruits.txt

✅ Shows all lines with the word apple.

📊 awk – Extract specific columns

Used to print selected fields (columns) from text.

awk '{print $1}' data.txt

✅ Prints the first word (or column) of each line.

👉 Combine with grep:

grep "apple" fruits.txt | awk '{print $2}'

✅ Finds lines with "apple" and shows their second column.

🛠️ sed – Stream Editor

Used for searching and replacing text.

sed 's/old/new/' filename.txt

✅ Replaces first 'old' with 'new' in each line.

👉 Replace all:

sed 's/old/new/g' filename.txt

🔍 find Command in Linux

The find command is used to search for files and directories in your Linux system based on name, size, date, type, etc. 🔎📁

✅ Basic Syntax: find [path] [options] [expression]

🌐 1. ping – Check if a host is reachable

ping google.com

📡 Sends ICMP packets to check if a server is online and how long it takes to respond.

✅ Use: Test network connection
🧪 Output: Response time (ms), packet loss

🌍 2. traceroute – Trace path of packets to a destination

traceroute google.com

🧭 Shows every hop (router) your packet passes through to reach the destination.

✅ Use: Find slow or failing network points
🔍 Output: Each step & delay in reaching a server

🕵️ 3. nslookup – Query DNS records

nslookup google.com

📖 Asks DNS servers for info like IP addresses of a domain.

✅ Use: Check domain name resolution
🌐 Output: IP address of a domain

🧠 4. dig – Advanced DNS lookup tool

dig google.com

🧪 Similar to nslookup but gives more detailed DNS info (like TTL, record types, etc.)

✅ Use: Deep dive into DNS records (A, MX, TXT, etc.)

📥 5. wget – Download files from the web

wget https://example.com/file.txt

📂 Saves files from the internet directly to your system.

✅ Use: Script-based or bulk file downloads
💡 Add -O filename to rename it while saving

🌐 6. curl – Transfer data from/to a server

curl https://example.com

🧰 Can download data, send forms, or access APIs. More flexible than wget.

✅ Use: Web testing, APIs, downloading
🔄 Supports GET, POST, PUT, DELETE, etc.