Day 02: Linux Commands

Essential Linux Commands

These commands help you interact with the Linux filesystem, create/remove files and directories, and view file contents.

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Navigation & Directory Commands

File & Directory Management

✅ Use rm -rf <dir> with caution — it will force delete everything inside.

Viewing and Writing to Files

✨ tail -f is commonly used in DevOps for log monitoring during deployments and debugging.

🧠 Tips:

• You can create and write to a file in one go using:

    echo "Hello World" > hello.txt
  

• View current directory contents neatly with:

    ls -lh
  

• Press Ctrl + C to exit from commands like tail -f or when you're stuck in terminal.

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Advanced Linux Commands (For DevOps & Power Users)

1. cp – Copy Files or Directories

cp <source> <destination>

• Copies files or folders from one place to another.

📌 Examples:

cp new_file.txt devops/             # Copy file into "devops" folder
cp devops/new_file.txt cloud/       # Copy file from devops to cloud
cp -r cloud/ devops/                # Copy entire "cloud" folder into "devops"

2. mv – Move or Rename

mv <source> <destination>

• Moves or renames files/directories.

📌 Examples:

mv new_file.txt ../cloud/           # Move file to another folder
mv devops/ linux_for_devops         # Rename the directory

3. wc – Word Count

wc <file>

Shows:

• Line count

• Word count

• Byte count

📌 Example:

cat new_file.txt            # Output: this is my file
wc new_file.txt             # Output: 1  4  16
                            # (1 line, 4 words, 16 bytes)

Can also be used with multiple files.

4. Hard Link vs Soft Link

🔗 What are Links? Links are like shortcuts to a file. There are two types:

🔧 Create a Hard Link:

ln original.txt hardlink.txt

🔧 Create a Soft Link:

ln -s original.txt softlink.txt

📌 Check Links:

ls -ltr             # Shows file type and link relationships

5. cut – Extract Bytes or Characters

cut -b 1 myfile.txt

• Extracts the first byte of each line.

• From this, it will output t.

6. tee – Output + Save Simultaneously

echo "hello" | tee new2File.txt

• Displays output in terminal.

• Also saves it into new2File.txt. Useful when you want to log something while still seeing it live.

7. sort – Sort Lines in a File

sort myfile.txt

• Sorts lines alphabetically.

• You can use flags like -r (reverse), -n (numeric sort).

8. diff – Compare Two Files

diff file1.txt file2.txt

• Shows the differences between two files.

• Very useful for config comparison, troubleshooting, etc.

9. vi – Vim/Vi Text Editor

vi <filename>

Basic Controls:

• i – Enter insert mode to type

• Esc – Exit insert mode

• :w – Write (save) the file

• :q – Quit

• :wq – Save and quit

Vi is essential when working on remote Linux servers without a GUI.

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Login Related: SSH & Remote Access

✅ What is SSH? SSH stands for Secure Shell — it is a secure way to remotely connect to another computer/server over a network.

• Default Port: 22

• It encrypts the connection, making it safe even over public networks.

• Commonly used in DevOps to connect to Linux servers (like EC2 in AWS).

🔑 SSH Key-Based Authentication (Public & Private Key) SSH works using a pair of cryptographic keys:

🔁 How It Works (Real Example) Let’s say:

• You're on your laptop.

• You want to connect to an AWS EC2 instance (server).

Behind the scenes:

1. You use your private key (on your laptop).

2. The EC2 server checks your public key (already stored on the instance).

3. If both match, the connection is allowed.

💡 AWS Key Pair (Real DevOps Scenario) When you launch an EC2 instance:

• AWS runs ssh-keygen in the background to create:

  • A public key → automatically added to the EC2 instance.

  • A private key → you download it as a .pem file.

🧭 Steps to SSH into an EC2 Instance (Linux/Mac Terminal)

1. Open Terminal.

2. Locate your .pem file (private key).

   • Make sure the file is in a secure location.

3. Set correct permission for the file.

   • Run this to make it private:

       chmod 400 your-key.pem
     

4. SSH Command to Connect:

    ssh -i path/to/your-key.pem ubuntu@<PublicDNS>
   

   • -i specifies the private key.

   • ubuntu is the default username for Ubuntu AMIs (it may vary).

   • <PublicDNS> is the server's DNS name (e.g., ec2-13-...amazonaws.com).

🖥️ Using SSH in Windows (via PuTTY) Windows doesn’t support .pem files directly in PowerShell. So:

1. Download PuTTY.

2. Convert .pem to .ppk using PuTTYgen.

3. Use PuTTY to connect by entering:

   • Hostname: ec2-user@<PublicDNS>

   • Auth: Browse and select the .ppk key file.

📌 Quick Notes:

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Disk Usage Commands (Storage Monitoring)

These commands help you monitor disk space — essential for system health, especially in production environments.

1. df – Disk Free

df

• Shows total, used, and available space on mounted file systems.

df -h

• Adds human-readable format (e.g., GB, MB instead of blocks). Use this to check if your root / or /home partitions are running out of space.

2. du – Disk Usage

du .

• Shows disk usage of the current directory and its contents.

du -sh <folder-name>

• -s: Summary, -h: Human-readable.

• Use to check how much space a particular folder is using.

3. ls -a – List All Files (Including Hidden)

ls -a

• Lists all files and folders, including hidden files (those starting with .).

🧠 Common Use Case Example:

df -h              # Check overall disk space
du -sh /var/log    # See log folder size
ls -a              # Find hidden `.config`, `.env` files

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Process Management Commands

Used to view, monitor, and kill running processes.

1. ps – Process Status

ps

• Shows currently running processes in current terminal session.

ps aux | grep <name>

• Lists all system processes, you can filter by name.

2. top – Real-time Process Monitoring

top

• Displays all running processes in real-time, including CPU and memory usage.

• Press q to quit.

3. fuser – Show Process Using a File/Directory

fuser .

• Shows process IDs (PIDs) accessing the current folder.

4. kill – Terminate a Process

kill -9 <pid>

• Sends SIGKILL signal to forcefully stop a process. Use ps or top to get the PID (Process ID).

5. free – Memory Usage

free

• Shows system memory info (RAM, swap).

free -h

• Human-readable format.

🌀 nohup – Run Commands That Ignore Hangups Used when running long commands or background jobs — even after you log out.

nohup <command>

📌 Example:

nohup free -h

• Output is saved in a file named nohup.out by default.

nohup df -h

• Appends disk usage result to the same file. Useful for scripts or logs that should run independently of the shell.

6. vmstat – Virtual Memory Statistics

vmstat

• Shows info about memory, CPU usage, processes, I/O.

vmstat -a

• Displays active and inactive memory details.

✅ Summary Table: