👈

Linux Basics

1️⃣ What is Linux?

Linux is an operating system, just like:

• Windows
• macOS

But Linux is:

• Free
• Open source
• Very powerful
• Mostly used on servers

👉 AWS uses Linux on most servers (EC2)

So, when you use AWS, you are actually using Linux machines.

2️⃣ Why Linux is important for AWS?

In AWS:

• EC2 instances mostly run Linux
• Docker runs on Linux
• Kubernetes runs on Linux
• CI/CD servers run on Linux

💡 If you know Linux:

• You can manage EC2 easily
• You can debug issues faster
• You can deploy applications confidently

3️⃣ Linux Distributions (Very Important)

Linux has many versions. These are called distributions (distros).

Common Linux distros in AWS:

👉 For AWS learning, focus on:

• Amazon Linux
• Ubuntu

4️⃣ Linux is Case-Sensitive ⚠

Linux treats:

File.txt ≠ file.txt
DEV ≠ dev

This is very important in AWS, especially:

• File names
• Paths
• Scripts

5️⃣ Linux Directory Structure (High-Level)

Linux files are organized like a tree.

Important folders you must know:

Example:

/home/ec2-user
/etc/nginx/nginx.conf
/var/log/messages

👉 In AWS, logs are often in /var/log

6️⃣ Root User vs Normal User

Root user

• Full access
• Can do anything
• Very dangerous if misused

Normal user (Recommended)

• Limited access
• Uses sudo for admin tasks

Example:

sudo yum install nginx

👉 On AWS EC2:

• Default user is NOT root

• Example users:

  • ec2-user (Amazon Linux)
  • ubuntu (Ubuntu)

7️⃣ Linux Shell & Terminal

Shell

• A program that understands your commands

Common shell:

• bash (most common)

Terminal

• The place where you type commands

In AWS:

• You connect to EC2
• You get a terminal
• You run Linux commands

8️⃣ Linux vs Windows (Simple Comparison)

AWS = Linux-first

9️⃣ What you should remember from Topic 1

✅ Linux is the backbone of AWS ✅ AWS EC2 mostly runs Linux ✅ Linux is case-sensitive ✅ Know basic directories ✅ Root vs normal user concept

File & Directory Management (Linux)

1️⃣ What is a File & Directory?

• File → Stores data Example: app.js, config.yml, log.txt

• Directory (folder) → Stores files and other folders Example: /home/ec2-user, /var/log

👉 On AWS EC2, everything you do is inside files & directories.

2️⃣ Current Working Directory

When you open a terminal, you are inside a directory.

Command:

pwd

📌 Shows where you are now

Example output:

/home/ec2-user

3️⃣ Listing Files & Folders

ls        # list files
ls -l     # detailed list
ls -a     # show hidden files
ls -la    # detailed + hidden

• l → long listing format (Shows detailed information about files)
• a → all files, including hidden files (those starting with .)

Columns explanation (left to right)

Example line:

-rw-rw-r--. 1 ec2-user ec2-user 53142881 Jan  9 13:41 EWA-1.0.0.jar

🔹 Column 1: File type + Permissions

-rw-rw-r--.

Breakdown:

File type (first character)

🔹 Column 2: Number of links

1

• For files → usually 1
• For directories → number of subdirectories + itself

🔹 Column 3: Owner (User)

ec2-user

• The user who owns the file

🔹 Column 4: Group

ec2-user

• The group that owns the file

🔹 Column 5: File size (in bytes)

53142881

• Size is always in bytes
• Example: 53142881 bytes ≈ 50.7 MB

🔹 Column 6–8: Date & Time (Last Modified)

Jan  9 13:41

• This shows last modification time
• If the file is old, you’ll see year instead of time.

🔹 Column 9: File / Directory name

EWA-1.0.0.jar

• Actual name of the file or directory
• Hidden files start with .

Example: .bashrc, .ssh

Special entries in your output

🔸 . (current directory)

drwx------. 3 ec2-user ec2-user 131 Jan  9 15:37 .

• Refers to current directory

🔸 .. (parent directory)

drwxr-xr-x. 3 root root 22 Jan  9 13:15 ..

• Refers to parent directory

4️⃣ Changing Directories

cd folder-name     # go inside folder
cd ..              # go back one level
cd ~               # go to home directory
cd /               # go to root

Example:

cd /var/log

5️⃣ Absolute vs Relative Paths (VERY IMPORTANT)

Absolute Path

• Starts from /
• Works from anywhere

Example:

/home/ec2-user/app/config.yml

Relative Path

• Based on current directory

Example:

config.yml
../logs/app.log

👉 In AWS scripts, absolute paths are safer.

6️⃣ Creating Files & Directories

touch file.txt         # create empty file
mkdir folder           # create folder
mkdir -p a/b/c         # create nested folders

• -p → Parents (Create parent directories as needed.)

7️⃣ Copying Files & Directories

cp file1 file2         # copy file
cp file folder/        # copy into folder
cp -r dir1 dir2        # copy directory

• First = OLD (source)
• Second = NEW (destination)

Example:

cp config.yml backup.yml

8️⃣ Moving & Renaming

mv oldname newname     # rename
mv file folder/        # move

Example:

mv app.log app-old.log

9️⃣ Deleting Files & Directories ⚠

rm file               # delete file
rm -r folder          # delete folder
rm -rf folder         # force delete (DANGEROUS)

• -r → Recursive (Remove the folder and everything inside it, recursively)
• -f → Force (Forcefully remove the folder and all its contents recursively, without asking for confirmation)

⚠ Important warning

• Linux has NO recycle bin
• rm -rf / can destroy the system

👉 Be extra careful on production EC2 servers.

🔟 Useful Shortcuts

cd .     # current directory
cd ..    # parent directory
cd ~     # home directory
cd /     # root directory

1️⃣1️⃣ What you must remember (Exam + Real AWS)

✅ Linux has no GUI on EC2 ✅ You manage everything via files & folders ✅ Paths matter (case-sensitive) ✅ rm -rf is dangerous ✅ Absolute paths are safer in AWS scripts

File Viewing & Editing (Linux)

1️⃣ Why this topic is important for AWS?

On AWS EC2, you will:

• Read log files
• Edit configuration files
• Check application output
• Debug errors

2️⃣ Viewing File Content (Basic)

cat – show entire file

cat file.txt

Use when:

• File is small

⚠ Not good for large log files.

less – best command (recommended)

less file.txt

Controls:

• ↑ ↓ → scroll
• Space → next page
• /error → search
• q → quit

more – simple pager

more file.txt

Less powerful than less.

3️⃣ Viewing Part of a File

head – beginning of file

head file.txt
head -n 20 file.txt

• -n → Number of lines (Show N number of lines, by defult 10).

tail – end of file

tail file.txt
tail -n 50 file.txt

🔥 tail -f – LIVE log monitoring (VERY IMPORTANT)

tail -f /var/log/nginx/access.log

• -f → Follow (keep following the file and show new lines as they are added)

👉 Used to:

• Monitor running applications
• Debug live issues on EC2

Press Ctrl + C to stop.

4️⃣ Editing Files in Linux

On AWS EC2, you usually edit files using:

• nano (easy)
• vi / vim (powerful)

5️⃣ nano Editor (Beginner Friendly)

nano file.txt

Controls:

• Type → edit
• Ctrl + O → save
• Enter → confirm
• Ctrl + X → exit

👉 Recommended when you are starting AWS.

6️⃣ vi Editor (Basic Knowledge Enough)

Open file:

vi file.txt

Modes in vi

• Normal mode → navigation
• Insert mode → editing
• Command mode → save/exit

Important commands:

i       → insert mode
Esc     → normal mode
:w      → save
:q      → quit
:wq     → save & quit
:q!     → quit without saving

8️⃣ What you must remember

✅ Use less for reading files ✅ Use tail -f for live logs ✅ Use nano for easy editing ✅ Know basic vi commands ✅ Use sudo for system files

Excellent 👍 Now we are at Topic 4: File Permissions & Ownership. This topic is CRITICAL for AWS. Many EC2 issues happen only because of permissions.

I’ll explain slowly, clearly, and with real AWS examples.

File Permissions & Ownership (Linux)

1️⃣ Why permissions are important in AWS?

On AWS EC2:

• Applications fail to start ❌
• Scripts don’t run ❌
• Logs can’t be written ❌

Most of the time the reason is: 👉 Wrong file permissions

2️⃣ Understanding File Permissions

Run:

ls -l

Example output:

-rwxr-xr-- 1 ec2-user ec2-user  512 app.sh

Let’s break it down.

3️⃣ Permission Structure (Very Important)

-rwx r-x r--
  │    │   │
  │    │   └── Others
  │    └────── Group
  └────────── User (Owner)

Permission letters:

• r → read
• w → write
• x → execute
• - → no permission

4️⃣ User, Group, Others

👉 Linux always checks permissions in this order: User → Group → Others

5️⃣ File vs Directory Permissions

File

• r → read file
• w → modify file
• x → run file (script)

Directory

• r → list files
• w → create/delete files
• x → enter directory

👉 Without x on a directory, you cannot cd into it.

6️⃣ Changing Permissions chmod (change mode)

Symbolic mode

chmod u+x file.sh              # add execute to user
chmod g-w file.txt             # remove write from group
chmod o+r file.txt             # add read to others
chmod u=rwx,g=rx,o=r file.txt  # set exact permissions
chmod a+x script.sh            # add execute to all
chmod a-r file.txt             # remove read from all
chmod a=r file.txt             # set read for all
chmod +x script.sh             # add execute to all

• a means all (user, group, others)
• u means user (owner)
• g means group
• o means others

Numeric mode (VERY IMPORTANT)

Example:

chmod 755 app.sh

Meaning:

User   → 7 (rwx)
Group  → 5 (r-x)
Others → 5 (r-x)

7️⃣ Making a Script Executable (AWS COMMON)

If you see:

permission denied

Fix:

chmod +x script.sh
./script.sh

👉 Common in user-data scripts.

8️⃣ File Ownership chown (change owner)

Change owner:

chown user file.txt  # user → new owner (user)

Change owner & group:

chown user:group file.txt  # user → new owner, group → new group

Example (AWS):

sudo chown ec2-user:ec2-user app.log 

9️⃣ Why sudo is needed

System files are owned by root.

Example:

sudo chmod 644 /etc/nginx/nginx.conf

Without sudo:

Permission denied

🔟 Common AWS Permission Issues

❌ App cannot write logs ❌ Script not executing ❌ Nginx cannot access files ❌ Docker volume permission issues

1️⃣1️⃣ Real AWS Example

ls -l /var/www/html
sudo chown -R nginx:nginx /var/www/html
sudo chmod -R 755 /var/www/html

• -R means recursive (apply the command to the folder AND everything inside it)

1️⃣2️⃣ What you MUST remember

• Permissions = User | Group | Others
• chmod controls access
• chown controls ownership
• 755 is very common in AWS
• sudo is required for system files

User & Group Management (Linux)

1️⃣ Why this topic is important for AWS?

On AWS EC2:

• Multiple people may access one server
• Applications should not run as root
• Security is very important

👉 Linux users & groups help control access

2️⃣ What is a User?

A user is an account that can:

• Login to Linux
• Run commands
• Own files
• Run applications

3️⃣ Root User vs Normal User

Root user

• Superuser
• Full access
• Can break the system

Normal user (Recommended)

• Limited permissions
• Uses sudo for admin tasks

👉 On AWS:

• You login as ec2-user or ubuntu
• You use sudo when needed

4️⃣ What is a Group?

A group is a collection of users.

Why groups?

• Easier permission management
• Used by applications (nginx, docker, etc.)

5️⃣ Important User Commands

Check current user

whoami

User details

id

6️⃣ Creating a User (Admin Task)

sudo useradd devuser
sudo passwd devuser

7️⃣ Switching Users

su devuser

Switch to root:

sudo su -

⚠ Be careful with root.

8️⃣ Deleting a User

sudo userdel devuser

9️⃣ Group Management

Create group

sudo groupadd devgroup

Add user to group

sudo usermod -aG devgroup devuser

Example (AWS common):

sudo usermod -aG docker ec2-user

🔟 AWS Real-Life Examples

• Add user for SSH access
• Add user to docker group
• Restrict access to logs
• Avoid using root

Process Management (Linux)

1️⃣ What is a Process?

A process is: 👉 A running program in Linux

Examples:

• Nginx running
• Java application running
• Node.js server running
• SSH session running

If a program is running → it has a process ID (PID).

2️⃣ Process ID (PID)

Every process has a unique number called PID.

Example:

nginx → PID 1345
java  → PID 2098

👉 PID is used to monitor or kill a process.

3️⃣ Foreground vs Background Process

Foreground Process

• Runs in terminal
• Blocks the terminal

Example:

sleep 30  # sleep for 30 seconds

Terminal is busy until process stops.

Background Process

• Runs in background
• Terminal is free

Example:

sleep 30 &  # run in background

4️⃣ Sending Process to Background

If a process is running:

• Press Ctrl + Z → pause
• Then run:

bg

5️⃣ Process States (High-Level)

👉 Zombie processes = badly written apps.

6️⃣ Monitoring Processes

ps – process snapshot

ps
output: 
    PID TTY          TIME CMD
  48615 pts/7    00:00:00 bash
  49301 pts/7    00:00:00 sleep
  49303 pts/7    00:00:00 ps

• PID (Process ID): Unique identifier for each running process.
• TTY (Terminal Type): The terminal associated with the process.
• TIME: The amount of CPU time the process has used.
• CMD (Command): The command that started the process.

ps aux

What this command means

• ps → show running processes
• a → processes for all users
• u → show user-oriented format (CPU, memory, etc.)
• x → include processes not attached to a terminal

Most common:

ps aux | less

ps -p PID # show specific process by PID

top – live monitoring (VERY IMPORTANT)

top

Shows:

• CPU usage
• Memory usage
• Running processes

Press q to quit.

7️⃣ Finding a Specific Process

ps aux | grep nginx
ps aux | grep java

Used when:

• App is running but not responding

8️⃣ Killing Processes (IMPORTANT)

Graceful stop

kill PID

Force stop (DANGEROUS)

kill -9 PID

⚠ Use -9 only if normal kill fails.

9️⃣ Kill by Name

pkill nginx
killall node

Useful when PID is unknown.

🔟 AWS Real-Life Examples

Nginx consuming high CPU

top
ps aux | grep nginx
sudo kill PID

1️⃣1️⃣ Background Jobs

List jobs:

jobs

Bring to foreground:

fg  

The fg command is used in Linux/Unix shells to resume a stopped or background job and bring it to the foreground.

1️⃣3️⃣ Background Processes in AWS (nohup)

What happens with & (background only)

• & runs the command in the background Example:

java -jar EWA-1.0.0.jar > app.log 2>&1 &

✅ What this does:

• Runs the app in the background
• You can still use the same terminal

❌ What happens when you:

• Close SSH
• Network disconnects
• Session times out

👉 Process gets killed

Why this happens (important)

Processes started with & still receive SIGHUP (hangup signal) when the terminal closes.

Linux says:

“Terminal gone → kill child processes”

What nohup does

Example nohup java -jar EWA-1.0.0.jar > app.log 2>&1 &

✅ What this does:

• Ignores SIGHUP
• Process keeps running after logout
• Fully detached from terminal

That’s why nohup = NO HANG UP

1️⃣2️⃣ What you MUST remember

• ✅ Process = running program
• ✅ Every process has PID
• ✅ top is your best friend
• ✅ kill stops processes
• ✅ Use kill -9 carefully
• ✅ Background processes are common in AWS

Networking Basics (Linux)

1️⃣ IP Address & Hostname

IP Address

An IP address uniquely identifies a machine on a network.

Example:

172.31.12.45

In AWS EC2:

• Private IP → inside VPC
• Public IP → access from internet

Check IP Address

ip a   # show IP addresses

or

ifconfig  # older command

Hostname

A hostname is the name of the machine.

Check hostname:

hostname

Set hostname:

sudo hostnamectl set-hostname my-ec2-server

2️⃣ Network Interfaces

A network interface connects your server to the network.

Example:

eth0
ens5

Check interfaces:

ip link

3️⃣ Ports & Services (VERY IMPORTANT)

Port

A port is a communication endpoint.

Common ports:

Check Open Ports

netstat -tulnp  # Show all TCP ports that are currently listening, with process info.
or
ss -tulnp  # Modern replacement for netstat

output:

State   Recv-Q  Send-Q  Local Address:Port  Peer Address:Port  Process
LISTEN  0       128           0.0.0.0:22         0.0.0.0:*
LISTEN  0       128              [::]:22            [::]:*
LISTEN  0       511                 *:80               *:*
LISTEN  0       100                 *:8080             *:*      users:(("java",pid=51635,fd=19))

4️⃣ Connectivity Testing

ping – test network reachability

ping google.com
ping 8.8.8.8

Stop with Ctrl + C.

⚠ AWS Security Groups may block ping.

curl – test web & APIs (VERY IMPORTANT)

curl http://localhost
curl http://public-ip
curl https://api.example.com

Used to:

• Test APIs
• Check if app is running

telnet / nc – test port connectivity

telnet localhost 80
nc -zv localhost 8080

5️⃣ Downloading from Internet

wget

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

curl download

curl -O https://example.com/file.zip

Used in:

• App setup
• User-data scripts
• CI/CD pipelines

6️⃣ AWS Real-Life Debugging Scenario

❌ App not accessible from browser

Steps:

ip a
ss -tulnp
curl localhost

If works locally but not from browser: 👉 Issue is Security Group / NACL, not Linux.

7️⃣ What you MUST remember

• ✅ IP identifies the server
• ✅ Ports expose services
• ✅ ss -tulnp shows listening ports
• ✅ curl is essential for AWS
• ✅ Download tools are used everywhere

Disk & Storage Management (Linux)

🧱 What is Disk & Storage Management in Linux?

Disk & Storage Management means:

• Detecting disks
• Creating partitions
• Formatting filesystems
• Mounting disks
• Managing space
• Monitoring disk usage
• Handling permissions & performance

👉 In simple words:

It is how Linux stores, organizes, and accesses your data on disks

🖴 Types of Storage Devices in Linux

🧭 Disk Naming in Linux

Linux treats everything as a file.

Check disks:

lsblk

📦 Partitions (Why needed?)

A partition divides a disk into logical sections.

Example:

• Disk: 1 TB
• Partition 1: OS
• Partition 2: Home
• Partition 3: Backup

View partitions:

lsblk
fdisk -l

🛠 Partitioning Tools

Create partition (example):

sudo fdisk /dev/sdb

Common fdisk commands:

• n → new partition
• p → print table
• w → write changes

🧬 Filesystem (VERY IMPORTANT)

A filesystem defines how data is stored & retrieved.

Format partition:

sudo mkfs.ext4 /dev/sdb1

📌 Mounting (Make storage usable)

Linux needs to mount a disk to access it.

Temporary Mount

sudo mount /dev/sdb1 /mnt/data

Check mounts:

df -h
mount

Unmount:

sudo umount /mnt/data

📌 Permanent Mount (fstab)

File:

/etc/fstab

Example entry:

/dev/sdb1  /data  ext4  defaults  0  2

Apply:

sudo mount -a

⚠️ Wrong fstab = system boot failure

📊 Disk Usage Monitoring

Check disk space

df -h

Check folder size

du -sh /var/log

Find large files

find / -size +1G

📂 Inodes (Often ignored but important)

Each file uses an inode.

Check inode usage:

df -i

Problem:

Disk shows free space but still “No space left”

Cause:

Inodes exhausted

🔐 Permissions & Ownership

Check:

ls -l

Format:

-rwxr-xr--

Change owner:

sudo chown user:group file

Change permission:

chmod 755 file

🔥 Logical Volume Management (LVM)

LVM allows:

• Resize disks
• Combine multiple disks
• Snapshot backups

LVM Components

Commands:

pvcreate /dev/sdb
vgcreate vg_data /dev/sdb
lvcreate -L 10G -n lv_data vg_data

Format & mount:

mkfs.ext4 /dev/vg_data/lv_data
mount /dev/vg_data/lv_data /data

☁️ Disk Management in Cloud (AWS EC2)

List disks:

lsblk

Extend EBS volume:

growpart /dev/xvda 1
resize2fs /dev/xvda1

🧯 Disk Errors & Health

Check filesystem:

fsck /dev/sdb1

SMART health:

smartctl -a /dev/sda

🚨 Common Disk Problems

🧠 Important Linux Directories

🧪 Real-World Example

Production server is down due to disk full

Steps:

df -h
du -sh /var/*
rm -rf /var/log/*.gz

🎯 Interview-Ready Questions

Q: What is the difference between partition & filesystem?

• Partition divides disk
• Filesystem organizes data

Q: What is LVM?

• Logical Volume Manager used to resize and manage disks dynamically

Q: What happens if fstab is wrong?

• System may fail to boot

🧾 Must-Know Commands (Cheat Sheet)

lsblk
df -h
du -sh
mount
umount
fdisk
mkfs
fsck
chmod
chown

Package Management (Linux)

📦 What is Package Management?

Package Management is the system used to:

• Install software
• Update software
• Remove software
• Manage dependencies
• Keep software secure and up to date

👉 In simple words:

It is how Linux installs and manages software safely

🧠 What is a Package?

A package is:

• A compiled application
• With configuration files
• With dependency information
• With version details

🏗 Why Package Management Exists

Without package management:

• Manual downloads
• Dependency hell
• Broken systems

With package management:

• Automatic dependency resolution
• Central repositories
• Security updates

🗂 Package Management Components

🧰 Common Linux Package Managers

👉 Amazon Linux is most common in AWS, so focus mainly on yum / dnf, but know apt basics.

1️⃣ yum – (Amazon Linux, RHEL, CentOS)

🔹 What is yum?

yum = Yellowdog Updater Modified It downloads software from repositories, resolves dependencies, and installs packages.

🔹 Update package list

sudo yum check-update

What it does:

• Checks for available updates
• Does NOT install anything

📌 Use before upgrading servers in AWS.

🔹 Install a package

sudo yum install nginx

Meaning:

• install → install software
• nginx → package name

AWS examples:

sudo yum install httpd      # Apache web server
sudo yum install docker    # Docker
sudo yum install git       # Git

🔹 Install without confirmation

sudo yum install nginx -y

-y = yes automatically

📌 Very common in EC2 user-data scripts

🔹 Remove a package

sudo yum remove nginx

Remove unused deps

sudo yum autoremove

Removes software (not always config files).

🔹 Update all packages

sudo yum update

or

sudo yum update -y

📌 Used during EC2 patching & security updates.

🔹 Search for a package

yum search docker

Finds package names from repositories.

🔹 Show package info

yum info nginx

Shows:

• Version
• Size
• Repository
• Description

🔹 List installed packages

yum list installed 

To check what you installed recently:

yum history

🔹 Check if a package is installed

yum list installed nginx

🔹 Clean cache

sudo yum clean all

Clears downloaded package cache.

📌 Used when yum behaves incorrectly.

2️⃣ dnf – (Amazon Linux 2023)

🔹 What is dnf?

dnf = Dandified YUM It is the modern replacement for yum.

👉 Commands are almost the same.

sudo dnf install nginx
sudo dnf remove nginx
sudo dnf update -y
dnf search docker
dnf info nginx

📌 If AWS exam mentions Amazon Linux 2023 → think dnf.

3️⃣ apt – (Ubuntu on EC2)

🔹 Update package index (VERY IMPORTANT)

sudo apt update

📌 Always run this before installing anything.

🔹 Install a package

sudo apt install nginx

🔹 Install with auto yes

sudo apt install nginx -y

🔹 Remove a package

sudo apt remove nginx

Remove + config:

sudo apt purge nginx

🔹 Upgrade installed packages

sudo apt upgrade

or

sudo apt upgrade -y

🔹 Full upgrade (handles dependencies)

sudo apt full-upgrade

🔹 Search package

apt search docker

🔹 Show package info

apt show nginx

🔹 List installed packages

apt list --installed

4️⃣ Repository Management (AWS Important)

🔹 What is a repository?

A repo is a server that stores packages.

AWS examples:

• Amazon Linux repos
• EPEL (Extra Packages for Enterprise Linux)

🔹 List yum repositories

yum repolist

or

dnf repolist

🔹 Enable Amazon Linux extras

sudo amazon-linux-extras list

Install from extras:

sudo amazon-linux-extras install docker

📌 VERY important for AWS EC2

5️⃣ Package Files (rpm & dpkg)

🔹 RPM (Amazon Linux / RHEL)

rpm -qa

List all installed rpm packages.

rpm -q nginx

Check if nginx installed.

🔹 DPKG (Ubuntu)

dpkg -l

dpkg -l nginx

6️⃣ Real AWS EC2 Examples 🔥

🔹 Install web server on EC2 (Amazon Linux)

sudo yum update -y
sudo yum install httpd -y
sudo systemctl start httpd
sudo systemctl enable httpd

🔹 Install Docker on EC2

sudo yum install docker -y
sudo systemctl start docker
sudo systemctl enable docker

🔹 EC2 User-Data Script Example

#!/bin/bash
yum update -y
yum install httpd -y
systemctl start httpd

📌 Package commands are heavily used in user-data.

7️⃣ AWS Exam Focus (Remember This)

• ✅ Know which OS uses which package manager
• ✅ Understand install, remove, update, search
• ✅ Know -y flag
• ✅ Know amazon-linux-extras
• ✅ Know difference between yum / dnf / apt

🧠 Simple Rule to Remember

Amazon Linux → yum / dnf Ubuntu → apt

🎯 Interview-Ready Questions

Q: What is a package manager?

A tool that installs, updates, removes, and manages software along with its dependencies.

Q: Difference between apt & yum?

• apt → Debian based
• yum/dnf → RHEL based

Q: What is a repository?

A centralized storage location for packages.

🏁 Final Summary

Package management in Linux ensures safe, fast, and dependency-aware software installation and maintenance, making Linux systems stable and secure.

Service Management (systemd)

🔧 What is Service Management?

Service Management means:

• Starting services (like nginx, mysql, ssh)
• Stopping services
• Restarting services
• Enabling services at boot
• Checking service health

👉 In simple words:

It is how Linux runs and controls background programs (services/daemons)

🧠 What is systemd?

systemd is the service manager in Linux.

• The init system of modern Linux
• The first process started by the kernel
• Runs with PID 1

It:

• Starts services
• Stops services
• Restarts services
• Starts services automatically at boot Check:

ps -p 1

Output:

systemd

🔹 Main command: systemctl

systemctl = system control

This is the only command you need to manage services.

🔄 What is a Service / Daemon?

A service (daemon) is a program that:

• Runs in background
• Starts at boot (optional)
• Provides functionality

Examples:

• ssh → Remote login
• nginx → Web server
• docker → Container engine
• mysql → Database

🗂 systemd Unit Types

systemd manages units.

1️⃣ Start a Service

sudo systemctl start httpd

Meaning:

• start → start the service now
• httpd → service name

📌 After starting, service runs until reboot (unless enabled).

2️⃣ Stop a Service

sudo systemctl stop httpd

Stops the service immediately.

📌 Website goes down if web server stops.

3️⃣ Restart a Service (VERY COMMON)

sudo systemctl restart httpd

Used when:

• Config file changes
• App update
• Debugging issues

📌 Most used command in real AWS work.

4️⃣ Reload a Service

sudo systemctl reload httpd

Difference:

📌 Not all services support reload.

5️⃣ Check Service Status ⭐ (MOST IMPORTANT)

systemctl status httpd

Shows:

• Is service running?
• Error messages
• Logs (last lines)
• PID

📌 First command to run when something breaks

6️⃣ Enable Service at Boot (AWS CRITICAL)

sudo systemctl enable httpd

Meaning:

• Service starts automatically when EC2 reboots

📌 Without this:

• After EC2 restart → website DOWN ❌

7️⃣ Disable Service at Boot

sudo systemctl disable httpd

Service will NOT start on reboot.

8️⃣ Start + Enable Together

sudo systemctl enable --now httpd

Meaning:

• Start service now
• Enable it for boot

📌 Clean & professional command.

9️⃣ List All Services

🔹 All running services

systemctl list-units --type=service

🔹 All services (enabled + disabled)

systemctl list-unit-files --type=service

🔟 Check if Service is Enabled

systemctl is-enabled httpd

Output:

• enabled
• disabled

1️⃣1️⃣ Check if Service is Active

systemctl is-active httpd

Output:

• active
• inactive
• failed

1️⃣2️⃣ View Service Logs (AWS Debugging)

journalctl -u httpd

Last logs only:

journalctl -u httpd -n 20

Live logs:

journalctl -u httpd -f

📌 Used when:

• Service fails to start
• Port issues
• Permission issues

1️⃣3️⃣ Common AWS Services & Names

📌 Service name ≠ package name sometimes.

1️⃣4️⃣ Real AWS EC2 Examples 🔥

✅ Install & run Apache (Amazon Linux)

sudo yum install httpd -y
sudo systemctl start httpd
sudo systemctl enable httpd

✅ After config change

sudo vi /etc/httpd/conf/httpd.conf
sudo systemctl restart httpd

✅ Debug service failure

systemctl status httpd
journalctl -u httpd

1️⃣5️⃣ User-Data Script Example (AWS)

#!/bin/bash
yum install httpd -y
systemctl start httpd
systemctl enable httpd

📌 systemctl works perfectly in user-data

1️⃣6️⃣ systemd Files (High Level – Exam Only)

Service files location:

/etc/systemd/system/
/usr/lib/systemd/system/

Example file:

httpd.service

You usually DON’T edit these in AWS, just manage them.

1️⃣7️⃣ systemd vs init (Interview / Exam)

🧠 One-line Rule

Install → Start → Enable → Check Status

🧩 Service States

⚙️ Anatomy of a Service File

Location:

/lib/systemd/system/

or

/etc/systemd/system/

Example: myapp.service

[Unit]
Description=My Custom App
After=network.target

[Service]
ExecStart=/usr/bin/python3 /app/main.py
Restart=always
User=ec2-user

[Install]
WantedBy=multi-user.target

🧠 Key Sections Explained

[Unit]

• Dependencies
• Startup order

[Service]

• Command to run
• Restart policy
• User/group

[Install]

• Boot target

🔁 Reload systemd (IMPORTANT)

After editing service file:

sudo systemctl daemon-reload
sudo systemctl restart myapp

⏱ systemd Timers (cron replacement)

Timer example

[Timer]
OnBootSec=5min
OnUnitActiveSec=1h

Enable timer:

sudo systemctl enable backup.timer
sudo systemctl start backup.timer

🧯 Common systemd Problems

Debug:

journalctl -xe

🧪 Common Admin Commands

🔹 1️⃣ systemctl reboot

systemctl reboot

What it does

👉 Reboots (restarts) the entire system

It is the same as:

• Turning the server OFF
• Then turning it ON again

What happens internally

• All running services are stopped safely
• Files are synced to disk
• Kernel restarts
• System boots again

When to use

• After kernel updates
• System stuck or unstable
• Configuration changes that need reboot

⚠️ Warning

• SSH connection will be lost
• On EC2 → instance restarts

🔹 2️⃣ systemctl poweroff

systemctl poweroff

What it does

👉 Shuts down the system completely

Think of it as:

“Turn off the server”

What happens

• Services stop
• System shuts down
• Power is off

On AWS EC2

⚠️ Important:

• Instance goes to stopped state
• You must start it again manually
• Public IP may change (if not elastic IP)

When to use

• Maintenance
• Cost saving (EC2)
• Permanent shutdown

🔹 3️⃣ systemctl daemon-reexec

systemctl daemon-reexec

This one is special 🧠 (often confusing)

What it does

👉 Restarts systemd itself, NOT the system

• Reloads the systemd binary
• Keeps services running
• Does NOT reboot

Why is this needed?

Used when:

• systemd package is updated
• systemd configuration changes
• systemctl behaves strangely

What does NOT happen

❌ No reboot ❌ No shutdown ❌ No service stop

🧪 Simple Comparison Table

⚠️ Permissions

All these commands need root access:

sudo systemctl reboot

sudo systemctl poweroff

sudo systemctl daemon-reexec

🎯 Interview-Ready Q&A

Q: What is systemd?

systemd is a system and service manager used in modern Linux distributions that initializes the system and manages services.

Q: What is PID 1?

The first process started by the kernel, responsible for starting all other services.

Q: Difference between reload & restart?

• Reload: re-read config
• Restart: stop and start

🧾 Must-Know Commands Cheat Sheet

systemctl status
systemctl start
systemctl stop
systemctl restart
systemctl enable
systemctl disable
journalctl

🏁 Final Summary

systemd is the backbone of modern Linux service management, controlling how services start, stop, restart, log, and behave during system boot and runtime.

Environment Variables & Shell

1️⃣ What are Environment Variables?

Environment variables are: 👉 Key–value pairs used by the system and applications.

Example:

JAVA_HOME=/usr/lib/jvm/java-17
PORT=8080
DB_HOST=localhost

👉 In AWS, environment variables are used everywhere.

2️⃣ Why Environment Variables are Important in AWS?

Used for:

• App configuration
• Database credentials
• Ports
• AWS SDK settings
• CI/CD pipelines

👉 Avoid hard-coding values in code.

3️⃣ Viewing Environment Variables

View all

env

or

printenv

View specific variable

echo $HOME
echo $PATH

4️⃣ PATH Variable (VERY IMPORTANT)

PATH tells Linux where to find commands.

Check PATH:

echo $PATH

Example output:

/usr/local/bin:/usr/bin:/bin

👉 If a command is not in PATH → “command not found”.

5️⃣ Setting Environment Variables (Temporary)

export APP_ENV=prod
export PORT=8080

Valid only:

• For current session
• Until terminal is closed

6️⃣ Setting Environment Variables (Permanent)

For current user

Edit:

nano ~/.bashrc

Add:

export APP_ENV=prod
export JAVA_HOME=/usr/lib/jvm/java-17

Apply changes:

source ~/.bashrc

System-wide (All users)

Edit:

sudo nano /etc/environment

Example:

APP_ENV=prod

7️⃣ Shell Configuration Files

Important shell files:

👉 AWS EC2 user config is usually in .bashrc.

8️⃣ Using Environment Variables in Apps

Example:

echo $DB_HOST

In shell script:

echo "Running in $APP_ENV mode"

9️⃣ AWS Real-Life Example

Java App on EC2

export DB_URL=jdbc:mysql://localhost:3306/appdb
export DB_USER=admin
export DB_PASS=secret

App reads these values at runtime.

🔟 Common AWS Problems

❌ App not picking config ✔ Check env variable ✔ Check .bashrc ✔ Restart service

1️⃣1️⃣ What you MUST remember

✅ Environment variables store config ✅ PATH controls command access ✅ export sets variables ✅ .bashrc is very important ✅ Restart shell or service to apply changes

Searching & Text Processing (Linux)

🔍 What is Searching & Text Processing?

Searching & Text Processing means:

• Finding files
• Searching text inside files
• Filtering output
• Modifying text streams
• Analyzing logs & data

👉 In simple words:

It is how Linux reads, searches, filters, and transforms text efficiently

Linux treats almost everything as text (logs, configs, outputs).

🧱 Core Philosophy (VERY IMPORTANT)

Linux tools follow:

Do one thing and do it well

Then you combine tools using pipes (|)

Example:

cat log.txt | grep ERROR | sort | uniq -c

📁 Searching Files (find & locate)

🔹 find – Real-time search

find /var/log -name "*.log"

By size:

find / -size +100M

By modified time:

find /home -mtime -1

Delete files:

find /tmp -type f -name "*.tmp" -delete

🔹 locate – Fast index-based search

locate nginx.conf

Update index:

sudo updatedb

⚠️ May not find newly created files until index updates

📄 Searching Text Inside Files (grep)

Basic search

grep "error" app.log

Case-insensitive:

grep -i error app.log

Recursive:

grep -R "password" /etc

Line numbers:

grep -n "failed" auth.log

Invert match:

grep -v "INFO" app.log

🔁 Regular Expressions (Regex Basics)

grep "^ERROR" app.log   ## line starts with ERROR
grep "failed$" app.log  ## line ends with failed
grep "[0-9]\{3\}" file  ## 3 digits

🧪 Filtering & Transforming Text

🔹 cut – Column extraction

cut -d: -f1 /etc/passwd

🔹 awk – Powerful text processing

awk '{print $1, $3}' file.txt

Condition:

awk '$3 > 100 {print $1}' data.txt

Sum column:

awk '{sum+=$2} END {print sum}' sales.txt

🔹 sed – Stream editor (modify text)

Replace:

sed 's/error/ERROR/g' app.log

Edit file:

sed -i 's/8080/9090/g' server.conf

Delete lines:

sed '5d' file.txt

📊 Sorting & Counting

🔹 sort

sort file.txt
sort -n numbers.txt
sort -r file.txt

🔹 uniq

uniq file.txt
sort file.txt | uniq
sort file.txt | uniq -c

🔹 wc – Word count

wc file.txt
wc -l file.txt

🔗 Pipes (|) – Power of Linux

grep ERROR app.log | wc -l

ps aux | grep nginx

📂 File Content Viewing

Live logs:

tail -f app.log

🧯 Real-World Use Cases

🔥 Find errors in logs

grep -i error /var/log/syslog

🔥 Top IPs hitting server

awk '{print $1}' access.log | sort | uniq -c | sort -nr | head

🔥 Kill process using port

netstat -tulpn | grep 8080

🧠 Common Mistakes

🎯 Interview-Ready Questions

Q: Difference between grep & find?

• grep → search text
• find → search files

Q: Why awk is powerful?

• It processes structured text & columns

Q: What is a pipe?

• Connects output of one command to input of another

🧾 Must-Know Commands Cheat Sheet

find
locate
grep
awk
sed
cut
sort
uniq
wc
head
tail

🏁 Final Summary

Searching & text processing tools allow Linux users to quickly find information, analyze logs, filter outputs, and automate data handling using simple yet powerful commands.

Compression & Archiving (Linux)

1️⃣ What is Archiving vs Compression?

Archiving

• Combines multiple files into one
• Example: .tar

Compression

• Reduces file size
• Example: .gz, .zip

👉 Very often used together:

.tar.gz

2️⃣ tar – Archiving Tool (MOST IMPORTANT)

Create archive

tar -cvf backup.tar folder/

Options:

• c → create
• v → verbose
• f → file name

Extract archive

tar -xvf backup.tar

3️⃣ Compressed Archives (tar.gz)

Create compressed archive

tar -czvf backup.tar.gz folder/

Extract compressed archive

tar -xzvf backup.tar.gz

4️⃣ gzip & gunzip

Compress file

gzip file.log

Creates:

file.log.gz

Decompress

gunzip file.log.gz

5️⃣ zip & unzip

Create zip

zip -r backup.zip folder/

Extract zip

unzip backup.zip

👉 zip is useful when sharing files with Windows users.

6️⃣ Backup Basics (AWS Context)

Common backup targets:

• Application files
• Logs
• Database dumps

Example:

tar -czvf app-backup.tar.gz /opt/app

Upload to S3:

aws s3 cp app-backup.tar.gz s3://my-bucket/

7️⃣ AWS Real-Life Example

Before EC2 termination

tar -czvf logs-backup.tar.gz /var/log

8️⃣ Common Mistakes

❌ Forgetting f option ❌ Extracting in wrong directory ❌ Overwriting files

9️⃣ What you MUST remember

✅ tar is most important ✅ .tar.gz is very common ✅ zip for cross-platform ✅ Backups are essential in AWS ✅ Always verify archive

SSH & Remote Access

🔐 What is SSH?

SSH (Secure Shell) is a protocol used to:

• Connect to remote servers securely
• Execute commands remotely
• Transfer files safely
• Manage servers over the network

👉 In simple words:

SSH lets you control another computer securely from your terminal

🌍 Why SSH is Important

Without SSH:

• Passwords sent in plain text
• Easy hacking

With SSH:

• Encrypted communication
• Secure authentication
• Industry standard for server access

🧠 How SSH Works (Concept)

1. Client sends connection request
2. Server responds with public key
3. Client verifies server
4. Authentication happens (password or key)
5. Encrypted session starts

🖥 SSH Basic Command

ssh user@server_ip

Example:

ssh ec2-user@13.233.45.10

🔑 SSH Authentication Methods

1️⃣ Password Authentication

ssh user@server

❌ Less secure

2️⃣ Key-Based Authentication (BEST PRACTICE)

Generate key

ssh-keygen

Copy key to server

ssh-copy-id user@server_ip

Or manually:

cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys

🔐 SSH Key Files

🔒 File Permissions (VERY IMPORTANT)

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

Wrong permissions = SSH login fails ❌

🚪 SSH Port & Config

Default port: 22

Config file:

/etc/ssh/sshd_config

Common settings:

PermitRootLogin no
PasswordAuthentication no
Port 2222

Restart SSH:

sudo systemctl restart sshd

📁 File Transfer with SSH

🔹 SCP

scp file.txt user@server:/path

Download:

scp user@server:/path/file.txt .

🔹 RSYNC (Preferred)

rsync -avz file.txt user@server:/path

🔄 SSH Tunneling & Port Forwarding

Local Port Forwarding

ssh -L 8080:localhost:80 user@server

Access remote port locally

Remote Command Execution

ssh user@server "df -h"

🧯 Common SSH Errors & Fixes

Fix host key:

ssh-keygen -R server_ip

☁️ SSH in AWS EC2 (Real-World)

Connect:

ssh -i mykey.pem ec2-user@ec2-ip

Permissions:

chmod 400 mykey.pem

🔐 SSH Security Best Practices

✔ Use key-based auth ✔ Disable root login ✔ Change default port ✔ Use firewall ✔ Use fail2ban

🧪 Advanced SSH Options

ssh -v user@server

ssh -i key.pem user@server

ssh -o StrictHostKeyChecking=no user@server

🔁 SSH Config File (Client Side)

~/.ssh/config

Example:

Host myserver
  HostName 13.233.45.10
  User ec2-user
  IdentityFile ~/.ssh/mykey.pem

Connect:

ssh myserver

🎯 Interview-Ready Q&A

Q: What is SSH?

SSH is a secure protocol used for remote access and command execution over encrypted connections.

Q: Password vs Key authentication?

• Password → weaker
• Key → stronger & secure

Q: What is known_hosts?

Stores server public keys to prevent MITM attacks

🧾 Must-Know Commands Cheat Sheet

ssh
ssh-keygen
ssh-copy-id
scp
rsync
chmod

🏁 Final Summary

SSH is the backbone of secure remote server management, enabling encrypted access, file transfers, and automation across Linux and cloud systems.

Bash Scripting Basics

🐚 What is Bash?

Bash (Bourne Again Shell) is:

• A command-line shell
• A scripting language
• The default shell on most Linux systems

👉 In simple words:

Bash lets you automate tasks by writing commands in a file

📜 What is a Bash Script?

A Bash script is:

• A text file
• Contains Linux commands
• Executed line by line

File extension:

.sh

Example:

backup.sh

🧱 Structure of a Bash Script

1️⃣ Shebang (VERY IMPORTANT)

#!/bin/bash

Tells system which interpreter to use

2️⃣ Commands

echo "Hello World"

▶️ Running a Bash Script

Make executable

chmod +x script.sh

Run

./script.sh

OR

bash script.sh

🔣 Variables in Bash

Declare variable

name="Dev"

Use variable

echo "Hello $name"

⚠️ No space around =

📥 User Input

read -p "Enter name: " name
echo "Hello $name"

🔁 Conditional Statements

if-else

if [ $age -gt 18 ]; then
  echo "Adult"
else
  echo "Minor"
fi

Common operators:

• -eq → equal
• -ne → not equal
• -gt → greater than
• -lt → less than

🔄 Loops

for loop

for i in 1 2 3
do
  echo $i
done

while loop

while true
do
  echo "Running..."
  sleep 1
done

🧠 Functions

greet() {
  echo "Hello $1"
}

greet Dev

📁 File & Directory Checks

if [ -f file.txt ]; then
  echo "File exists"
fi

if [ -d /var/log ]; then
  echo "Directory exists"
fi

🔎 Command Line Arguments

echo "Script name: $0"
echo "First arg: $1"
echo "All args: $@"

Run:

./script.sh arg1 arg2

🔄 Exit Status

ls /tmp
echo $?

0 → success Non-zero → error

🔗 Pipes & Redirection

Redirect output

ls > file.txt

Append

ls >> file.txt

Redirect error

ls invalid 2> error.txt

🧪 Debugging Bash Scripts

bash -x script.sh

Inside script:

set -x

Stop on error:

set -e

🧯 Common Mistakes

☁️ Real-World Use Case

Backup Script Example

#!/bin/bash
tar -czf backup.tar.gz /home/dev
echo "Backup completed"

🎯 Interview-Ready Questions

Q: What is Bash?

Bash is a Unix shell and scripting language used to automate system tasks.

Q: Difference between $@ and $*?

• $@ → separate arguments
• $* → single string

Q: What is shebang?

It specifies the interpreter for the script.

🧾 Must-Know Bash Commands

echo
read
if
for
while
case
function

🏁 Final Summary

Bash scripting allows you to automate repetitive Linux tasks, manage servers efficiently, and build powerful command-line tools using simple scripts.

Linux Security Basics

1️⃣ Firewall Basics

A firewall controls: 👉 Which traffic is allowed or blocked.

Linux firewalls:

• iptables (low-level)
• firewalld
• ufw (Ubuntu)

Check firewall status

Ubuntu:

sudo ufw status

Amazon Linux:

sudo systemctl status firewalld

👉 In AWS, Security Groups act as the main firewall, but Linux firewall still matters.

2️⃣ File Permission Security

Permissions prevent:

• Unauthorized access
• Accidental deletion
• Data leaks

Best practices:

• No 777 permissions
• Use 755 for folders
• Use 644 for files

Example:

chmod 600 id_rsa

3️⃣ SSH Security (VERY IMPORTANT)

Best practices:

• Use key-based authentication
• Disable password login
• Avoid root login

SSH config file:

/etc/ssh/sshd_config

Important settings:

PermitRootLogin no
PasswordAuthentication no

Restart SSH:

sudo systemctl restart sshd

4️⃣ Running Services Securely

Never run apps as root ❌

Run as:

• nginx
• appuser
• docker

Check service user:

ps aux | grep nginx

5️⃣ Principle of Least Privilege

Give:

• Only required access
• Only required permissions

Example:

• App user → app folder only
• Log user → logs only

6️⃣ AWS Security Context

👉 All layers matter.

7️⃣ Common AWS Security Mistakes

❌ SSH open to 0.0.0.0/0 ❌ Root login enabled ❌ World-writable files ❌ Hard-coded secrets

8️⃣ AWS Real-Life Example

Secure SSH:

chmod 400 mykey.pem
sudo nano /etc/ssh/sshd_config
sudo systemctl restart sshd

9️⃣ What you MUST remember

✅ Security is layered ✅ Use key-based SSH ✅ Restrict file permissions ✅ Don’t run apps as root ✅ AWS + Linux security together

Logs & Troubleshooting (Linux)

1️⃣ What are Logs?

Logs are files that record:

• System activity
• Errors
• Application events

👉 Logs tell you what went wrong and why.

2️⃣ System Logs

System logs show:

• Boot issues
• Service failures
• OS-level errors

Common locations:

View logs:

less /var/log/messages

3️⃣ Application Logs

Each application has its own logs.

Examples:

• Nginx:

  /var/log/nginx/access.log
  /var/log/nginx/error.log
  

• Docker:

  docker logs container_id
  

• Java App:

  app.log
  

4️⃣ Reading Error Logs (MOST IMPORTANT)

View last errors

tail -n 50 error.log

Live error tracking

tail -f error.log

Search errors

grep "ERROR" error.log

5️⃣ Service Logs (systemd)

journalctl -u nginx
journalctl -u nginx -f

👉 Used when service fails to start.

6️⃣ Basic Troubleshooting Flow (VERY IMPORTANT)

When something breaks:

Step 1: Check service

systemctl status nginx

Step 2: Check logs

journalctl -u nginx

Step 3: Check port

ss -tulnp

Step 4: Test locally

curl localhost

Step 5: Check permissions

ls -l

Step 6: Check disk

df -h

7️⃣ AWS Real-Life Example

❌ Website not loading

Checklist:

• Is service running?
• Is port open?
• Is app listening?
• Are logs showing errors?
• Is Security Group correct?

8️⃣ Common AWS Errors & Logs

9️⃣ What you MUST remember

✅ Logs explain failures ✅ Always check logs first ✅ tail -f for live issues ✅ Follow troubleshooting steps ✅ AWS issues = Linux + AWS configs

Linux for Cloud (AWS Context)

☁️ What Does “Linux for Cloud (AWS)” Mean?

It means:

Using Linux to run, manage, secure, and scale cloud resources on AWS

In AWS:

• Most servers (EC2) run Linux
• Most services assume Linux knowledge
• Cloud automation = Linux + scripting

👉 If you know Linux well, AWS becomes easy

🧠 Why Linux Is So Important in AWS?

🖥 Linux in AWS = EC2

EC2 = Virtual Linux Server

Popular Linux AMIs:

• Amazon Linux 2 / Amazon Linux 2023
• Ubuntu 20.04 / 22.04
• RHEL
• CentOS (legacy)

🔑 Accessing Linux EC2 (SSH)

ssh -i key.pem ec2-user@EC2_PUBLIC_IP

Permissions:

chmod 400 key.pem

Default users:

📁 Linux Filesystem in Cloud Servers

Important directories:

Cloud admins live inside /var and /etc

📦 Package Management (Cloud Must-Know)

Amazon Linux:

sudo yum install nginx

Ubuntu:

sudo apt install nginx

Update system:

sudo yum update -y

🔧 Service Management (Production Critical)

AWS apps run as services

sudo systemctl start nginx
sudo systemctl enable nginx
sudo systemctl status nginx

Logs:

journalctl -u nginx

📊 Monitoring & Logs (Very Important in Cloud)

Check system usage:

top
htop
free -m
df -h

View logs:

tail -f /var/log/messages
tail -f /var/log/syslog

AWS CloudWatch often reads these logs.

💾 Storage in AWS (Linux View)

AWS Storage → Linux Disk

Check disks:

lsblk
df -h

Mount EBS:

mount /dev/xvdf /data

🔐 Linux Security in AWS

🔹 SSH Security

• Key-based auth
• Disable root login
• Change SSH port

🔹 Permissions

chmod 600 key.pem
chown ec2-user file

🔹 Firewall

• Security Groups (AWS)
• iptables / firewalld (Linux)

🌐 Networking (Linux + AWS)

Check IP:

ip a

Check ports:

ss -tuln

Test connectivity:

curl localhost
ping google.com

AWS Networking concepts mapped to Linux:

🧪 Automation with Bash (Cloud Core Skill)

Example startup script:

#!/bin/bash
yum install -y nginx
systemctl start nginx
systemctl enable nginx

Used in:

• EC2 User Data
• CI/CD pipelines
• Auto Scaling

🔄 Scaling & Linux

In Auto Scaling:

• Linux boots
• User-data runs
• App starts automatically

Your Linux setup must be:

• Idempotent
• Fast
• Error-free

☁️ Linux + AWS Services Mapping

🧯 Common Cloud Linux Problems

🎯 Interview-Ready Q&A

Q: Why Linux is preferred in AWS?

Linux is lightweight, secure, open-source, and well suited for cloud scalability and automation.

Q: How do you secure EC2?

• SSH keys
• Security Groups
• OS hardening
• Patch updates

Q: What is user-data?

A script executed at first boot to configure the Linux instance.

🧾 Must-Know Linux Commands for AWS

ssh
lsblk
df -h
top
systemctl
journalctl
curl
chmod
chown

🏁 Final Summary (Very Important)

Linux is the backbone of AWS cloud computing. Mastering Linux means you can deploy, secure, scale, and troubleshoot cloud infrastructure confidently.