Introduction to Linux
Linux File System Hierarchy
Basic Linux Commands
File Permissions and Ownership
Text Processing Commands
Process Management
Introduction to Shell Scripting
Variables and Input
Conditional Statements
Loops
Functions

1. Introduction to Linux

What is Linux?

Linux is an open-source, Unix-like operating system kernel first released by Linus Torvalds in 1991. Unlike Windows or macOS, Linux is highly customizable and powers:

Servers (90% of the internet runs on Linux)
Cloud computing (AWS, Google Cloud, Azure)
Embedded systems (Routers, Smart TVs, Android)
Supercomputers (All top 500 supercomputers run Linux)

Why Use Linux?

✔ Free and Open-Source (No licensing fees)
✔ Secure (Less prone to malware)
✔ Lightweight (Runs on old hardware)
✔ Highly Customizable (Choose your own desktop environment)

Popular Linux Distributions

Distro	Best For
Ubuntu	Beginners
Debian	Stability
CentOS	Servers
Arch Linux	Advanced Users
Kali Linux	Cybersecurity

2. Linux File System Hierarchy

Linux follows a standard directory structure:

Directory	Purpose
`/`	Root directory
`/bin`	Essential binaries (ls, cp)
`/etc`	Configuration files
`/home`	User directories
`/var`	Variable data (logs)
`/tmp`	Temporary files
`/usr`	User programs

Example:

ls /  # List root directory

3. Basic Linux Commands

Command	Description	Example
`pwd`	Print current directory	`pwd`
`cd`	Change directory	`cd /home/user`
`ls`	List files	`ls -l`

File Operations

Command	Description	Example
`touch`	Create file	`touch file.txt`
`cp`	Copy file	`cp file.txt backup/`
`mv`	Move/rename	`mv file.txt newname.txt`
`rm`	Delete file	`rm file.txt`

Viewing Files

cat file.txt      # Display entire file
less file.txt    # Scroll through file
head -n 5 file.txt # Show first 5 lines
tail -f log.txt  # Follow log in real-time

4. File Permissions and Ownership

Linux uses permissions to control file access:

-rw-r--r-- 1 user group 1024 Jan 1 10:00 file.txt

Permissions: rw- (user), r-- (group), r-- (others)
Change permissions:
```
  chmod 755 script.sh  # rwxr-xr-x
```
Change ownership:
```
  chown user:group file.txt
```

5. Text Processing Commands

Command	Description	Example
`grep`	Search text	`grep "error" log.txt`
`sed`	Find & replace	`sed 's/old/new/g' file.txt`
`awk`	Text processing	`awk '{print $1}' file.txt`
`sort`	Sort lines	`sort file.txt`
`uniq`	Remove duplicates	`uniq file.txt`

6. Process Management

Command	Description	Example
`ps`	List processes	`ps aux`
`top`	Live process monitor	`top`
`kill`	Terminate process	`kill -9 PID`
`bg` / `fg`	Background/Foreground jobs	`bg %1`

7. Introduction to Shell Scripting

A shell script is a program that automates tasks using shell commands.

Create Your First Script

#!/bin/bash
echo "Hello, World!"

Save as hello.sh
Make executable:
```
  chmod +x hello.sh
```
Run:
```
  ./hello.sh
```

8. Variables and Input

#!/bin/bash
name="Linux"
echo "Welcome to $name!"

# User input
read -p "Enter your name: " username
echo "Hello, $username!"

9. Conditional Statements

#!/bin/bash
if [ $1 -gt 10 ]; then
  echo "Greater than 10"
else
  echo "10 or less"
fi

10. Loops

For Loop

for i in {1..5}; do
  echo "Number: $i"
done

While Loop

count=1
while [ $count -le 5 ]; do
  echo "Count: $count"
  ((count++))
done

11. Functions

#!/bin/bash
greet() {
  echo "Hello, $1!"
}
greet "Alice"

The Ultimate Guide to Linux and Shell Scripting: BEGINNER FRIENDLY

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