#!/bin/bash
# This is the shebang line, indicating this is a bash script

# Comments start with #
echo "Hello, World!"  # This is an inline comment

# Assigning variables (no spaces around =)
name="John Doe"
age=30

# Using variables
echo "My name is $name and I am $age years old."

# Command substitution
current_date=$(date +"%Y-%m-%d")
echo "Today is $current_date"

# Arithmetic operations
result=$((5 + 3))
echo "5 + 3 = $result"

# Environmental variables
echo "Home directory: $HOME"
echo "Current user: $USER"

# Basic input
read -p "Enter your name: " user_name

# Input with a default value
read -p "Enter your age [30]: " age
age=${age:-30}

# Silent input (for passwords)
read -s -p "Enter password: " password

# If-else statement
if [ "$age" -ge 18 ]; then
    echo "You are an adult."
elif [ "$age" -ge 13 ]; then
    echo "You are a teenager."
else
    echo "You are a child."
fi

# Test operators
# -eq (equal), -ne (not equal), -lt (less than), -le (less than or equal)
# -gt (greater than), -ge (greater than or equal)

# File test operators
if [ -f "file.txt" ]; then
    echo "File exists"
fi
if [ -d "/path/to/directory" ]; then
    echo "Directory exists"
fi
# -r (readable), -w (writable), -x (executable), -s (not empty)

# For loop
for i in {1..5}; do
    echo "Iteration $i"
done

# While loop
counter=0
while [ $counter -lt 5 ]; do
    echo "Counter: $counter"
    ((counter++))
done

# Until loop
until [ $counter -eq 10 ]; do
    echo "Counter: $counter"
    ((counter++))
done

# Loop over array elements
fruits=("apple" "banana" "orange")
for fruit in "${fruits[@]}"; do
    echo "I like $fruit"
done

# Loop over command output
for file in $(ls); do
    echo "File: $file"
done

# Defining a function
greet() {
    echo "Hello, $1!"
}

# Calling a function
greet "Alice"

# Function with return value
is_even() {
    if [ $(($1 % 2)) -eq 0 ]; then
        return 0  # true
    else
        return 1  # false
    fi
}

# Using function return value
if is_even 4; then
    echo "4 is even"
fi

string="Hello, World!"
echo "Length: ${#string}"
echo "Uppercase: ${string^^}"
echo "Lowercase: ${string,,}"
echo "Substring: ${string:7:5}"  # World
echo "Replace: ${string/World/Universe}"

# Declaring arrays
declare -a fruits=("apple" "banana" "orange")
numbers=(1 2 3 4 5)

# Accessing elements
echo "Second fruit: ${fruits[1]}"

# Array length
echo "Number of fruits: ${#fruits[@]}"

# Looping through array
for number in "${numbers[@]}"; do
    echo "Number: $number"
done

# Adding elements
fruits+=("grape")

# Removing elements
unset fruits[1]

case "$1" in
    start)
        echo "Starting the service..."
        ;;
    stop)
        echo "Stopping the service..."
        ;;
    restart)
        echo "Restarting the service..."
        ;;
    *)
        echo "Usage: $0 {start|stop|restart}"
        exit 1
        ;;
esac

# Exit on error
set -e

# Print commands before execution
set -x

# Error handling with trap
trap 'echo "Error on line $LINENO"' ERR

# Redirecting errors to a file
command 2> error.log

# Redirecting both stdout and stderr
command &> output.log

echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "All arguments: $@"
echo "Number of arguments: $#"

# Reading a file line by line
while IFS= read -r line; do
    echo "Line: $line"
done < input.txt

# Writing to a file
echo "New content" > output.txt
echo "Appended content" >> output.txt

# Checking file existence
if [ -e "file.txt" ]; then
    echo "File exists"
fi

# Comparing files
if cmp -s "file1.txt" "file2.txt"; then
    echo "Files are identical"
fi

# Running a command in the background
long_running_command &

# Getting the PID of the last background process
bg_pid=$!

# Waiting for a background process to finish
wait $bg_pid

# Killing a process
kill $bg_pid

# Running a command with a timeout
timeout 5s some_command