Control Flow

#!/bin/bash

# Basic if statement
if [ $age -ge 18 ]; then
    echo "Adult"
fi

# If-else
if [ $score -ge 60 ]; then
    echo "Pass"
else
    echo "Fail"
fi

# If-elif-else
if [ $grade -ge 90 ]; then
    echo "A"
elif [ $grade -ge 80 ]; then
    echo "B"
elif [ $grade -ge 70 ]; then
    echo "C"
else
    echo "F"
fi

-eq    # Equal to
-ne    # Not equal to
-gt    # Greater than
-lt    # Less than
-ge    # Greater than or equal to
-le    # Less than or equal to

# Examples
[ $a -eq $b ]    # Is a equal to b?
[ $x -gt 10 ]    # Is x greater than 10?

=      # Equal to
!=     # Not equal to
-z     # String is empty
-n     # String is not empty
<      # Less than (alphabetically)
>      # Greater than (alphabetically)

# Examples
[ "$str1" = "$str2" ]    # Are strings equal?
[ -z "$str" ]            # Is string empty?
[ -n "$str" ]            # Is string not empty?

-e     # File exists
-f     # Is a regular file
-d     # Is a directory
-r     # File is readable
-w     # File is writable
-x     # File is executable
-s     # File is not empty

# Examples
if [ -f "config.txt" ]; then
    echo "Config file exists"
fi

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

if [ ! -e "file.txt" ]; then
    echo "File does not exist"
fi

&&     # AND
||     # OR
!      # NOT

# Examples
if [ $age -ge 18 ] && [ $age -le 65 ]; then
    echo "Working age"
fi

if [ $x -lt 0 ] || [ $x -gt 100 ]; then
    echo "Out of range"
fi

if [ ! -d "backup" ]; then
    mkdir backup
fi

# Pattern matching
if [[ $filename == *.txt ]]; then
    echo "Text file"
fi

# Regular expressions
if [[ $email =~ ^[a-zA-Z0-9]+@[a-zA-Z0-9]+\.[a-zA-Z]{2,}$ ]]; then
    echo "Valid email format"
fi

# No word splitting (safer)
if [[ $var == "hello world" ]]; then
    echo "Match"
fi

# Logical operators (easier to read)
if [[ $age -ge 18 && $citizen == "yes" ]]; then
    echo "Can vote"
fi

#!/bin/bash

read -p "Enter a letter: " letter

case $letter in
    [aeiou])
        echo "Vowel"
        ;;
    [0-9])
        echo "Digit"
        ;;
    [A-Z])
        echo "Uppercase letter"
        ;;
    *)
        echo "Other character"
        ;;
esac

#!/bin/bash
# Menu system

echo "Select an option:"
echo "1) Start service"
echo "2) Stop service"
echo "3) Restart service"
echo "4) Exit"

read -p "Choice: " choice

case $choice in
    1|start)
        echo "Starting service..."
        ;;
    2|stop)
        echo "Stopping service..."
        ;;
    3|restart)
        echo "Restarting service..."
        ;;
    4|exit|quit)
        echo "Goodbye!"
        exit 0
        ;;
    *)
        echo "Invalid option"
        exit 1
        ;;
esac

# Loop over a list
for item in apple banana cherry; do
    echo "Fruit: $item"
done

# Loop over command output
for file in *.txt; do
    echo "Processing $file"
    cat "$file"
done

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

# Traditional for loop
for ((i=0; i<10; i++)); do
    echo "Number: $i"
done

# Countdown
for ((i=10; i>=1; i--)); do
    echo "$i"
    sleep 1
done
echo "Liftoff!"

# Safe way to loop through files with spaces
while IFS= read -r -d '' file; do
    echo "Found: $file"
done < <(find . -name "*.log" -print0)

# Simple file loop
for file in /var/log/*.log; do
    if [ -f "$file" ]; then
        echo "Log file: $file"
    fi
done

# Basic while loop
counter=1
while [ $counter -le 5 ]; do
    echo "Count: $counter"
    ((counter++))
done

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

# Infinite loop with break
while true; do
    read -p "Enter command (quit to exit): " cmd
    if [ "$cmd" = "quit" ]; then
        break
    fi
    echo "You entered: $cmd"
done

#!/bin/bash
# Wait for file to appear

echo "Waiting for file.txt..."
while [ ! -f "file.txt" ]; do
    sleep 1
    echo "Still waiting..."
done
echo "File found!"
            

# Basic until loop (opposite of while)
counter=1
until [ $counter -gt 5 ]; do
    echo "Count: $counter"
    ((counter++))
done

# Wait until service is ready
until curl -s http://localhost:8080 > /dev/null; do
    echo "Waiting for service..."
    sleep 2
done
echo "Service is ready!"

# break - exit the loop
for i in {1..10}; do
    if [ $i -eq 5 ]; then
        break  # Exit loop when i is 5
    fi
    echo $i
done

# continue - skip to next iteration
for i in {1..10}; do
    if [ $((i % 2)) -eq 0 ]; then
        continue  # Skip even numbers
    fi
    echo $i  # Only prints odd numbers
done

# Nested loops with break
for i in {1..3}; do
    for j in {1..3}; do
        if [ $j -eq 2 ]; then
            break  # Only breaks inner loop
        fi
        echo "$i,$j"
    done
done

#!/bin/bash

echo "Choose your favorite fruit:"
select fruit in "Apple" "Banana" "Cherry" "Quit"; do
    case $fruit in
        "Apple")
            echo "You chose Apple"
            ;;
        "Banana")
            echo "You chose Banana"
            ;;
        "Cherry")
            echo "You chose Cherry"
            ;;
        "Quit")
            echo "Goodbye!"
            break
            ;;
        *)
            echo "Invalid option $REPLY"
            ;;
    esac
done