Basic Conditional Structures

When to Use Conditional Statements

• When your script needs to make decisions based on user input
• When you need to handle different cases or scenarios
• When you want to check if files or directories exist before using them
• When you need to validate input before processing it
• When you need to handle errors or unexpected conditions

Common Conditional Structures

Practical Examples

# Basic if statement
number=5
if [[ $number -gt 3 ]]; then
    echo "The number is greater than 3"
fi

# if-else statement
number=2
if [[ $number -gt 3 ]]; then
    echo "The number is greater than 3"
else
    echo "The number is not greater than 3"
fi

# if-elif-else statement
number=3
if [[ $number -gt 3 ]]; then
    echo "The number is greater than 3"
elif [[ $number -eq 3 ]]; then
    echo "The number is equal to 3"
else
    echo "The number is less than 3"
fi

# Nested if statement
number=4
if [[ $number -gt 0 ]]; then
    if [[ $number -lt 10 ]]; then
        echo "The number is between 1 and 9"
    else
        echo "The number is 10 or greater"
    fi
else
    echo "The number is zero or negative"
fi

Test Commands: [[ ]] vs [ ]

When to Use Different Test Commands

• Use [[ ]] when writing scripts specifically for Bash
• Use [ ] when writing scripts that need to run on multiple Unix-like systems
• Use [[ ]] when you need pattern matching or advanced string operations
• Use [ ] for maximum portability across different shells
• Use [[ ]] when working with variables that might contain spaces or be empty

Comparison of Test Commands

Practical Examples

# String comparison with [ ]
VAR="hello"
if [ "$VAR" = "hello" ]; then
    echo "Matched with single brackets"
fi

# String comparison with [[ ]]
VAR="hello"
if [[ $VAR == "hello" ]]; then
    echo "Matched with double brackets"
fi

# Logical operators with [ ]
if [ "$num" -gt 5 -a "$num" -lt 10 ]; then
    echo "Number is between 6 and 9 (using single brackets)"
fi

# Logical operators with [[ ]]
if [[ $num -gt 5 && $num -lt 10 ]]; then
    echo "Number is between 6 and 9 (using double brackets)"
fi

# Pattern matching (only works with [[ ]])
filename="document.txt"
if [[ $filename == *.txt ]]; then
    echo "Text file detected"
fi

# Regex matching (only works with [[ ]])
VAR="hello123"
if [[ $VAR =~ ^hello[0-9]+$ ]]; then
    echo "Matches regex pattern"
fi

Comparison Operators

When to Use Different Comparison Types

• Use string comparisons when working with text
• Use numeric comparisons when working with numbers
• Use file test operators when checking file properties
• Use string pattern matching when searching for specific patterns
• Use logical operators to combine multiple conditions

Common Comparison Operators

Practical Examples

# String comparison
name="Alice"
if [[ $name == "Alice" ]]; then
    echo "Hello Alice!"
fi

# Numeric comparison
age=25
if [[ $age -ge 18 ]]; then
    echo "You are an adult"
fi

# String emptiness test
input=""
if [[ -z $input ]]; then
    echo "No input provided"
fi

# File test
if [[ -f "config.txt" ]]; then
    echo "Config file exists, reading settings..."
    cat config.txt
else
    echo "Creating default config file..."
    echo "default_setting=true" > config.txt
fi

# Combining conditions with logical operators
temperature=72
humidity=65
if [[ $temperature -gt 70 && $humidity -lt 80 ]]; then
    echo "Weather is warm and comfortable"
fi

# String pattern matching
filename="document.pdf"
if [[ $filename == *.pdf ]]; then
    echo "PDF document detected"
elif [[ $filename == *.txt ]]; then
    echo "Text file detected"
else
    echo "Unknown file type"
fi

Advanced Conditional Techniques

When to Use Advanced Techniques

• When you need to handle complex decision trees
• When you want to create more concise and readable code
• When checking multiple related conditions
• When you need to validate input against patterns
• When your script needs to adapt to different environments

Advanced Conditional Techniques

Practical Examples

# Short-circuit evaluation
[[ -f config.txt ]] && echo "Config file exists"
[[ -f config.txt ]] || echo "Config file missing"

# Command success as a condition
if grep -q "error" log.txt; then
    echo "Errors found in log file"
fi

# Testing command-line arguments
if [[ $# -lt 2 ]]; then
    echo "Error: Not enough arguments"
    echo "Usage: $0 filename count"
    exit 1
fi

# Parameter expansion with default
name=${1:-"Guest"}  # Use "Guest" if $1 is not provided
echo "Hello, $name!"

# Complex condition with multiple checks
file=$1
if [[ -f $file && -r $file && $file == *.txt ]]; then
    echo "Reading text file: $file"
    cat "$file"
else
    echo "File $file is not a readable text file"
fi