Skip to main content

Control Structures

Conditionals​

If Statements​

# Basic if statement
if [ condition ]; then
    echo "True"
fi

# If-else
if [ condition ]; then
    echo "True"
else
    echo "False"
fi

# If-elif-else
if [ condition1 ]; then
    echo "First condition"
elif [ condition2 ]; then
    echo "Second condition"
else
    echo "Default"
fi

# Compact form
[ condition ] && echo "True" || echo "False"

Test Conditions​

# File tests
if [ -f "$file" ]; then echo "File exists"; fi
if [ -d "$dir" ]; then echo "Directory exists"; fi
if [ -r "$file" ]; then echo "File is readable"; fi
if [ -w "$file" ]; then echo "File is writable"; fi
if [ -x "$file" ]; then echo "File is executable"; fi
if [ -s "$file" ]; then echo "File is not empty"; fi

# String tests
if [ -z "$string" ]; then echo "String is empty"; fi
if [ -n "$string" ]; then echo "String is not empty"; fi
if [ "$str1" = "$str2" ]; then echo "Strings are equal"; fi
if [ "$str1" != "$str2" ]; then echo "Strings are different"; fi

# Numeric tests
if [ "$num1" -eq "$num2" ]; then echo "Equal"; fi
if [ "$num1" -ne "$num2" ]; then echo "Not equal"; fi
if [ "$num1" -lt "$num2" ]; then echo "Less than"; fi
if [ "$num1" -le "$num2" ]; then echo "Less or equal"; fi
if [ "$num1" -gt "$num2" ]; then echo "Greater than"; fi
if [ "$num1" -ge "$num2" ]; then echo "Greater or equal"; fi

Advanced Conditionals​

# Double brackets (bash extended test)
if [[ $string =~ ^[0-9]+$ ]]; then
    echo "String is numeric"
fi

if [[ $string == *pattern* ]]; then
    echo "String contains pattern"
fi

# Logical operators
if [[ condition1 && condition2 ]]; then
    echo "Both conditions are true"
fi

if [[ condition1 || condition2 ]]; then
    echo "At least one condition is true"
fi

if [[ ! condition ]]; then
    echo "Condition is false"
fi

# Arithmetic conditions
if (( num > 0 && num < 100 )); then
    echo "Number is between 0 and 100"
fi

Case Statements​

Basic Case Statement​

case $variable in
    pattern1)
        echo "Matched pattern1"
        ;;
    pattern2)
        echo "Matched pattern2"
        ;;
    *)
        echo "Default case"
        ;;
esac

Case Statement Patterns​

# Multiple patterns
case $input in
    y|Y|yes|Yes|YES)
        echo "User said yes"
        ;;
    n|N|no|No|NO)
        echo "User said no"
        ;;
    *)
        echo "Invalid input"
        ;;
esac

# Pattern matching
case $filename in
    *.txt)
        echo "Text file"
        ;;
    *.jpg|*.png|*.gif)
        echo "Image file"
        ;;
    [0-9]*.log)
        echo "Log file starting with digit"
        ;;
    *)
        echo "Unknown file type"
        ;;
esac

# Range patterns
case $grade in
    [90-100])
        echo "A"
        ;;
    [80-89])
        echo "B"
        ;;
    [70-79])
        echo "C"
        ;;
    *)
        echo "F"
        ;;
esac

Loops​

For Loops​

# Basic for loop
for i in 1 2 3 4 5; do
    echo "Number: $i"
done

# For loop with sequence
for i in {1..10}; do
    echo "Count: $i"
done

# For loop with step
for i in {1..10..2}; do
    echo "Odd number: $i"
done

# For loop with files
for file in *.txt; do
    echo "Processing: $file"
done

# For loop with command substitution
for user in $(cut -d: -f1 /etc/passwd); do
    echo "User: $user"
done

# For loop with arrays
fruits=("apple" "banana" "cherry")
for fruit in "${fruits[@]}"; do
    echo "Fruit: $fruit"
done

C-style For Loop​

# C-style for loop
for ((i=1; i<=10; i++)); do
    echo "Counter: $i"
done

# Multiple variables
for ((i=1, j=10; i<=j; i++, j--)); do
    echo "i=$i, j=$j"
done

# Infinite loop
for ((;;)); do
    echo "Press Ctrl+C to exit"
    sleep 1
done

While Loops​

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

# While reading from file
while read line; do
    echo "Line: $line"
done < file.txt

# While with condition
while [ -f "running.flag" ]; do
    echo "Process is running..."
    sleep 1
done

# Infinite while loop
while true; do
    echo "Press Ctrl+C to exit"
    sleep 1
done

Until Loops​

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

# Until with condition
until [ -f "stop.flag" ]; do
    echo "Working..."
    sleep 1
done

Loop Control​

# Break and continue
for i in {1..10}; do
    if [ $i -eq 5 ]; then
        continue    # Skip iteration
    fi
    if [ $i -eq 8 ]; then
        break       # Exit loop
    fi
    echo "Number: $i"
done

# Break/continue with nested loops
for i in {1..3}; do
    for j in {1..3}; do
        if [ $j -eq 2 ]; then
            break 2    # Break out of both loops
        fi
        echo "i=$i, j=$j"
    done
done

Functions​

Basic Function Definition​

# Function definition
function greet() {
    echo "Hello, $1!"
}

# Alternative syntax
greet() {
    echo "Hello, $1!"
}

# Function call
greet "World"

Function Parameters​

# Function with parameters
calculate() {
    local num1=$1
    local num2=$2
    local operation=$3

    case $operation in
        add)
            echo $((num1 + num2))
            ;;
        subtract)
            echo $((num1 - num2))
            ;;
        multiply)
            echo $((num1 * num2))
            ;;
        *)
            echo "Unknown operation"
            return 1
            ;;
    esac
}

# Function call
result=$(calculate 10 5 add)
echo "Result: $result"

Function Variables​

# Local variables
my_function() {
    local local_var="I'm local"
    global_var="I'm global"

    echo "Inside function: $local_var"
    echo "Inside function: $global_var"
}

# Variable scope
global_var="Initial value"
my_function
echo "Outside function: $global_var"
# echo "Outside function: $local_var"  # This would cause an error

Return Values​

# Return numeric value
is_even() {
    local number=$1
    if (( number % 2 == 0 )); then
        return 0    # Success (true)
    else
        return 1    # Failure (false)
    fi
}

# Using return value
if is_even 42; then
    echo "42 is even"
else
    echo "42 is odd"
fi

# Return string through echo
get_system_info() {
    local os=$(uname -s)
    local arch=$(uname -m)
    echo "$os-$arch"
}

system_info=$(get_system_info)
echo "System: $system_info"

Function Arrays​

# Function returning array
get_files() {
    local directory=$1
    local -a files

    while IFS= read -r -d '' file; do
        files+=("$file")
    done < <(find "$directory" -type f -print0)

    printf '%s\n' "${files[@]}"
}

# Using function with array
while IFS= read -r file; do
    echo "File: $file"
done < <(get_files "/tmp")

Advanced Control Structures​

Select Statement​

# Menu selection
select option in "Option 1" "Option 2" "Option 3" "Quit"; do
    case $option in
        "Option 1")
            echo "You selected Option 1"
            ;;
        "Option 2")
            echo "You selected Option 2"
            ;;
        "Option 3")
            echo "You selected Option 3"
            ;;
        "Quit")
            break
            ;;
        *)
            echo "Invalid option"
            ;;
    esac
done

Nested Structures​

# Nested loops and conditions
for file in *.txt; do
    if [ -f "$file" ]; then
        echo "Processing: $file"

        while read line; do
            if [[ $line == *"pattern"* ]]; then
                echo "Found pattern in: $line"
            fi
        done < "$file"
    fi
done

Error Handling​

Exit Status​

# Check command success
if command; then
    echo "Command succeeded"
else
    echo "Command failed with exit code $?"
fi

# Set exit status
my_function() {
    if [ condition ]; then
        return 0    # Success
    else
        return 1    # Failure
    fi
}

Trap Signals​

# Cleanup function
cleanup() {
    echo "Cleaning up..."
    rm -f temp_file.txt
    exit 1
}

# Trap signals
trap cleanup SIGINT SIGTERM

# Main script
while true; do
    echo "Working... (Press Ctrl+C to exit)"
    sleep 1
done

Error Handling in Functions​

# Function with error handling
safe_copy() {
    local source=$1
    local dest=$2

    if [ ! -f "$source" ]; then
        echo "Error: Source file '$source' does not exist" >&2
        return 1
    fi

    if ! cp "$source" "$dest"; then
        echo "Error: Failed to copy '$source' to '$dest'" >&2
        return 1
    fi

    echo "Successfully copied '$source' to '$dest'"
    return 0
}

# Using function with error handling
if ! safe_copy "file1.txt" "file2.txt"; then
    echo "Copy operation failed"
    exit 1
fi

Best Practices​

Function Organization​

# Function documentation
# Description: Validates email address format
# Parameters: $1 - email address to validate
# Returns: 0 if valid, 1 if invalid
validate_email() {
    local email=$1
    local regex='^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'

    if [[ $email =~ $regex ]]; then
        return 0
    else
        return 1
    fi
}

Defensive Programming​

# Check parameter count
my_function() {
    if [ $# -ne 2 ]; then
        echo "Usage: my_function <param1> <param2>" >&2
        return 1
    fi

    local param1=$1
    local param2=$2

    # Function logic here
}

# Validate input
process_number() {
    local input=$1

    if ! [[ $input =~ ^[0-9]+$ ]]; then
        echo "Error: '$input' is not a valid number" >&2
        return 1
    fi

    # Process the number
    echo "Processing number: $input"
}

Performance Considerations​

# Use appropriate loop types
# For known ranges: for i in {1..100}
# For file processing: while read
# For arrays: for item in "${array[@]}"

# Avoid unnecessary subshells
# Bad: result=$(echo $var | sed 's/old/new/')
# Good: result=${var/old/new}

# Use built-in tests
# Bad: if [ $(echo $var | wc -c) -gt 10 ]
# Good: if [ $\{#var} -gt 10 ]