Function Exit Status Codes

Scripts use exit to define exit status codes and terminate executoin. Functions use return to define exit status codes and terminate execution.

#!/bin/bash
# Function with conditional exit status
check_file() {
    if [ -f "/etc/hosts" ]; then
        echo "File exists"
    else
        return 2
    fi
}
check_file
echo $?

Parameterized Functions

Functions can accept parameters passed during invocation:

#!/bin/bash
greet_user() {
    echo "Hello $1"
}
greet_user $1

Multiple parameters example:

#!/bin/bash
greet_multiple() {
    echo "Hello $1 $2"
}
greet_multiple $1 $2

Positional Parameters in Functions

Funtcions hendle all positional parameters using special variables:

#!/bin/bash
display_params() {
    echo "Script name: $0"
    echo "Parameter count: $#"
    echo "First parameter: $1"
    echo "All parameters: $*"
    echo "Exit status: $?"
}
display_params $*

Case Statement Syntax

Case statements provide pattern matching for conditional execution:

#!/bin/bash
case "$1" in
    "start"|"START")
        echo "Service started"
        ;;
    "stop"|"STOP")
        echo "Service stopped"
        ;;
    *)
        echo "Invalid command"
        ;;
esac

Character Type Detection

Use case statements to identify character types:

#!/bin/bash
case "$1" in
    [0-9])
        echo "Digit detected: $1"
        ;;
    [a-zA-Z])
        echo "Letter detected: $1"
        ;;
    *)
        echo "Invalid input"
        exit 1
        ;;
esac

Menu Systems with Case

Convert menu systems to use case statements for cleaner logic:

#!/bin/bash
# Check script files
check_lamp=`[ -s /server/scripts/lamp.sh ] && echo valid || echo invalid`
check_lnmp=`[ -s /server/scripts/lnmp.sh ] && echo valid || echo invalid`

# Validate executables
case "$check_lamp" in
    valid)
        echo -e "\033[32mLAMP script executable\033[0m"
        ;;
    *)
        echo -e "\033[31mLAMP script not executable!\033[0m"
        ;;
esac

cat << EOF
$(echo -e "\033[35m (1).[install lamp]\033[0m")
$(echo -e "\033[36m (2).[install lnmp]\033[0m")
$(echo -e "\033[33m (3).[exit]\033[0m")
EOF

read -p "Select option: " choice
case "$choice" in
    1)
        echo "Installing LAMP..."
        /server/scripts/lamp.sh
        ;;
    2)
        echo "Installing LNMP..."
        /server/scripts/lnmp.sh
        ;;
    3)
        echo "Exiting..."
        exit
        ;;
    *)
        echo "Invalid selection"
        exit 1
        ;;
esac

Nginx Service Management

Create comprehensive Nginx management using case and functions:

#!/bin/bash
start_nginx() {
    nginx
    echo "Nginx started successfully"
}

stop_nginx() {
    if [ $(ss -lntup | grep nginx | wc -l) -eq 0 ]; then
        echo "Nginx already stopped"
    else
        pkill nginx
        echo "Nginx stopped"
    fi
}

restart_nginx() {
    stop_nginx
    sleep 1
    start_nginx
    echo "Nginx restarted"
}

case "$1" in
    start)
        start_nginx
        ;;
    stop)
        stop_nginx
        ;;
    restart)
        restart_nginx
        ;;
    *)
        echo "Usage: $0 {start|stop|restart}"
        exit 1
        ;;
esac

While Loop Constructs

While loops execute commands as long as conditions remain true:

#!/bin/bash
# Sum numbers from 1 to 100
number=0
sum=0
while [ $number -lt 100 ]; do
    let number++
    let sum=$sum+$number
done
echo "Sum: $sum"

File Processing with While

Process files line by line:

#!/bin/bash
# Calculate average from age file
sum_age=0
count=0
while read -r current_line; do
    individual_age=$(echo $current_line | awk '{print $2}')
    let sum_age=$sum_age+$individual_age
    let count++
done < age.txt

average=$(echo "$sum_age/$count" | bc)
echo "Average age: $average"

Interactive Menus with While

Create persistent menus using infinite loops:

#!/bin/bash
while true; do
    cat << MENU
$(echo -e "\033[32m 1. Option A\033[0m")
$(echo -e "\033[33m 2. Option B\033[0m")
$(echo -e "\033[34m 3. Exit\033[0m")
MENU
    
    read -p "Choose option: " selection
    
    case $selection in
        1)
            echo "Selected option A"
            ;;
        2)
            echo "Selected option B"
            ;;
        3)
            echo "Goodbye"
            exit 0
            ;;
        *)
            echo "Invalid choice, try again"
            ;;
    esac
done

Number Guessing Game

Implement interactive games with while loops:

#!/bin/bash
# Generate random number
target=$((RANDOM % 100 + 1))
attempts=0

echo "Guess the number between 1-100!"

while true; do
    read -p "Enter your guess: " guess
    
    # Validate input
    if ! [[ "$guess" =~ ^[0-9]+$ ]]; then
        echo "Please enter a valid number"
        continue
    fi
    
    let attempts++
    
    if [ $guess -eq $target ]; then
        echo "Correct! Number was $target"
        echo "Attempts: $attempts"
        break
    elif [ $guess -gt $target ]; then
        echo "Too high!"
    else
        echo "Too low!"
    fi
done

System Information Menu

Create comprehensive system monitoring tools:

#!/bin/bash
while true; do
    cat << INFO_MENU
$(echo -e "\033[32m [h]. Help\033[0m")
$(echo -e "\033[32m [f]. Login info\033[0m")
$(echo -e "\033[32m [d]. Disk info\033[0m")
$(echo -e "\033[32m [m]. Memory usage\033[0m")
$(echo -e "\033[32m [u]. System load\033[0m")
$(echo -e "\033[35m [q]. Quit\033[0m")
INFO_MENU
    
    read -p "Select option: " option
    
    case $option in
        h)
            echo "System information menu"
            sleep 2
            clear
            ;;
        f)
            echo "Active sessions: $(who | wc -l)"
            who
            ;;
        d)
            df -h
            ;;
        m)
            free -h
            ;;
        u)
            uptime
            ;;
        q)
            echo "Goodbye!"
            exit 0
            ;;
        *)
            echo "Invalid option"
            ;;
    esac
done