Advanced Bash Scripting: Arrays, Associative Arrays, and Select Menus

#!/bin/bash
# Indexed arrays - ordered lists

# Create arrays (multiple ways)
servers=("web-01" "web-02" "db-01" "cache-01")
files=(*.txt)  # Create from glob pattern
empty=()       # Empty array

# Access elements (zero-indexed)
echo "First server: ${servers[0]}"      # web-01
echo "Second server: ${servers[1]}"     # web-02
echo "All servers: ${servers[@]}"       # web-01 web-02 db-01 cache-01
echo "Array length: ${#servers[@]}"     # 4

# Modify arrays
servers[4]="cache-02"                    # Add at index 4
servers+=("monitor-01")                  # Append to end
unset servers[2]                         # Remove index 2 (db-01)

# Slice arrays (start:length)
echo "First two: ${servers[@]:0:2}"

# Loop through arrays
echo "=== Server List ==="
for server in "${servers[@]}"; do
    echo "Server: $server"
done

# Loop with index
for i in "${!servers[@]}"; do
    echo "Index $i: ${servers[$i]}"
done

#!/bin/bash
# Associative arrays - key-value pairs (Bash 4.0+)

# Declare associative array
declare -A server_ip
declare -A config

# Add key-value pairs
server_ip["web-01"]="10.0.1.10"
server_ip["web-02"]="10.0.1.11"
server_ip["db-01"]="10.0.2.10"

# Or declare all at once
declare -A server_role=(
    ["web-01"]="webserver"
    ["web-02"]="webserver"
    ["db-01"]="database"
)

# Access values
echo "web-01 IP: ${server_ip["web-01"]}"
echo "db-01 role: ${server_role["db-01"]}"

# Get all keys or values
echo "All servers: ${!server_ip[@]}"
echo "All IPs: ${server_ip[@]}"

# Loop through associative array
echo "=== Server Configuration ==="
for server in "${!server_ip[@]}"; do
    echo "$server -> ${server_ip[$server]} (${server_role[$server]})"
done

# Check if key exists
if [[ -v server_ip["web-03"] ]]; then
    echo "web-03 exists"
else
    echo "web-03 not found"
fi

# Remove entry
unset server_ip["web-02"]

#!/bin/bash
# Select menu - interactive choice menu

# Simple select menu
options=("Start Service" "Stop Service" "Restart Service" "View Status" "Exit")

echo "Choose an option:"
select choice in "${options[@]}"; do
    case $choice in
        "Start Service")
            echo "Starting service..."
            # systemctl start myservice
            ;;
        "Stop Service")
            echo "Stopping service..."
            ;;
        "Restart Service")
            echo "Restarting service..."
            ;;
        "View Status")
            echo "Service is running"
            ;;
        "Exit")
            echo "Goodbye!"
            break
            ;;
        *)
            echo "Invalid option $REPLY"
            ;;
    esac
done

# Select with server selection
declare -A servers=(
    ["web-01"]="10.0.1.10"
    ["web-02"]="10.0.1.11"
    ["db-01"]="10.0.2.10"
)

PS3="Select a server to ping: "  # Prompt
echo "Available servers:"
select server in "${!servers[@]}" "Quit"; do
    if [[ "$server" == "Quit" ]]; then
        echo "Exiting..."
        break
    elif [[ -n "$server" ]]; then
        echo "Pinging ${server} at ${servers[$server]}..."
        ping -c 2 "${servers[$server]}"
    else
        echo "Invalid selection"
    fi
done

#!/bin/bash
# Here documents - multi-line text blocks

# Basic here document
cat << EOF
This is a multi-line
block of text
that preserves formatting
and line breaks.
EOF

# Here document with variables
name="John"
cat << EOF
Hello $name,
This is your report.
Today is $(date).
EOF

# Here document with command substitution
cat << EOF
Current directory: $(pwd)
User: $(whoami)
Files: $(ls | wc -l)
EOF

# Here document with tab suppression (<<-)
cat <<- EOF
	This line has a leading tab
	This line is indented
	But the output won't show the tabs
EOF

# Create file from here document
cat > config.yml << 'EOF'
# This is a YAML configuration file
database:
  host: localhost
  port: 5432
  user: admin
  password: ${PASSWORD}  # This won't expand (quoted EOF)
app:
  name: myapp
  version: 1.0
EOF

# Multi-line variable using here document
read -r -d '' help_text << 'HELP'
Usage: myscript [OPTIONS]

Options:
  -h, --help     Show this help
  -v, --verbose  Enable verbose output
  -f, --file     Specify input file

Examples:
  myscript -f data.txt
  myscript --verbose
HELP

echo "$help_text"

#!/bin/bash
# Process substitution - treat command output as a file

# Compare two directories
diff <(ls /etc) <(ls /usr/local/etc)

# Count lines in command output
wc -l <(grep -r "ERROR" /var/log/)

# Read from process substitution
while IFS= read -r line; do
    echo "Processing: $line"
done < <(ps aux | grep nginx)

# Combine multiple command outputs
paste <(ls -1 dir1/) <(ls -1 dir2/)

# Real-world example: Compare server configs
diff <(ssh user@server1 'cat /etc/nginx/nginx.conf') \
     <(ssh user@server2 'cat /etc/nginx/nginx.conf')

# Another useful pattern: Join files
join <(sort file1.txt) <(sort file2.txt)

#!/bin/bash
# server_manager.sh - Advanced server management CLI

declare -A SERVERS=(
    ["web-01"]="10.0.1.10"
    ["web-02"]="10.0.1.11"
    ["web-03"]="10.0.1.12"
    ["db-01"]="10.0.2.10"
    ["db-02"]="10.0.2.11"
    ["cache-01"]="10.0.3.10"
)

declare -A SERVER_ROLES=(
    ["web-01"]="web"
    ["web-02"]="web"
    ["web-03"]="web"
    ["db-01"]="database"
    ["db-02"]="database"
    ["cache-01"]="cache"
)

# Color codes
RED='\033[0;31m'
GREEN='\033[0;32m'
YELLOW='\033[1;33m'
BLUE='\033[0;34m'
NC='\033[0m'

# Function: Show help
show_help() {
    cat << 'HELP'
Server Management CLI

COMMANDS:
  list              List all servers
  ping <server>     Ping a server
  ssh <server>      SSH into a server
  status <server>   Check server status
  roles             Show servers by role
  health            Run health check on all servers
  exec <server> <cmd> Execute command on server
  help              Show this help
  exit              Exit program

EXAMPLES:
  server_manager.sh list
  server_manager.sh ping web-01
  server_manager.sh exec web-01 "df -h"
HELP
}

# Function: List all servers
list_servers() {
    echo -e "${BLUE}Available Servers:${NC}"
    echo "----------------------------------------"
    printf "%-15s %-15s %-10s\n" "SERVER" "IP ADDRESS" "ROLE"
    echo "----------------------------------------"
    for server in "${!SERVERS[@]}"; do
        printf "%-15s %-15s %-10s\n" "$server" "${SERVERS[$server]}" "${SERVER_ROLES[$server]}"
    done
}

# Function: Ping server
ping_server() {
    local server="$1"
    local ip="${SERVERS[$server]}"
    
    if [[ -z "$ip" ]]; then
        echo -e "${RED}Error: Server '$server' not found${NC}"
        return 1
    fi
    
    echo -e "${YELLOW}Pinging $server ($ip)...${NC}"
    ping -c 3 "$ip"
}

# Function: SSH to server
ssh_server() {
    local server="$1"
    local ip="${SERVERS[$server]}"
    
    if [[ -z "$ip" ]]; then
        echo -e "${RED}Error: Server '$server' not found${NC}"
        return 1
    fi
    
    echo -e "${GREEN}Connecting to $server ($ip)...${NC}"
    ssh "$USER@$ip"
}

# Function: Check server status
check_status() {
    local server="$1"
    local ip="${SERVERS[$server]}"
    
    if [[ -z "$ip" ]]; then
        echo -e "${RED}Error: Server '$server' not found${NC}"
        return 1
    fi
    
    if ping -c 1 -W 2 "$ip" > /dev/null 2>&1; then
        echo -e "${GREEN}✓ $server is ONLINE${NC}"
        return 0
    else
        echo -e "${RED}✗ $server is OFFLINE${NC}"
        return 1
    fi
}

# Function: Show servers by role
show_by_role() {
    local role
    
    echo -e "${BLUE}Select a role:${NC}"
    select role in "web" "database" "cache" "all"; do
        case $role in
            web|database|cache)
                echo -e "\n${YELLOW}Servers with role '$role':${NC}"
                for server in "${!SERVERS[@]}"; do
                    if [[ "${SERVER_ROLES[$server]}" == "$role" ]]; then
                        echo "  - $server (${SERVERS[$server]})"
                    fi
                done
                break
                ;;
            all)
                list_servers
                break
                ;;
            *)
                echo -e "${RED}Invalid option${NC}"
                ;;
        esac
    done
}

# Function: Run health check on all servers
health_check() {
    echo -e "${BLUE}Running health check on all servers...${NC}\n"
    
    local online=0
    local offline=0
    
    for server in "${!SERVERS[@]}"; do
        if check_status "$server" > /dev/null 2>&1; then
            echo -e "${GREEN}✓ $server${NC}"
            ((online++))
        else
            echo -e "${RED}✗ $server${NC}"
            ((offline++))
        fi
    done
    
    echo -e "\n${BLUE}Summary:${NC} $online online, $offline offline"
}

# Function: Execute remote command
exec_command() {
    local server="$1"
    local cmd="$2"
    local ip="${SERVERS[$server]}"
    
    if [[ -z "$ip" ]]; then
        echo -e "${RED}Error: Server '$server' not found${NC}"
        return 1
    fi
    
    if [[ -z "$cmd" ]]; then
        echo -e "${RED}Error: No command specified${NC}"
        return 1
    fi
    
    echo -e "${YELLOW}Executing on $server ($ip): $cmd${NC}"
    ssh "$USER@$ip" "$cmd"
}

# Main menu
main() {
    # If arguments provided, run in command-line mode
    if [[ $# -gt 0 ]]; then
        case "$1" in
            list) list_servers ;;
            ping) ping_server "$2" ;;
            ssh) ssh_server "$2" ;;
            status) check_status "$2" ;;
            roles) show_by_role ;;
            health) health_check ;;
            exec) exec_command "$2" "$3" ;;
            help) show_help ;;
            *) echo -e "${RED}Unknown command: $1${NC}"; show_help ;;
        esac
        exit 0
    fi
    
    # Interactive mode
    echo -e "${GREEN}=== Server Management CLI ===${NC}"
    echo -e "Type 'help' for commands, 'exit' to quit\n"
    
    while true; do
        read -p "server-manager> " cmd args
        
        case "$cmd" in
            list) list_servers ;;
            ping) ping_server "$args" ;;
            ssh) ssh_server "$args" ;;
            status) check_status "$args" ;;
            roles) show_by_role ;;
            health) health_check ;;
            exec) 
                read -r server command <<< "$args"
                exec_command "$server" "$command"
                ;;
            help) show_help ;;
            exit|quit) echo "Goodbye!"; break ;;
            "") continue ;;
            *) echo -e "${RED}Unknown command: $cmd${NC}";;
        esac
        echo ""
    done
}

# Run main function with all arguments
main "$@"