SwiftPing.sh

#!/bin/bash

# Enable strict mode for robust error handling. This will:
# - exit immediately if a command exits with a non-zero status (-e)
# - treat unset variables as an error (-u)
# - ensure the return value of a pipeline is the status of the last command
#   to exit with a non-zero status, or zero if all commands exit successfully (-o pipefail)
set -euo pipefail

# Function to display usage instructions for the script.
display_usage() {
    echo "Usage: ./SwiftPing.sh <IP_PREFIX> [START_OCTET] [END_OCTET]"
    echo "Example 1 (full /24 subnet): ./SwiftPing.sh 192.168.1"
    echo "Example 2 (custom range): ./SwiftPing.sh 192.168.1 10 50"
    echo ""
    echo "Note: Hostname resolution requires 'dig' command to be installed."
}

# Function to check if required commands (ping, dig) are installed on the system.
check_dependencies() {
    local missing_commands=""

    # Check for 'ping'
    if ! command -v ping &>/dev/null; then
        missing_commands+="ping "
    fi

    # If any commands are missing, print an error and exit.
    if [ -n "$missing_commands" ]; then
        echo "Error: The following required commands are not installed: $missing_commands"
        echo "Please install them and try again."
        exit 1
    fi
}

# --- Main Script Logic Starts Here ---

# Run dependency checks before proceeding.
check_dependencies

# Check for the correct number of arguments provided by the user.
# The script expects 1 argument (IP_PREFIX) or 3 arguments (IP_PREFIX, START_OCTET, END_OCTET).
if [ "$#" -lt 1 ] || [ "$#" -gt 3 ]; then
    display_usage # Show usage if arguments are incorrect.
    exit 1
fi

IP_PREFIX="$1"     # Assign the first argument as the IP network prefix.
START_OCTET=1      # Default starting IP octet.
END_OCTET=254      # Default ending IP octet.

# If 3 arguments are provided, use them for a custom IP range.
if [ "$#" -eq 3 ]; then
    START_OCTET="$2" # Assign the second argument as the custom start octet.
    END_OCTET="$3"   # Assign the third argument as the custom end octet.

    # Validate that START_OCTET and END_OCTET are numeric.
    if ! [[ "$START_OCTET" =~ ^[0-9]+$ && "$END_OCTET" =~ ^[0-9]+$ ]]; then
       echo "Error: START_OCTET and END_OCTET must be numbers."
       display_usage
       exit 1
    fi

    # Validate that START_OCTET and END_OCTET are within a valid range (1-254).
    if [ "$START_OCTET" -lt 1 ] || [ "$START_OCTET" -gt 254 ] || \
       [ "$END_OCTET" -lt 1 ] || [ "$END_OCTET" -gt 254 ]; then
       echo "Error: START_OCTET and END_OCTET must be between 1 and 254 (inclusive)."
       display_usage
       exit 1
    fi

    # Validate that START_OCTET is not greater than END_OCTET.
    if [ "$START_OCTET" -gt "$END_OCTET" ]; then
        echo "Error: START_OCTET cannot be greater than END_OCTET."
        display_usage
        exit 1
    fi
fi

echo "Scanning $IP_PREFIX.$START_OCTET to $IP_PREFIX.$END_OCTET..."
echo "Active hosts found:"
echo "-------------------"

# Array to store Process IDs (PIDs) of background 'ping' operations.
PIDS=()

# Loop through each IP address in the specified range.
for ip in $(seq "$START_OCTET" "$END_OCTET"); do
    # Run the ping and resolution logic in a subshell in the background.
    # This allows for parallel scanning.
    (
        TARGET_IP="$IP_PREFIX.$ip"
        # Perform a single ping with a 1-second timeout.
        # Redirect stdout and stderr to /dev/null to keep the console clean.
        if ping -c 1 -W 1 "$TARGET_IP" &>/dev/null; then
            HOSTNAME="" # Initialize hostname to empty.

            # Attempt hostname resolution only if 'dig' command is available.
            if command -v dig &>/dev/null; then
                # Use 'dig -x' for reverse DNS lookup and 'head -n 1' to get only the first result.
                HOSTNAME=$(dig -x "$TARGET_IP" +short | head -n 1)
            fi

            # Print the result using printf to ensure a newline after each entry.
            # If a hostname is found, print it in parentheses; otherwise, just print the IP.
            if [ -n "$HOSTNAME" ]; then
                printf "%s (%s)\n" "$TARGET_IP" "$HOSTNAME"
            else
                printf "%s\n" "$TARGET_IP"
            fi
        fi
    ) & # Run the entire subshell block in the background.
    PIDS+=($!) # Add the PID of the background process to the array.
done

# Wait for all background 'ping' processes to complete.
# '|| true' is added to prevent 'set -e' from exiting the script if a background process
# finishes with a non-zero exit status (e.g., if a ping fails, but we don't want to halt the entire scan).
for pid in "${PIDS[@]}"; do
    wait "$pid" || true
done

echo "-------------------"
echo "Scan complete."
echo "Note: Some hostnames might not resolve if 'dig' is not installed or available."