Open Ports Scanner using NMAP

A minimal Bash script that scans open TCP ports on one or more hosts using nmap. Designed for quick diagnostics, learning, and lightweight network visibility.

Features

• Scans multiple hosts from a file
• Supports:
• Top common ports (default)
• All TCP ports (1–65535)
• Outputs only open ports
• Clean, script-friendly output format: host port
• Read-only (does not modify any files)

Verify if NMAP is installed

nmap --version

Input File Format

The script expects a plain text file containing one host per line.

Example: hosts.txt

127.0.0.1
localhost
1.1.1.1
google.com

# comments and empty lines are ignored

Supported entries:

• IP addresses
• Hostnames
• localhost
• 127.0.0.1

Usage

Basic scan (top 1000 common ports)

./all-open-ports-nmap.sh hosts.txt

Scan top N ports

./all-open-ports-nmap.sh hosts.txt --top 100

Scan all TCP ports (1–65535)

./all-open-ports-nmap.sh hosts.txt --all

Output Format

Each open port is printed on its own line:

127.0.0.1 3306
1.1.1.1 53
1.1.1.1 80
1.1.1.1 443

If a host has no open ports, nothing is printed for that host.

Output is sent to stdout (terminal). To save results:

./all-open-ports-nmap.sh hosts.txt --all > results.txt

Script: all-open-ports-nmap.sh

#!/usr/bin/env bash
set -euo pipefail

# Scan open TCP ports on hosts listed in a file
# Output format: "host port"

MODE="top"
TOPN="1000"

HOSTS_FILE="${1:-}"
if [[ -z "$HOSTS_FILE" || ! -f "$HOSTS_FILE" ]]; then
  echo "Usage: $0 <hosts_file> [--top N | --all]" >&2
  exit 1
fi
shift || true

while [[ $# -gt 0 ]]; do
  case "$1" in
    --top) MODE="top"; TOPN="${2:-1000}"; shift 2 ;;
    --all) MODE="all"; shift ;;
    -h|--help)
      echo "Usage: $0 <hosts_file> [--top N | --all]"
      exit 0
      ;;
    *) echo "Unknown option: $1" >&2; exit 1 ;;
  esac
done

command -v nmap >/dev/null 2>&1 || {
  echo "Error: nmap is required (install with: brew install nmap)" >&2
  exit 1
}

while IFS= read -r raw || [[ -n "$raw" ]]; do
  host="$(echo "$raw" | sed 's/^[[:space:]]*//;s/[[:space:]]*$//')"
  [[ -z "$host" || "$host" =~ ^# ]] && continue

  if [[ "$MODE" == "all" ]]; then
    nmap -p- --open -T4 "$host" 2>/dev/null \
      | awk -v h="$host" '/^[0-9]+\/tcp[[:space:]]+open/ {
          split($1,a,"/");
          print h, a[1]
        }'
  else
    nmap --top-ports "$TOPN" --open -T4 "$host" 2>/dev/null \
      | awk -v h="$host" '/^[0-9]+\/tcp[[:space:]]+open/ {
          split($1,a,"/");
          print h, a[1]
        }'
  fi
done < "$HOSTS_FILE"