TCP port scanner in Bash

strobe.bash

#!/bin/bash

# strobe.bash
# Bash TCP/IP port scanner.

# http://www.catonmat.net/blog/tcp-port-scanner-in-bash/
# alarm() by Coder.C, scan() by Peteris Krumins, wrapped by Yesmar.

# Usage: strobe.bash <target> <protocol> <low[-high][,...]>
# You can specify 80 or 22,80 or 22,80,90-100,10000-25000,43001
# It's best to do this: strobe.bash localhost tcp 22,80,443 2>&1 | grep open
# This script requires the presence of /dev/tcp.

script=$(basename "$0")

function usage() {
  echo "Usage: $script <target> <proto> <low[-high][,...]>"
}

function alarm() {
  timeout=$1; shift;
  bash -c "$@" &
  pid=$!
  {
    sleep "$timeout"
    kill $pid 2> /dev/null
  } &
  wait $pid 2> /dev/null
  return $?
}

scan() {
  if [[ -z $1 || -z $2 || -z $3 ]]; then
    usage
    return
  fi

  local host=$1
  local proto=$2
  local ports=()
  case $3 in
    *,*) IFS=, read -ra ports <<< "$3" ;;
    *-*) IFS=- read -r start end <<< "$3"
         for ((port=start; port <= end; port++)); do ports+=($port); done ;;
    *)   ports+=($3) ;;
  esac

  for port in "${ports[@]}"; do
    alarm 1 "echo >/dev/$proto/$host/$port &&
      echo \"port $port/$proto is open\"" ||
      echo "port $port/$proto is closed"
  done
}

if [ $# -lt 2 ]; then
  usage
  exit 1
else
  scan "$@"
  exit 0
fi

This prints N in about a second:

alarm 1 "echo >/dev/tcp/google.com/230" && echo "Y" || echo "N"

This still returns almost immediately and prints Y:

alarm 60 "echo >/dev/tcp/google.com/80" && echo "Y" || echo "N"

This script has been useful in scanning minicomputers with flaky TCP/IP stacks.