shutdown on lan

sol.sh

#!/bin/bash

INTERFACE=ens33       # interface we're listening on
NUM_PACKETS=3         # number of packets sent at once
TIME_THRESHOLD=3      # max seconds between packets
PACKET_THRESHOLD=3    # packets needed to run command
COMMAND="echo DONE!"  # command to run

packet_count=0
last_timestamp=0

# parse ifconfig output:
# ens33     Link encap:Ethernet  HWaddr 00:0c:29:a1:6a:27
[[ $(ifconfig "$INTERFACE") =~ HWaddr[[:space:]](.{17}) ]]

# matched MAC address
mac="${BASH_REMATCH[1]}"

# transform MAC address to regex for ngrep by replacing all : with \x
# i.e. 00:11:22:33:44:55 to \x00\x11\x22\x33\x44\x55
mac_regex="\x${mac//:/\\x}"

while true; do
  # wait for NUM_PACKETS wol packets addressed to our MAC address on UDP port 9
  ngrep -d "$INTERFACE" -n "$NUM_PACKETS" -q -x "\xff{6}($mac_regex){16}" "udp and dst port 9" >/dev/null 2>&1
  # got'em, check time difference from last packet
  timestamp=$(date +%s)
  if [ "$last_timestamp" -ne "0" ]; then  # $last_timestamp != 0
    diff=$(($timestamp - $last_timestamp))
    if [ "$diff" -le "$TIME_THRESHOLD" ]; then  # $diff <= $TIME_THRESHOLD
      (( packet_count += 1))
      if [ "$packet_count" -eq "$PACKET_THRESHOLD" ]; then  # $packet_count == $PACKET_THRESHOLD
        echo "$timestamp: packet threshold reached, running \"$COMMAND\""
        eval "$COMMAND"
        exit 0
      else
        echo "$timestamp: got packet, count is $packet_count"
      fi
    else
      echo "$timestamp: time threshold exceeded (${diff}s), restarting"
      packet_count=1
    fi
  else
    echo "$timestamp: first packet"
    packet_count=1
  fi
  last_timestamp="$timestamp"
done