watch and execute

watch.sh

#!/bin/bash

# monitor STDIN
# run command when interval of lines come in smaller than sepecific duration

DURATION=2 # in seconds
LINES_COUNT=1
COMMAND="echo Aha!"

SECONDS=$DURATION
COUNTER=$LINES_COUNT

while true; do
  read line
  if (( SECONDS >= DURATION )); then
    SECONDS=0
    COUNTER=$LINES_COUNT
    continue
  fi

  COUNTER=$((COUNTER-1))
  if (( COUNTER <= 0 )); then
    eval $COMMAND
    SECONDS=$DURATION
    COUNTER=$LINES_COUNT
  fi
done