Poor man's guard: a simple, one line, watch function. For Makefiles or shell scripts.

watch.mk

watch = while true; do (inotifywait -qre close_write --format "%w%f" . | grep $1) && $2; done

all:
	$(call watch, 'test/.*.txt', echo "some stuff has changed...")

watch.sh

#!/bin/bash

function watch {
  while true; do
    (inotifywait -qre close_write --format "%w%f" . | grep $1) && $2;
  done
}

watch 'test/.*.txt'  'echo some stuff has changed...'