#!/bin/bash
# hook into inotify to watch a file, and generate
# diffs for changes in real-time
#
# requires inotify-tools (apt-get install inotify-tools)
#
# @author Filipe Dobreira <filipe.dobreira@ez.no>
usage() {
  echo "Usage:"
  echo "  in-diff <file>|help"

  exit 1
}

# check arguments:
if [ "$1" == "" ] || [ ! -f "$1" ] || [ "$1" == "help" ]; then
  usage
fi

# check dependencies:
command -v inotifywait >/dev/null 2>&1 || { echo >&2 "inotifywait(1) not available, please install inotify-tools"; exit 1; }

FILE="$1"
SNAP=$(tempfile)

# copy the initial snapshot to a temp file
cp "$FILE" "$SNAP"

watch () {
  # start an inotifywatch loop:
  while RES=$(inotifywait -q -e modify $FILE); do	
    diff "$FILE" "$SNAP" -d
    cp "$FILE" "$SNAP"
  done
}

# Keep watching for changes, even if the file is deleted.
while true; do
  if [ ! -f "$FILE" ]; then
    echo "" > "$SNAP"
  else
    watch
  fi
done