inotify-sub

#!/bin/sh
# Copyright 2016 W. Trevor King <wking@tremily.us>
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# usage: inotify-sub [OPTIONS] [DIRECTORY]
#
# Subscribe to messages being published under DIRECTORY, printing
# events to stdout as they are published.  If EVENT is set, only print
# events matching that label.  If it is not set, print all events.

EVENT=
EVENT_DIR=
WATCHING=0
LAST_OLD_EVENT=
MONITOR=0
HISTORIC=0

show_matching_event() {
	FILENAME="${1}"
	FILE_EVENT="${FILENAME#[^_]*_}"
	if test -z "${EVENT}" -o "_${EVENT}" = "_${FILE_EVENT}"
	then
		cat "${EVENT_DIR}/${FILENAME}" &&
		return 0
	fi
	return 1
}

less_than() {
	if test "_${1}" = "_${2}"
	then
		return 1
	fi
	A=$(printf '%s\n%s\n' "${1}" "${2}")
	B=$(echo "${A}" | sort)
	test "${A}" = "${B}"
	return "${?}"
}

while test -n "${1}"
do
	case "${1}" in
	-m|--monitor)
		MONITOR=1
		;;
	-e|--event)
		shift
		EVENT="${1}"
		;;
	-H|--historic)
		HISTORIC=1
		;;
	-*)
		cat <<-EOF >&2
			unrecognized argument: '${1}'

			usage: inotify-sub [OPTIONS] [DIRECTORY]

			Options:

			-m, --monitor
			  Instead of exiting after receiving a single event, execute
			  indefinitely.  The default behavior is to exit after the first
			  event occurs.

			-e <event>, --event <event>
			  Listen for specific events only.  If ommitted, all events are
			  listened for.

			-H, --historic
			  Replay historic events.  If ommitted, only new events are
			  output.
		EOF
		exit 1
	esac
	shift
done

DIRECTORY="${1:-.}"
FIFO_DIR="${DIRECTORY}/fifo"
EVENT_DIR="${DIRECTORY}/events"

mkdir -p "${EVENT_DIR}" &&
mkdir -p "${FIFO_DIR}" &&
inotifywait -m -e moved_to --format '%f' "${EVENT_DIR}" 2>&1 |
while read -r FILENAME
do
	if test "${WATCHING}" -eq 0
	then
		if test 'Watches established.' = "${FILENAME}"
		then
			WATCHING=1
			if test "${HISTORIC}" -eq 1
			then
				DATE=$(TZ=UTC date --iso-8601=ns) &&
				mkfifo -m 0600 "${FIFO_DIR}/${DATE}"
				ls "${EVENT_DIR}" | (
					LAST_OLD_EVENT=
					while read -r FNAME
					do
						LAST_OLD_EVENT="${FNAME}"
						if show_matching_event "${FNAME}" && test "${MONITOR}" -eq 0
						then
							break
						fi
					done &&
					echo "${LAST_OLD_EVENT}" >> "${FIFO_DIR}/${DATE}"
				) &
				LAST_OLD_EVENT=$(cat "${FIFO_DIR}/${DATE}") &&
				wait &&
				rm "${FIFO_DIR}/${DATE}" &&
				if test -n "${LAST_OLD_EVENT}" -a "${MONITOR}" -eq 0
				then
					pkill --parent "${$}" inotifywait &&
					break
				fi
			fi
		fi
	elif test -z "${LAST_OLD_EVENT}" ||
		less_than "${LAST_OLD_EVENT}" "${FILENAME}"
	then
		if show_matching_event "${FILENAME}" && test "${MONITOR}" -eq 0
		then
			pkill --parent "${$}" inotifywait &&
			break
		fi
	fi
done