See https://github.com/umlaeute/v4l2loopback/issues/109#issuecomment-617638198

rtsp_on_demand.sh

#!/usr/bin/env bash
set -e

DEVICE_NUMBER="9"
DEVICE_FILE="/dev/video${DEVICE_NUMBER}"
RTSP_URL="rtsp://username:password@WIFIFCAM:554"

# GitHub: iddo
# https://github.com/umlaeute/v4l2loopback/issues/109#issuecomment-617638198

# This script creates a video loopback device with an
# on-demand RTSP stream connected to it, which allows saving resources when not in use.

# This work is licensed under the Creative Commons Attribution 3.0 Unported License.
# To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/
# or send a letter to Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.

function finish {
	# Your cleanup code here
	if [ -n "${PID}" ]; then # streamer is already running
		echo "--- Stopping RTSP streamer"
		kill "${PID}" && sleep 2 || true
	fi
	echo "--- Removing loaded v4l2loopback dkms"
	sudo modprobe -r v4l2loopback
}
trap finish EXIT

if [ ! -e "${DEVICE_FILE}" ]; then

	echo "--- Loading v4l2loopback with device ${DEVICE_FILE}"
	sudo modprobe v4l2loopback exclusive_caps=1 video_nr=${DEVICE_NUMBER} card_label="RTSP Stream"

	echo "--- Locking output format"
	sudo v4l2-ctl --device "${DEVICE_FILE}" --set-ctrl keep_format=1
	ffmpeg -rtsp_transport tcp -i "${RTSP_URL}" -f v4l2 "${DEVICE_FILE}" &
	PID="${!}"
	echo "--- PID of ffmpeg is '${PID}'"
	sleep 5

	echo "--- Setting device timeout (blank video if connection is lost)"
	sudo v4l2-ctl --device "${DEVICE_FILE}" --set-ctrl timeout=1500
fi

# Trigger usage check in 1 second so that if the camera is not used the streamer will stop
(sleep 1; touch "${DEVICE_FILE}")&

echo "--- Waiting for video device usage"
sudo inotifywait -e OPEN,CLOSE -m "${DEVICE_FILE}" |
while read dir op file; do
	if [ -n "${PID}" ]; then # streamer is already running
		if [ "$(lsof -t "${DEVICE_FILE}" | grep -v "${PID}" | wc -l)" -eq "0" ]; then # no more clients
			echo "--- No more clients, stopping RTSP streamer"
			kill "${PID}"
			unset PID
		fi
	elif [ "$(lsof -t "${DEVICE_FILE}" | wc -l)" -gt "0" ]; then # new clients
		echo "--- Detected usage of camera, spinning up RTSP streamer"
		ffmpeg -rtsp_transport tcp -i "${RTSP_URL}" -vf format=pix_fmts=yuv420p -f v4l2 "${DEVICE_FILE}" &
		# TODO add disconnection detection and restart streamer
		PID="${!}"
		echo "--- PID of ffmpeg is '${PID}'"
	fi
done