Bash script to run something at dawn

dawn.sh

#!/bin/bash

civ_tw=$(curl -s 'https://api.sunrise-sunset.org/json?lat=X&lng=Y&date=today&formatted=0' | json_pp | grep civil_twilight_begin | awk '{print $3}' | sed -e 's/[",]//g')

date_sec_ct=$(date -d ${civ_tw} +%s)
date_now=$(date +%s)
sleepsec=$((${date_sec_ct} - ${date_now}))
end_at=$((${date_sec_ct} + 3600))
echo "Sleeping for ${sleepsec}"
sleep ${sleepsec}

cd ~/dawn
while /usr/bin/true
do
    fn=$(date +%H.%M.%S.jpg)
    ffmpeg -y  -rtsp_transport tcp -i rtsp://X.X.X.X:5000/stream0 -vframes 1 ~/dawn/${fn}
    now=$(date +%s)
    if [ ${now} -gt ${end_at} ]; then
	exit 0
    fi
    sleep 30
done