Bash script to find the total duration of videos in a folder

totaltime.sh

ext=".mp4"
ffmpeg_location="/usr/local/Cellar/ffmpeg/3.1.2/bin/ffmpeg"
for file in *$ext
do
 duration=$("$ffmpeg_location" -i "${file}" 2>&1 | grep Duration | cut -d ' ' -f 4 | sed s/,//)
 hours=10#$(echo "$duration" | cut -d":" -f1)
 minutes=10#$(echo "$duration" | cut -d":" -f2)
 seconds=10#$(echo "$duration" | cut -d":" -f3 | cut -d"." -f1)
 millis=10#$(echo "$duration" | cut -d":" -f3 | cut -d"." -f2)
 ((totalMillis=millis+seconds*1000+minutes*60000+hours*3600000))
 ((sum=sum+totalMillis))
done

let avSeconds=$sum/1000
let avMillis=$sum-$avSeconds*1000;

let avMinutes=$avSeconds/60;
let avSeconds=$avSeconds-$avMinutes*60;

let avHours=$avMinutes/60;
let avMinutes=$avMinutes-$avHours*60;
str=""
if [[ $avHours -gt 0 ]]; then
str="$avHours hours, "
fi
if [[ $avMinutes -gt 0 ]]; then
str="${str} ${avMinutes} minutes"
fi
if [[ $avSeconds -gt 0 ]]; then
str="${str} and ${avSeconds}.${avMillis} seconds"
fi
echo "$str"