Bash Script for calculating total time in files from stdin

sum=0
count=0
while read line; do
duration=$(/opt/local/bin/ffmpeg -i "${line}" 2>&1 | grep Duration | cut -d ' ' -f 4 | sed s/,//)
hours=$(echo $duration | cut -d":" -f1)
minutes=$(echo $duration | cut -d":" -f2)
seconds=$(echo $duration | cut -d":" -f3 | cut -d"." -f1)
millis=$(echo $duration | cut -d":" -f3 | cut -d"." -f2)
let totalMillis=$millis+$seconds*1000+$minutes*60000+$hours*3600000
let count=$count+1
let sum=$sum+$totalMillis
done < /dev/stdin
echo "$count"
let avSeconds=$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