bash shell計算時間差

function getTiming() {  
    start=$1
    end=$2
    start_s=$(echo $start | cut -d '.' -f 1)  
    start_ns=$(echo $start | cut -d '.' -f 2)  
    end_s=$(echo $end | cut -d '.' -f 1)  
    end_ns=$(echo $end | cut -d '.' -f 2)  
    time=$(( ( 10#$end_s - 10#$start_s ) * 1000 + ( 10#$end_ns / 1000000 - 10#$start_ns / 1000000 ) ))  
    echo "$time ms"

}

start=$(date +%s.%N)
end=$(date +%s.%N)
runtime=$(getTiming $start $end)
echo "runtime: "$runtime