计算linux执行时间差

[root@localhost fred]# cat date_test.sh
#!/bin/bash

date
date_start=`date +%s`
date_end=`date +%s`
date
min=`expr $date_end - $date_start`

echo "cost is "`expr $date_end - $date_start`" secs."


echo cost is `expr $min / 60` mins.