shell 日期减法 自加

#!/bin/bash

logname=/home/test.log

filename=/opt/test.txt
count=0

while true; do
    LASTLINE=`date +"%Y-%m-%d %H:%M:%S"`
    ls $filename 
    CURTIME=`date +"%Y-%m-%d %H:%M:%S"`
    Sys_data=`date -d "$CURTIME" +%s`
    In_data=`date -d "$LASTLINE" +%s`
    interval=`expr $Sys_data - $In_data`
    ((count +=1))
    if (( $interval > 1 || $count==10 ))
        then
        count=0
        echo "ls $filename  cost time: $interval" >> $logname
        date >> $logname
    fi
    sleep 2
done