shell中bc expr [ ] (( ))的使用方法
http://blog.chinaunix.net/uid-20671208-id-3552751.html
http://blog.163.com/panfeng_pf/blog/static/1119608542009212423455/
http://blog.chinaunix.net/uid-20671208-id-3552751.html
http://blog.163.com/panfeng_pf/blog/static/1119608542009212423455/