cal 命令：
用法：
usage: cal [-jy] [[month] year]
cal [-j] [-m month] [year]
ncal [-Jjpwy] [-s country_code] [[month] year]
ncal [-Jeo] [year]
1.显示当月日历 cal
2.显示当月日历是当前年的第几天 cal -j
3.显示当前年日历 cal -y
4.显示指定年月日历 cal 1 2019
5.竖向排列日历 ncal
6.显示本周是第几周 ncal -w
7.显示国家码 ncal -p