ln 命令使用格式：
ln [options] source dist
[options]：
-s Make symlinks instead of hardlinks
-f Remove existing destinations
-n Don't dereference symlinks - treat like normal file
-b Make a backup of the target (if exists) before link operation
-S suf Use suffix instead of ~ when making backup files
使用例子：
ln -f /sbin/ifconfig /bin/busybox