mkdir命令是常用的命令，用来建立空目录，它还有2个常用参数：
- -m, --mode=模式 设定权限<模式> (类似 chmod)，而不是 rwxrwxrwx 减 umask
- -p, --parents 需要时创建上层目录，如目录早已存在则不当作错误
下面是英文原版
-m, --mode=MODE set file mode (as in chmod), not a=rwx - umask
-p, --parents no error if existing, make parent directories as needed
-v, --verbose print a message for each created directory
-Z set SELinux security context of each created directory
to the default type
--context[=CTX] like -Z, or if CTX is specified then set the SELinux
or SMACK security context to CTX
--help display this help and exit
--version output version information and exit