The following groups are read: mysqldump client
The following options may be given as the first argument:
--no-defaults Don't read default options from any option file, except for login file.
--defaults-file=# Only read default options from the given file #.
--defaults-extra-file=# Read this file after the global files are read.
--defaults-group-suffix=# Also read groups with concat(group, suffix)
--login-path=# Read this path from the login file.
1）mysqldump 作为一个客户端工具，它会去上诉目录中寻找 my.cnf 文件，然后读取该文件中 [mysqldump] 和 [client] 下面的选项；
2）那些 defaults 相关的选项都是为了另外指定 配置文件和登录文件，极少使用；
-u, --user=name User for login if not current user.
-p, --password[=name] Password to use when connecting to server. If password is not given it's solicited on the tty.
-h, --host=name Connect to host.
-P, --port=# Port number to use for connection.
--protocol=name The protocol to use for connection (tcp, socket, pipe, memory).