输入问号(?)，回车，就可以查看可以使用的命令

ftp> ?
Commands may be abbreviated. Commands are:

!               cr              mdir            proxy           send
$               delete          mget            sendport        site
account         debug           mkdir           put             size
append          dir             mls             pwd             status
ascii           disconnect      mode            quit            struct
bell            form            modtime         quote           system
binary          get             mput            recv            sunique
bye             glob            newer           reget           tenex
case            hash            nmap            rstatus         trace
ccc             help            nlist           rhelp           type
cd              idle            ntrans          rename          user
cdup            image           open            reset           umask
chmod           lcd             passive         restart         verbose
clear           ls              private         rmdir           ?
close           macdef          prompt          runique
cprotect        mdelete         protect         safe

如果不清楚某个命令的功能，可以使用 help 命令来查看其功能描述，　如输入help ls 会返回

list contents of remote directory