FTP ACCT命令的作用是什么？

FTP协议有许多可使用的命令：

ABOR - abort a file transfer
CWD - change working directory
DELE - delete a remote file
LIST - list remote files
MDTM - return the modification time of a file
MKD - make a remote directory
NLST - name list of remote directory
PASS - send password
PASV - enter passive mode
PORT - open a data port
PWD - print working directory
QUIT - terminate the connection
RETR - retrieve a remote file
RMD - remove a remote directory
RNFR - rename from
RNTO - rename to
SITE - site-specific commands
SIZE - return the size of a file
STOR - store a file on the remote host
TYPE - set transfer type
USER - send username

及

我感兴趣的是ACCT命令。即使它不是在所有FTP服务器中都实现，它也必须来自某个地方。它的目的是什么

从RFC 959：

     ACCOUNT (ACCT)

        The argument field is a Telnet string identifying the user's
        account.  The command is not necessarily related to the USER
        command, as some sites may require an account for login and
        others only for specific access, such as storing files.  In
        the latter case the command may arrive at any time.

        There are reply codes to differentiate these cases for the
        automation: when account information is required for login,
        the response to a successful PASSword command is reply code
        332.  On the other hand, if account information is NOT
        required for login, the reply to a successful PASSword
        command is 230; and if the account information is needed for
        a command issued later in the dialogue, the server should
        return a 332 or 532 reply depending on whether it stores
        (pending receipt of the ACCounT command) or discards the
        command, respectively.

这对我也没有多大帮助。有人费心解释这件事吗

是否可以在服务器上传递要匹配的字符串？

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即创建具有不同访问权限或其他权限的组？用户指定用于授权的用户。一旦授权，用户可能需要在其拥有的各种帐户之间进行选择。虽然这可能不是大多数服务器的常见用例，但请记住，ftp不仅适用于每个用户只有一个帐户的系统，而且它是一种非常古老的协议，是在大型机上开发的，在分离授权和帐户时可能不同或更灵活的虚拟机和其他虚拟机。

所讨论的字符串、其含义和用途取决于ftp服务器软件。它可以识别登录的操作系统用户，或者其他类型的帐户。当它进入RFC时，它的意图是什么？能够识别要使用的帐户，与用户规范分开。那么用户和帐户之间的区别是什么？（…）USER指定用于授权的用户。一旦授权，用户可能需要在其拥有的各种帐户之间进行选择。虽然这可能不是大多数服务器的常见用例，但请记住，ftp不仅适用于每个用户只有一个帐户的系统，而且它是一个非常古老的协议，是针对大型机、虚拟机和其他设备开发的，在分离授权和帐户时，这些设备可能不同或更灵活。

     ACCOUNT (ACCT)

        The argument field is a Telnet string identifying the user's
        account.  The command is not necessarily related to the USER
        command, as some sites may require an account for login and
        others only for specific access, such as storing files.  In
        the latter case the command may arrive at any time.

        There are reply codes to differentiate these cases for the
        automation: when account information is required for login,
        the response to a successful PASSword command is reply code
        332.  On the other hand, if account information is NOT
        required for login, the reply to a successful PASSword
        command is 230; and if the account information is needed for
        a command issued later in the dialogue, the server should
        return a 332 or 532 reply depending on whether it stores
        (pending receipt of the ACCounT command) or discards the
        command, respectively.