FTP Replies, Reply Code Format and Important Reply Codes
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Each time that the User-PI sends a command to the Server-PI over the control connection, the server sends back a reply. FTP replies serve three main purposes. First, they serve as confirmation that the server received a command. Second, they tell the user device whether the command was accepted or not, and if an error occurred, what it was. Third, they communicate various types of information to the user of the session, such as the status of a transfer.
For a human user, a string of reply text would be sufficient to satisfy the requirements above, and FTP replies do include descriptive text. But having only a text string would make it difficult or impossible for FTP software on the client side to interpret results coming from the server. FTP was designed to also allow software applications to interact with each other over the FTP command link. For this reason, the protocol designed its reply system around the use of reply codes.
FTP reply codes are three-digit numeric responses that can be easily interpreted by a computer program. They are also useful for human users who are familiar with FTP, because they communicate at a glance the results of various operations. While each FTP server implementation may differ in the text sent for each type of reply, the reply codes are used in a consistent manner based on the specifications of the FTP standard. It is therefore the codes that are examined to determine the results of a command; the text is just descriptive.
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