FTP (File Transfer Protocol) is a protocol — meaning a standard language that lets two machines communicate — used so that computers of different types (or with different operating systems) can transfer files over a network that uses TCP/IP.
FTP operates in a client/server environment, meaning that the remote machine is configured as a server, and consequently waits for the other machine to request a service from it. In UNIX, the service is provided by what is called a daemon, a small task that runs in the background. The FTP daemon is called ftpd.
There are many FTP client programs, some of which are run from a command-line (such as the command ftp, a standard installed in many operating systems), but a large majority allow the user to manipulate files via a graphical interface (such as CuteFTP), which makes file transfers more user-friendly.
The FTP protocol is used for transferring one file at a time, in either direction, between the client machine (the one which initiated the connection, i.e. the calling machine) and the server machine (which provided the FTP service, i.e. the called machine). The FTP protocol can also perform other actions, such as creating and deleting directories (only if they are empty), listing files, deleting and renaming files, etc.
Do not confuse the separate concepts of the protocol and its implementation!
A protocol is a set of rules and procedures which have been defined in order to standardise communications, while the implementation is software programming which follows (more or less) the protocol's recommendations. For this reason, FTP has been implemented across numerous platforms, based on the specifications of the protocol.
The command "ftp" is available across various platforms, including UNIX, Windows and Linux. The command initiates an FTP session, and is usually run as follows:
ftp server_name
server_name represents the name or IP address of the remote machine that the user wants to connect to. The target machine must, of course, have an FTP service.
Once the connection has been initialised, a few lines of text appear on the screen. The first line lets you know that you have connected to an FTP server, the next lines welcome you to it, and may indicate which kind of FTP site it is (i.e, what sort of files it hosts or which organisation owns it), or instructions for users.
In FTP, each line begins with a number that represents either success or failure. For a welcome message, the line might be preceded by the number 220, which means "the service is ready for the new user."
The server asks you to enter your user name (also called a login or identification), in order to set access rights (such as read/write privileges). After the user name has been accepted, a line beginning with the number 331 invites you to input your password, which is masked, meaning that it doesn't appear on the screen.
In some cases the server may be public, in which case you can log in anonymously, and you will therefore have to log in as "anonymous" (or "guest"). For public servers, custom dictates that the user enters his/her email address as the password, but you can enter whatever you choose.
Once the password has been accepted, a message will show if connection has been established or not, in which case a reason will be given (for example, the site may have reached its maximum number of users allowed at a time, in which case the message "No more users allowed" appears).
Once logged in, the FTP site waits for the user to enter commands describing actions to perform.
The normal FTP commands are:
| Order | Description |
|---|---|
| help | Displays all commands supported by the FTP server. |
| status | Used for showing some of the client machine's settings |
| binary | This command switches you from ASCII mode (sending text documents) to binary mode (sending binary files, i.e. non-text files like images or programs) |
| ascii | Switches from binary mode to ASCII mode. This is the default mode. |
| type | Displays the current transfer mode (binary or ASCII) |
| user | Allows you to log in on the current FTP server again using a different user name. You will then be requested to enter a new password. |
| ls | Lists all files found in the current directory. The command "ls -l" gives additional information on the files. |
| pwd | Displays the full name of the current directory |
| CD | The command means change directory, and is used for changing to a different directory. The command "cd .." is used to access the parent directory |
| mkdir | The command mkdir (in UNIX, or md in Microsoft) is used for creating a directory within the current directory. The use of this command is reserved for users with access allowing it. |
| rmdir | The command rmdir (in UNIX, or rmd in Microsoft) is used for deleting a directory within the current directory. The use of this command is reserved for users with access allowing it. |
| get | This command is used to retrieve a file found on the server.
|
| put | This command is used to send a local file to the server
|
| open | Logs out and opens a new session on another FTP server |
| close | Logs out, leaving the FTP client active |
| bye | Disconnects the FTP client from the server and puts it into inactive mode |
| quit | Disconnects the FTP client from the server and puts it into inactive mode |
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