1 An overview of tramp

After the installation of tramp into your GNU Emacs, you will be able to access files on remote machines as though they were local. Access to the remote file system for editing files, version control, and dired are transparently enabled.

Your access to the remote machine can be with the rsh, rlogin, telnet programs or with any similar connection method. This connection must pass ASCII successfully to be usable but need not be 8-bit clean.

The package provides support for ssh connections out of the box, one of the more common uses of the package. This allows relatively secure access to machines, especially if ftp access is disabled.

Under Windows, tramp is integrated with the PuTTY package, using the plink program.

The majority of activity carried out by tramp requires only that the remote login is possible and is carried out at the terminal. In order to access remote files tramp needs to transfer their content to the local machine temporarily.

tramp can transfer files between the machines in a variety of ways. The details are easy to select, depending on your needs and the machines in question.

The fastest transfer methods for large files rely on a remote file transfer package such as rcp, scp, rsync or (under Windows) pscp.

If the remote copy methods are not suitable for you, tramp also supports the use of encoded transfers directly through the shell. This requires that the mimencode or uuencode tools are available on the remote machine. These methods are generally faster for small files.

tramp is still under active development and any problems you encounter, trivial or major, should be reported to the tramp developers. See Bug Reports.

Behind the scenes

This section tries to explain what goes on behind the scenes when you access a remote file through tramp.

Suppose you type C-x C-f and enter part of an tramp file name, then hit <TAB> for completion. Suppose further that this is the first time that tramp is invoked for the host in question. Here's what happens:

• tramp discovers that it needs a connection to the host. So it invokes ‘telnet host’ or ‘rsh host -l user’ or a similar tool to connect to the remote host. Communication with this process happens through an GNU Emacs buffer, that is, the output from the remote end goes into a buffer.
• The remote host may prompt for a login name (for telnet). The login name is given in the file name, so tramp sends the login name and a newline.
• The remote host may prompt for a password or pass phrase (for rsh or for telnet after sending the login name). tramp displays the prompt in the minibuffer, asking you for the password or pass phrase.

  You enter the password or pass phrase. tramp sends it to the remote host, followed by a newline.

• tramp now waits for the shell prompt or for a message that the login failed.

  If tramp sees neither of them after a certain period of time (a minute, say), then it issues an error message saying that it couldn't find the remote shell prompt and shows you what the remote host has sent.

  If tramp sees a ‘login failed’ message, it tells you so, aborts the login attempt and allows you to try again.

• Suppose that the login was successful and tramp sees the shell prompt from the remote host. Now tramp invokes /bin/sh because Bourne shells and C shells have different command syntaxes.¹

  After the Bourne shell has come up, tramp sends a few commands to ensure a good working environment. It turns off echoing, it sets the shell prompt, and a few other things.

• Now the remote shell is up and it good working order. Remember, what was supposed to happen is that tramp tries to find out what files exist on the remote host so that it can do filename completion.

  So, tramp basically issues cd and ls commands and also sometimes echo with globbing. Another command that is often used is test to find out whether a file is writable or a directory or the like. The output of each command is parsed for the necessary operation.

• Suppose you are finished with filename completion, have entered C-x C-f, a full file name and hit <RET>. Now comes the time to transfer the file contents from the remote host to the local host so that you can edit them.

  See above for an explanation of how tramp transfers the file contents.

  For inline transfers, tramp issues a command like ‘mimencode -b /path/to/remote/file’, waits until the output has accumulated in the buffer that's used for communication, then decodes that output to produce the file contents.

  For external transfers, tramp issues a command like the following:

            rcp user@host:/path/to/remote/file /tmp/tramp.4711
  

  It then reads the local temporary file /tmp/tramp.4711 into a buffer and deletes the temporary file.

• You now edit the buffer contents, blithely unaware of what has happened behind the scenes. (Unless you have read this section, that is.) When you are finished, you type C-x C-s to save the buffer.
• Again, tramp transfers the file contents to the remote host either inline or external. This is the reverse of what happens when reading the file.

I hope this has provided you with a basic overview of what happens behind the scenes when you open a file with tramp.