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1 An overview of TRAMP

TRAMP is for transparently accessing remote files from within Emacs. TRAMP enables an easy, convenient, and consistent interface to remote files as if they are local files. TRAMP’s transparency extends to editing, version control, and dired.

TRAMP can access remote hosts using any number of access methods, such as ssh, scp, telnet, and related programs. If these programs can successfully pass ASCII characters, TRAMP can use them. TRAMP does not require or mandate 8-bit clean connections.

TRAMP’s most common access method is through ssh, a more secure alternative to ftp and other older access methods.

TRAMP on MS Windows operating systems is integrated with the PuTTY package, and uses the plink program.

TRAMP mostly operates transparently in the background using the connection programs. As long as these programs enable remote login and can use the terminal, TRAMP can adapt them for seamless and transparent access.

TRAMP temporarily transfers a remote file’s contents to the local host editing and related operations. TRAMP can also transfer files between hosts using standard Emacs interfaces, a benefit of direct integration of TRAMP in Emacs.

TRAMP can transfer files using any number of available host programs for remote files, such as rcp, scp, rsync or (under MS Windows) pscp. TRAMP provides easy ways to specify these programs and customize them to specific files, hosts, or access methods.

For faster small-size file transfers, TRAMP supports encoded transfers directly through the shell using mimencode or uuencode provided such tools are available on the remote host.

TRAMP behind the scenes

Accessing a remote file through TRAMP entails a series of actions, many of which are transparent to the user. Yet some actions may require user response (such as entering passwords or completing file names). One typical scenario, opening a file on a remote host, is presented here to illustrate the steps involved:

C-x C-f to initiate find-file, enter part of the TRAMP file name, then hit TAB for completion. If this is the first time connecting to that host, here’s what happens:

• TRAMP invokes ‘telnet host’ or ‘ssh -l user host’ and establishes an external process to connect to the remote host. TRAMP communicates with the process through an Emacs buffer, which also shows output from the remote host.
• The remote host may prompt for a login name (for telnet, for example) in the buffer. If on the other hand, the login name was included in the file name portion, TRAMP sends the login name followed by a newline.
• The remote host may then prompt for a password or passphrase (for ssh or for telnet). TRAMP displays the password prompt in the minibuffer. TRAMP then sends whatever is entered to the remote host, followed by a newline.
• TRAMP now waits for either the shell prompt or a failed login message.

  If TRAMP does not receive any messages within a timeout period (a minute, for example), then TRAMP responds with an error message about not finding the remote shell prompt. If there are any messages from the remote host, TRAMP displays them in the buffer.

  For any ‘login failed’ message from the remote host, TRAMP aborts the login attempt, and repeats the login steps.

• Upon successful login, if TRAMP recognizes the shell prompt from the remote host, TRAMP prepares the shell environment by turning off echoing, setting the shell prompt, and other housekeeping chores.

  Note that for the remote shell, TRAMP invokes /bin/sh. The remote host must recognize ‘exec /bin/sh’ and execute the appropriate shell. This shell must support Bourne shell syntax.

• TRAMP executes cd and ls commands to find which files exist on the remote host. TRAMP sometimes uses echo with globbing. TRAMP checks if a file or directory is writable with test. After each command, TRAMP parses the output from the remote host for completing the next operation.
• After remote file name completion, TRAMP transfers the file contents from the remote host.

  For inline transfers, TRAMP sends a command, such as ‘mimencode -b /path/to/remote/file’, waits until the output has accumulated in the buffer, then decodes that output to produce the file’s contents.

  For external transfers, TRAMP sends a command as follows:

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

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

• Edit, modify, change the buffer contents as normal, and then save the buffer with C-x C-s.
• TRAMP transfers the buffer contents to the remote host in a reverse of the process using the appropriate inline or external program.

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