tramp: Overview
1 An overview of TRAMP
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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 ‘rsh’, ‘rlogin’, ‘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 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 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
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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 connection
to that host, here’s what happens:
• TRAMP invokes ‘telnet HOST’ or ‘rsh HOST -l USER’ 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 pass phrase (for
‘rsh’ 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 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 again.
• Upon successful login and TRAMP recognizes the shell prompt from
the remote host, TRAMP prepares the shell environment by turning
off echoing, setting 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, decodes that output to produce the file’s contents.
For external transfers, TRAMP sends a command as follows:
rcp 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 wth ‘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.
Info Catalog
tramp: Top
tramp: Obtaining Tramp