Emacs TRAMP

TRAMP is a package providing an abstraction layer that can be used for accessing remote files on different machines. I say "abstraction layer" because it's not just a simple library for reading and writing files, it hooks into Emacs at a low enough level that other packages need not be aware of it in order to use it.

TRAMP stands for Transparent Remote (file) Access, Multiple Protocol

So what is this and how do I use it?

While TRAMP is built in to Emacs, it does require some external tools, so if you can, install these:

• ssh
• scp
• nc (optional)
• rsh/rlogin/telnet (if you hate security)
• ftp (optional)
• mimencode or uuencode (both optional)
• smbclient (optional)

And if you're on Windows:

• plink
• psch

Which are part of the Putty suite.

Tramp has a special syntax that you use when opening files in order to access the file as if it were local:

/{method}:{user}@{host}#{port}:{remote-file-path}

Not all the parts are required, so we'll look at a lot of examples

There are a bunch of methods that you can use with TRAMP, here are some of them

Most of the time, if you are on OS X or Linux, you'll want to use ssh or scp, and plink if you're on windows. If you really want to see the whole list, you can check tramp-methods.

Imagine you have a server remoteFoo and you want to edit a file in /etc/hosts, first you would invoke find-files with C-x C-f, and then type:

/ssh:user@remoteFoo:/etc/hosts

Hit enter, and now you have a buffer that coordinates to this file. You can also leave out the "user" part if you use the same user as your current username:

/ssh:remoteFoo:/etc/hosts

Whenever you save the file, tramp will transfer it back to the remote machine, you're technically editing a buffer that is a local (temporary) file, which is being transferred back and forth as needed by Emacs.

You can turn on tramp's logging by setting tramp-verbose to a number between 0 and 10 (inclusive), the default value is 3, with increasing values being more and more verbose:

Sometimes you run into trouble with tramp, it hangs, or it can't connect to a machine, this can especially happen if you forcibly disconnect the connection (like closing your laptop lid) while a session is open, in that case, you might need to clean up tramp's internal accounting before opening a new tramp session.

There are two "magic" cleanup commands:

• tramp-cleanup-all-buffers

  This kills all the remote buffers that tramp has around

• tramp-cleanup-all-connections

  Flushes all of tramp's internal things, like password, file, and connection caches, as well as internal buffers.

Have you ever accidentally opened a file as your user when in order to change it, you have to be root? Tramp can help!

(defun edit-file-with-sudo ()
  "Take the file currently being edited, and open it as root with `sudo'."
  (interactive)
  (let ((file-name (buffer-file-name)))
    (when file-name
      (find-alternate-file (concat "/sudo::" file-name)))))

This will re-open the buffer you're currently editing, but as root.

If you didn't want to be "root", but a different user, you could use the "su" method instead, with a username, so C-x C-f and then:

/su:test@localhost:/home/test/hi

The "sudo" method might allow you to do this also, but "su" saves two whole characters!

Now, it might not sound very efficient to make a connection, and in fact, it's not, this is why tramp caches a bunch of things by using ssh options to keep a connection around. It does this when tramp-use-ssh-controlmaster-options is set to "t", which it is by default.

Tramp will use things like ControlMaster, ControlPath, and ControlPersist to keep a connection around for a certain time.

In some cases though, you don't want this, you want to use your configuration options. Things that you stuck in ~/.ssh/config instead of tramps. If you set tramp-use-ssh-controlmaster-options to "nil" it will use those options, here are the ones I recommend:

Host *
  ControlMaster auto
  ControlPath ~/.ssh/master-%r@%h:%p
  # ControlPersist 30
  ForwardAgent yes
  ServerAliveInterval 60

ControlMaster allows sharing multiple sessions over a single network connection, "auto" means to try and use a master connection, or create one if it doesn't exist. This is what can speed up tramp.

ControlPath is the path for the socket user for the master control.

ControlPersist keeps the connection open for a certain amount of time, "0" or "yes" keeps it open forever, while a number means the number of seconds before automatically closing it.

Usually then, I'll "persist" a connection by ssh-ing to the machine in a terminal or shell buffer, and then tramp doesn't have to re-negotiate the connection every time I make a change to a buffer, it just has to transfer the file.

Since tramp isn't just for editing files, there are a lot of really neat things you can do with it. This isn't an exhaustive list, these are just the ones I use the most often.

As a bonus, here's a non-network connection, where I can look at the contents of my phone using tramp:

sudo adb devices

List of devices attached
LGD851d5b7eb08	device

And then I should be able to connect with this:

/adb::/

(DEMO)

In case it's useful to anyone, here's my configuration, it's pretty minimal.

(use-package tramp
  :defer 5
  :config
  (with-eval-after-load 'tramp-cache
    (setq tramp-persistency-file-name "~/.emacs.d/tramp"))
  (setq tramp-default-method "ssh"
        tramp-default-user-alist '(("\\`su\\(do\\)?\\'" nil "root"))
        tramp-adb-program "adb"
        ;; use the settings in ~/.ssh/config instead of Tramp's
        tramp-use-ssh-controlmaster-options nil
        ;; don't generate backups for remote files opened as root (security hazzard)
        backup-enable-predicate
        (lambda (name)
          (and (normal-backup-enable-predicate name)
               (not (let ((method (file-remote-p name 'method)))
                      (when (stringp method)
                        (member method '("su" "sudo"))))))))

  (use-package tramp-sh
    :config
    (add-to-list 'tramp-remote-path "/usr/local/sbin")
    (add-to-list 'tramp-remote-path "/opt/java/current/bin")
    (add-to-list 'tramp-remote-path "/opt/gradle/current/bin")
    (add-to-list 'tramp-remote-path "~/bin")))