blog/content/posts/Editing-remote-files-with-emacs-comfortably.adoc

---

title: "Editing remote files with Emacs, comfortably" description: "Edit remote files with Emacs using SSH and a wrapper for emacsclient." date: "2019-05-08T15:59:00+02:00" draft: false tags: ["emacs", "ssh"] ---

It took me a long time to collect all the bits and pieces I needed to make editing remote files with Emacs work the way I want, with a simple command that works via SSH. I hope I can save you some time by stitching it here together into a tutorial. I assume you use use-package in my examples.

Emacs server & TRAMP

We start with Emacs’s good old inbuilt server. The default is to use an UNIX domain socket; We have to change that to TCP to be able to receive input from our remote hosts. The server will bind to 127.0.0.1. Pick a strong password that is exactly 64 characters long and a port above 1023. I chose 51313 because if we substitute the digits for letters in the Latin alphabet, we get E M A C. The server will create the file ~/.emacs.d/server/server with the IP, port and password in it. This file needs to be distributed to every host that should be able to access the server.

{{< highlight elisp >}}
;; Run server if:
;; - Our EUID is not 0,
;; - We are not logged in via SSH,
;; - It is not already running.
(unless (equal (user-real-uid) 0)
  (unless (getenv "SSH_CONNECTION")
    (use-package server
      :init
      (setq server-use-tcp t
            server-port 51313
            server-auth-key ; 64 chars, saved in ~/.emacs.d/server/server.
            "looph8oow3Aph5ahje1eek1aish3Ohthu4Paengae0iketohGhaemi2iek5ae4ee")
      :config
      (unless (eq (server-running-p) t) ; Run server if not t.
          (server-start)))))
{{< / highlight >}}

The server expects filenames as input, we can’t just feed it the file. The package TRAMP allow us to use remote file paths with Emacs with the help of SSH. I have modified tramp-password-prompt-regexp to look for verification code prompts from the Google Authenticator PAM module.

Note	My modification overwrites the original value of tramp-password-prompt-regexp, which has a bunch of localized variants of “password” in it. You can view the original value with C-h v tramp-password-prompt-regexp.

{{< highlight elisp >}}
(use-package tramp
  :custom
  (tramp-use-ssh-controlmaster-options nil) ; Don't override SSH config.
  (tramp-default-method "ssh")    ; ssh is faster than scp and supports ports.
  (tramp-password-prompt-regexp   ; Add verification code support.
   (concat
    "^.*"
    (regexp-opt
     '("passphrase" "Passphrase"
       "password" "Password"
       "Verification code")
     t)
    ".*:\0? *")))
{{< / highlight >}}

SSH

In order to avoid having to enter our password again and again, we can edit our SSH configuration to reuse existing connections. The following configuration will create an UNIX domain socket per host and re-use that for all further connections to this host. It will also forward the Emacs server port, that we picked earlier, to every host we connect to. We will have to create ~/.ssh/sockets/ before we use the new configuration.

Warning

These sockets allow for unauthenticated access to every host you are connected to. While this is very convenient, it is also a security risk. The sockets are only usable by your user and root (file mode 0600).

Warning

Everyone on the remote host can connect to the port you forward. They will still need the password, but you might not want to do this if you don’t trust the other users.

{{< highlight cfg >}}
Host fc??:* fd??:* 192.168.* server1.example.com server2.example.com
    # Reuse connections.
    ControlMaster auto
    # Close socket 600s after after last connection closes.
    ControlPersist 600
    # Set path for sockets.
    ControlPath ~/.ssh/sockets/%r@%h-%p
    # Forward Emacs-server port.
    RemoteForward 127.0.0.1:51313 127.0.0.1:51313
{{< / highlight >}}

Wrapper for emacsclient

Using file paths in TRAMP notation gets annoying really quick. Thankfully Andy Skelton created a wrapper script; I extended it with the ability to become root using sudo and an option to use it with local servers. This file needs to be distributed to every host that should be able to access the server.

emacsremote

{{< highlight bash >}}
#!/bin/bash
# Open file on a remote Emacs server.
# https://andy.wordpress.com/2013/01/03/automatic-emacsclient/ with added sudo.

params=()
sudo=0
local=0

for p in "${@}"; do
    if [[ "${p}" == "-n" ]]; then
        params+=( "${p}" )
    elif [[ "${p:0:1}" == "+" ]]; then
        params+=( "${p}" )
    elif [[ "${p}" == "--sudo" ]]; then
        # This only works if the user can access the file.
        sudo=1
    elif [[ "${p}" == "--local" ]]; then
        # Use local server, for use with --sudo.
        local=1
    else
        if [[ $(id -u) -eq 0 || ${sudo} -eq 1 ]]; then
            if [[ ${local} -eq 0 ]]; then
                params+=( "/ssh:$(hostname -f)|sudo:$(hostname -f):"$(readlink -f $p) )
            else
                params+=( "/sudo:localhost:"$(readlink -f $p) )
            fi
        else
            params+=( "/ssh:$(hostname -f):"$(readlink -f $p) )
        fi
    fi
done
emacsclient "${params[@]}"
{{< / highlight >}}

Shell configuration

Now we should set VISUAL and EDITOR to the wrapper and set up some nice, short aliases. In my examples I assume we called our wrapper emacsremote.

Note	I wrote the following code for Zsh, but it should also work for Bash.

{{< highlight zsh >}}
# Set preferred editor.
if which emacsclient > /dev/null; then
    VISUAL="$(which emacsclient) -a emacs"
    if [[ -n "${SSH_CONNECTION}" ]]; then # Logged in via SSH.
        if which emacsremote > /dev/null; then
            VISUAL="$(which emacsremote)"
        fi
    elif [[ $(id -u) -eq 0 ]] && which emacsremote > /dev/null; then
        # Edit files as root in the Emacs instance run by the current user.
        VISUAL="$(which emacsremote) --sudo --local"
    fi
elif which emacs > /dev/null; then
    VISUAL="$(which emacs)"
elif which vim > /dev/null; then
    VISUAL="$(which vim)"
elif which nano > /dev/null; then
    VISUAL="$(which nano)"
fi
export VISUAL
export EDITOR="${VISUAL}"
{{< / highlight >}}

{{< highlight zsh >}}
if [[ "${VISUAL}" =~ "emacs(client|remote)" ]]; then
    alias e="${VISUAL} -n"
    if [[ "${VISUAL}" =~ "emacsremote$" ]]; then
        # Don't block the terminal until the file is closed.
        alias se="${VISUAL} -n --sudo"
    elif which emacsremote >/dev/null &&  [[ -z "${SSH_CONNECTION}" ]]; then
        # Edit files as root in the Emacs instance run by the current user.
        alias se="$(which emacsremote) -n --sudo --local"
    fi
else
    alias e="${VISUAL}"
    alias se="sudo ${VISUAL}"
fi
{{< / highlight >}}

To detect SSH connections after using sudo -i, we have to tell sudo to preserve the environment variable SSH_CONNECTION.

{{< highlight zsh >}}
echo 'Defaults env_keep += "SSH_CONNECTION"' >> /etc/sudoers.d/ssh_vars
{{< / highlight >}}