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"]
comtodon: 9ibPpjSsYeNmHrbBDc
---

:source-highlighter: pygments

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
https://github.com/jwiegley/use-package[use-package] in my examples.

== Emacs server & TRAMP

We start with Emacs's good old inbuilt
https://www.gnu.org/software/emacs/manual/html_node/emacs/Emacs-Server.html[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
https://www.w3.org/Daemon/User/Installation/PrivilegedPorts.html[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.

[source,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)))))
----

The server expects filenames as input, we can't just feed it the file. The
package https://www.gnu.org/software/tramp/[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
https://github.com/google/google-authenticator-libpam[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`.

[source,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? *")))
----

== SSH

In order to avoid having to enter our password again and again, we can edit our
https://linux.die.net/man/5/ssh_config[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.

[source,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
----

== Wrapper for emacsclient

Using file paths in TRAMP notation gets annoying really quick. Thankfully Andy
Skelton created a
https://andy.wordpress.com/2013/01/03/automatic-emacsclient/[wrapper script]; I
extended it with the ability to https://stackoverflow.com/a/16408592[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.

.https://dotfiles.tastytea.de/bin/emacsremote[emacsremote]
[source,shell]
----
#!/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
        sudo=1
    elif [[ "${p}" == "--local" ]]; then
        # Use local server, for use with --sudo.
        local=1
    else
        # Setting field separator to newline so that filenames with spaces will
        # not be split up into 2 array elements.
        OLDIFS=${IFS}
        IFS=$'\n'

        if [[ $(id -u) -eq 0 || ${sudo} -eq 1 ]]; then
            if [[ ${local} -eq 0 ]]; then
                params+=( "/ssh:$(hostname -f)|sudo:$(hostname -f):"$(realpath -m "${p}") )
            else
                params+=( "/sudo:localhost:"$(realpath -m "${p}") )
            fi
        else
            params+=( "/ssh:$(hostname -f):"$(realpath "${p}") )
        fi

        IFS=${OLDIFS}
    fi
done

emacsclient -f ~/.emacs.d/server/server "${params[@]}"
----

I had to add `[[ "${TERM}" = "dumb" ]] && unsetopt zle` to my Zsh configuration
to prevent TRAMP connections from hanging all the time. Thanks to Darius for
https://emacs.stackexchange.com/a/45810[their answer on StackExchange].

== 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`. The
argument `-f` causes emacsclient to not try to use UNIX domain sockets (and
print an error message).

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

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

[source,shell]
----
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="${VISUAL} -n --sudo"
    elif command -v emacsremote >/dev/null &&  [[ -z "${SSH_CONNECTION}" ]]; then
        # Edit files as root in the Emacs instance run by the current user.
        alias se="$(command -v emacsremote) -n --sudo --local"
    fi
else
    alias e="${VISUAL}"
    alias se="sudo ${VISUAL}"
fi
----

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

[source,shell]
----
echo 'Defaults env_keep += "SSH_CONNECTION"' >> /etc/sudoers.d/ssh_vars
----

== Updates

* Updated 2019-05-12: Add `-f` argument to emacsclient.
* Updated 2019-10-06: Support files with spaces in emacsremote and allow to
  open files the user can't read (for use with emacsremote --sudo).
* Updated 2019-10-17: Added Zsh-hack to prevent hanging TRAMP-connections.