When using ssh in term-mode, how do I get the remote system's hostname for emacs?

Question

I regularly use term-mode then ssh into a remote machine to run bash commands. I want to display the remote machine's hostname in my modeline. I added the relevant code for the modeline but I'm having trouble getting the right values. user-login-name and system-name always give me my local machine's information, since that's where Emacs itself is running.

If it's relevant, I'm using doom-modeline.

Answer 1

You'll need some assistance from the remote server for that, but the key functionality already exists in the form of the directory-tracking support implemented in term.el, so you can leverage that.

The gist of it is that the process filter term-emulate-terminal has support for custom Emacs-specific \eAnSiT escape sequences (which it passes to term-handle-ansi-terminal-messages) which can be issued by the shell in order to set Emacs variables term-ansi-at-dir, term-ansi-at-host, and term-ansi-at-user. These are then used for setting default-directory (in particular), but obviously the term-ansi-at-host value also facilitates your requirement.

Refer to the commentary (M-x finder-commentary RET term RET), and isearch for directory tracking. You'll find some code to add to your ~/.bashrc files (not only on your local host, but also on each remote host you will ssh to).

You can then use something like this to include the reported hostname in the mode line:

(add-to-list 'mode-line-misc-info '(term-ansi-at-host (" @" term-ansi-at-host)))

The suggested bash config doesn't actually recover from exiting ssh (you'll be back on the original host, but it will be reporting the host you just exited from), so you'll probably want to enhance that. This is only lightly tested, but:

# Emacs term.el directory tracking support.
dirtrack () {
    printf '\033AnSiTc %s\n' "$PWD"
    printf '\033AnSiTh %s\n' "$HOSTNAME"
    printf '\033AnSiTu %s\n' "$USER"
}
: ${HOSTNAME=$(uname -n)}
USER=$(whoami)
case $TERM in
    eterm*)
        cd()    { command cd    "$@" && printf '\033AnSiTc %s\n' "$PWD"; }
        pushd() { command pushd "$@" && printf '\033AnSiTc %s\n' "$PWD"; }
        popd()  { command popd  "$@" && printf '\033AnSiTc %s\n' "$PWD"; }
        ssh()   { command ssh   "$@" && dirtrack; }
        dirtrack
esac

Or if you only want the hostname info, and don't actually care about the directory-tracking support, then maybe just:

# Emacs term.el hostname tracking support.
: ${HOSTNAME=$(uname -n)}
case $TERM in
    eterm*)
        printf '\033AnSiTh %s\n' "$HOSTNAME"
        ssh() { command ssh "$@" && printf '\033AnSiTh %s\n' "$HOSTNAME"; }
esac

(I haven't tested that though, and it looks like term-handle-ansi-terminal-messages will still do some things with only the hostname being set, so it might be bad to not set everything.)

Or you might even consider making your shell prompts issue these escape sequences instead of using the above approaches. That would be redundant for most commands, but is potentially more robust (for handling arbitrary wrappers around ssh for instance). You'd have to experiment.