Vterm fails to get tty when invoking tmux, but only when vterm is started in a remote location

[mtekman]

Issue

If you start vterm from a remote org-mode document, then it spawns a remote shell at that location (which is fantastic!)

However, if you try to access a tmux session via vterm, then it fails with the error:

open terminal failed: can't use /dev/tty

You can get around this however by spawning vterm locally, and then ssh-ing into the machine, and then running tmux, which will work successfully

Steps to reproduce

1. Within emacs, in a remote document spawn vterm
2. Run tmux new-session -t test
3. Witness error message

Steps to work around

1. Within Emacs, in a local document spawn vterm
2. SSH into the remote machine from this vterm buffer
3. Run tmux new-session -t test
4. Witness no error message

---

Is it to do with the SSH_CONNECTION being the same one that Tramp uses?

[jixiuf]

Maybe it's a bug of tmux,
see https://unix.stackexchange.com/questions/404173/shell-command-tmux-throws-cant-use-dev-tty-error/512979

( exec </dev/tty; exec <&1; tmux )

works for me.

[ning-y]

I have the same issue. The fix does work for vterm/tmux, but if I use other terminal emulators, I do not run into the /dev/tty error to begin with.

Don't know enough to comment if its a vterm or tmux bug, though.

[raymond-w-ko]

I did some investigation and I think this is a tramp feature/bug. The issue happens here: tramp-sh.el

tramp has something called heredoc, which is used to support arguments that exceed command line length.

However, when it thinks it need heredoc it changes stdin (0) to /dev/tty here unconditionally, resulting in mismatched fd 0 vs fd 1 and 2.

I have only started learning Emacs like a few weeks ago, so I have no idea it doing this is a good or bad thing.

Thankfully there is a simple workaround. When it determines if it should enter heredoc mode, it checks is the program argument is surrounded by single quotes, so you can do something like this:

(setq vterm-tramp-shells '(("docker" "sh")
                           ("ssh" "'zsh'")))

Now tmux works!

In fact, this probably should be the default along with sshx (and maybe all the *sh) once someone more experienced than me has checked this reasoning.

[aramirezreyes]

Thanks for the insights! Both @raymond-w-ko and @jixiuf solutions work.