Problems with UTF-8 when attaching to a tmux session over ssh

Question

Scenario:

I have a server, on which a tmux session is running. Let's call that session tmux_session in the rest of this post.

Attaching to that session can be done in two, slightly different ways:

1. By first ssh-ing to the server:

   user@host$ ssh [email protected]
   

   and then attaching to the named session:

   user@server$ tmux a -d -t tmux_session
   

2. By ssh-ing and attaching at the same time:

   user@host$ ssh [email protected] -t tmux -a -d -t tmux_session
   

Symptoms:

Method №1 works as expected. My LANG is set to C.UTF-8, echo-ing it inside tmux_session works fine, and it's reported correctly. Unicode input of characters like "¹²³¤", works perfectly fine.

Method №2 yields a session, which is unable to display Unicode characters, beyond simple ASCII. All other characters get replaced by _ (an underscore). However when entering a special character, the character is actually entered into whatever application is running inside tmux_session, I confirmed it using editors, an IRC client, etc. When I detach and re-attach using method №1, the previously entered character is displayed correctly.

This problem when using method №2, only happens when attaching to the session with the ssh command, and it only affects the display of the characters, not the actual characters being entered.

Some of the output inside tmux is kinda broken both times, though. When I enter echo $LANG inside tmux_session using either method, I see this:

$ echo $LANG                            
cho C.UTF-8C.UTF-8

Something is garbled, for some reason the input line is (partially) repeated. I read in another post on superuser.com, that one should try by first deleting .tmux.conf. I don't have a .tmux.conf, so not having that file doesn't seem to alleviate the issue for me. In any event, $LANG being set to C.UTF-8 is correct.

This garbled output does not happen, when just ssh-ing to the server.

Answer 1

You're checking the locale settings inside the tmux sessions, but not those that tmux itself receives.

server.ltd likely doesn't have AcceptEnv LANG LC_* in its sshd_config or/and you don't have SendEnv LANG LC_* in your ssh_config, so the fact that your local system and terminal is using UTF-8 as the charset is not transmitted to the remote tmux client.

You could work around it by doing:

ssh -t [email protected] "
  set -o allexport
  $(locale | grep -v '"')
  exec tmux -a -d -t tmux_session"

(assuming the local shell and the login shell of the remote user are POSIX-like)

Or just hardcode the UTF-8 locale of your choice if you know your terminal is talking in UTF-8:

ssh -t [email protected] 'exec env LANG=C.UTF-8 tmux -a -d -t tmux_session'

(a syntax understood by all shells).

Answer 2

Use tmux -u to force output of utf-8 even if $LANG is not set correctly.