tmux: versions later than 3.4 break default shell behaviour on darwin

[x123]

Describe the bug

The following commit 80931ba pushing tmux to 3.5 appears to cause the users shell to not be respected inside of a tmux session on nix-darwin, always falling back to /bin/sh no matter if the default-shell option is set correctly or not.

Steps To Reproduce

Steps to reproduce the behavior:

1. Use tmux 3.5 or later (commit 80931ba or newer) on a nix-darwin system
2. Observe that you are always defaulted to /bin/sh, despite having another shell defined as default (both natively via chsh or by forcing default-shell in tmux)

Expected behavior

tmux respects and uses the correct user shell as it did in 3.4 and below.

Additional context

For now I need to revert to tmux 3.4, and I'm using the following overlay:
x123/dotfiles@1808a2e#diff-4c16758b5c5c0626884ec3692e2130e50cc772d436e95e2e0257fbee75b9cfe9R6

Notify maintainers

@matthiasbeyer

Metadata

$ nix run nixpkgs#nix-info -- -m
 - system: `"aarch64-darwin"`
 - host os: `Darwin 24.0.0, macOS 15.0.1`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.18.8`
 - channels(root): `"nixpkgs"`
 - nixpkgs: `/nix/store/fpivx4sjcp2vk4rp9nhliln5cwcp3kc6-source`

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[Atemu]

Is this an upstream bug?

[matthiasbeyer]

I like to believe that this is an upstream bug. Is there a way we can confirm that this is not an nix issue?

[x123]

Good point, I don't have a way to replicate it ATM because all my machines are nixos or nix-darwin. There is a comment here that I think may be involved in the issue:

"Revert to using /bin/sh for #(), run-shell and if-shell - only popups change to use default-shell." (in tmux/tmux#4162)

I will comment on this issue and see if anyone else notices this behavior.

[x123]

According to upstream tmux (tmux/tmux#4162) this appears to be a bug in tmux-sensible which is being implemented in my case by the home-manager programs.tmux directives.

"Note that a fix in 3.5 and 3.5a to correctly set SHELL for run-shell and if-shell reveals a bug in the tmux-sensible script, causing it to set default-shell to /bin/sh. This can be worked around by adding this to .tmux.conf after the tmux-sensible script is invoked:"

There is already a bug report in with home-manager: nix-community/home-manager#5952

Probably possible to close this one now, and wait for the HM fix.

[charlottia]

@x123 thanks for documenting this issue and what you tracked down so clearly, stopped me pulling my hair out for too long! :)