Respect the user's login shell when spawning a new pane

[himkt]

idea / problem

In sessions where SHELL is not in the environment (kubectl exec, docker exec, chroot, some CI), new panes start /bin/sh. The interactive shell shows $SHELL from the user's passwd entry without exporting it, so herdr never inherits it:

$ env -u SHELL bash -c 'echo $SHELL'
/bin/bash          # bash fills $SHELL from the passwd entry

$ env -u SHELL bash -c 'printenv SHELL'
                   # no output — not exported, children never see it

The current herdr does not use login shell stored in passwd. Therefore, I need to launch a shell that I want to use every time after spawning the pane even though that is already launched on the active pane.

herdr/src/pane.rs

Lines 1206 to 1227 in 8d2af4e

	fn pane_shell(configured_shell: &str) -> String {
	pane_shell_from(configured_shell, std::env::var("SHELL").ok())
	}
	fn pane_shell_from(configured_shell: &str, env_shell: Option<String>) -> String {
	let configured_shell = configured_shell.trim();
	if !configured_shell.is_empty() {
	return configured_shell.to_string();
	}
	#[cfg(windows)]
	{
	let _ = env_shell;
	default_pane_shell()
	}
	#[cfg(not(windows))]
	env_shell
	.map(|shell| shell.trim().to_string())
	.filter(|shell| !shell.is_empty())
	.unwrap_or_else(default_pane_shell)
	}

requested change

Respect a login shell stored in passwd. In my understanding, the behavior would be compatible with tmux so I think this request is (hopefully) reasonable.

https://github.com/tmux/tmux/blob/e50545f4516bebb2efb027d077895bd8ca9727a3/tmux.c#L90-L92

(I created the patch on #1516)

why you want this

On a container that has no exported SHELL environment, it's expensive to run cd path/to/workspace and bash every time on spawning a new pane.