devenv shell: support fish as interactive shell

[schickling]

Problem

devenv shell currently only supports bash as the interactive shell. For users whose default shell is fish, this means devenv shell drops them into bash instead of fish, losing their fish configuration, functions, completions, and prompt.

Context

Issue #36 tracks zsh support, and recent work (by @sandydoo and @mcdonc) has made progress on alternative shell support. Fish is another commonly used shell that would benefit from the same treatment.

As devenv moves toward replacing direnv (#2061), native fish support in devenv shell becomes a prerequisite for fish users to migrate away from the use devenv direnv integration.

Expected behavior

devenv shell should detect the user's preferred shell (via $SHELL or similar) and launch fish when appropriate, with the devenv environment correctly exported.

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This issue was created on behalf of @schickling

[schickling]

Worth noting: even with the devenv shell --no-reload fish workaround, the hot-reload mechanism (landed on main via #2061) is entirely bash-specific — it relies on PROMPT_COMMAND and declare -p -x for env diffing. This means fish users are forced to choose between:

1. devenv shell — get hot-reload but stuck in bash
2. devenv shell --no-reload fish — get fish but lose hot-reload

For fish, the reload hook would likely need to use fish_prompt or fish_preexec/fish_postexec events instead of PROMPT_COMMAND.

Native fish support in devenv shell should ideally include hot-reload compatibility, otherwise fish users get a degraded experience compared to bash.

[scheminglunatic]

Yes, also have similar issue I believe; for not just fish but also zsh, nu, etc. other non-bash shells. Default to bash does add to the friction.

[lluchs]

Translating the BashDialect to a FishDialect would be fairly straightforward, but the tricky part is actually activating the devenv environment. The environment is passed in as a bash script and sourced by the BashDialect:

devenv/devenv-shell/src/dialect/bash.rs

Lines 37 to 38 in 07cd36d

	# Source the devenv environment
	source "{env_script_path}"

We'd need to either emulate what direnv is doing (run the activation script in a bash subprocess, export the environment from there and apply it within the fish shell), or generate a fish activation script directly. I believe generating a fish (or other shells) activation script is very possible, but needs a somewhat larger refactoring to pass an appropriate ShellDialect everywhere.

[domenkozar]

https://devenv.sh/blog/2026/05/07/devenv-21-nix-with-zsh-fish-and-nushell-via-libghostty/

[pbek]

Nice, I wonder if 2.1 will also solve my, devenv direnv in fish issues. But I guess I'll switch to the new hook. 👍🏻