+title: buffer-env --- Buffer-local process environments

+html:

+html:

+html:

+html:

With this package, you can teach Emacs to call the correct version of external programs such as linters, compilers and language servers on a /per-project/ basis. Thus you can work on several projects in parallel with no undue interference and switch seamlessly between them.

** Basic setup *** On the project side Your project settings should go into a shell script named =.envrc= which exports a suitable =PATH=, as well as any other desired environment variables. Place this script at the root directory of your project.

This follows the approach of the popular [[https://direnv.net/][direnv]] program, and is mostly compatible with it. However, buffer-env is entirely independent of direnv so it is not possible to use direnv-specific features in the =.envrc= scripts --- at least not directly.

Alternatively, it is possible to configure buffer-env to directly support other environment setup methods, such as Python virtualenvs, =.env= files or certain build tools. See below for details.

*** On the Emacs side The usual way to activate this package in Emacs is by including the following in your init file:

+begin_src emacs-lisp

(add-hook 'hack-local-variables-hook #'buffer-env-update) (add-hook 'comint-mode-hook #'buffer-env-update)

+end_src

In this way, any buffer potentially affected by [[https://www.gnu.org/software/emacs/manual/html_node/emacs/Directory-Variables.html][directory-local variables]], as well as Comint buffers, will also be affected by buffer-env. It is also possible to add =buffer-env-update= only to specific major-mode hooks, or call it interactively.

** How this package works When called interactively, =buffer-env-update= asks for a script file and executes it (in the sense detailed below). The role of the script is to set some environment variables. Then the Emacs variables =process-environment= and =exec-path= are made buffer-local and their values are set so as to replicate the environment defined by the script.

When =buffer-env-update= is called from a hook, a file named =buffer-env-script-name= is looked up in the current directory or one of its parents. If found, the same procedure as in the interactive case takes place. Otherwise, nothing happens.

It remains to clarify what “executing a script” means in the paragraphs above. Normally, it simply means to execute the script as a shell script and collect all the exported variables. However, certain script names are treated specially. These are:

• =.env=: These files, used by Docker, Node.js and others, are simple lists of =VARIABLE=value= pairs. They are still executed as shell scripts (which dictates when and how quotes are to be used, for instance), but no =export= statements are needed.
• =guix.scm= and =manifest.scm=: The development environment of the Guix package is loaded and exported to Emacs. Make sure you have entered =guix shell= at least once before to install the dependencies, otherwise you may block Emacs for a long time.
• =flake.nix= and =shell.nix=: These files are used by the Nix package manager and are handled similarly to Guix.
• =pyproject.toml=: If you are using [[https://python-poetry.org/][Poetry]], [[https://hatch.pypa.io/][Hatch]] or [[https://pdm.fming.dev][PDM]], buffer-env can infer the project environment from them.
• =*.ps1=: Similar to a regular shell script, but interpreted by PowerShell.

For instructions on how to extend this list, see the documentation of the variable =buffer-env-command-alist=.

** Integration with other environment management mechanisms *** Python virtualenvs In most cases, the easiest way to interface with Python virtualenvs is to create an =.envrc= file with the following contents:

+begin_src bash

source path-to-virtualenv/bin/activate

+end_src

You can also call =buffer-env-update= interactively and select the =activate= script directly.

However, if you want to avoid writing =.envrc= scripts and you create virtualenvs in a predictable place, say in a =.venv= directory at the root of each project, you can say

+begin_src emacs-lisp

(setq buffer-env-script-name ".venv/bin/activate") ;; alternatively, try to find a .envrc file first (setq buffer-env-script-name '(".envrc" ".venv/bin/activate"))

+end_src

Note that it is also possible to provide an absolute path for =buffer-env-script-name=, and it is possible to specify it as a buffer- or directory-local variable.

*** .env files To load the environment defined by a =.env= file, you can select it interactively with =buffer-env-update=. To automate the process, set =buffer-env-script-name= to =".env"=, either globally, dir-locally or buffer-locally.

*** Direnv Buffer-env is /mostly/ compatible with direnv; specifically, it assumes =.envrc= is a regular shell script, so you can't directly use anything from direnv's library of helper functions. A workaround is to use the following configuration:

+begin_src emacs-lisp

(with-eval-after-load 'buffer-env (add-to-list 'buffer-env-command-alist '("/\.envrc\'" . "direnv exec . env -0")))

+end_src

If you need tighter integration with direnv, you may want to check out the [[https://github.com/purcell/envrc][envrc]] package.

** Compatibility issues Most Emacs packages are not written with the possibility of a buffer-local process environment in mind. This leads to issues with a few commands; specifically, those which start an external process after switching to a different buffer or remote directory. Examples include:

• =compile= and =project-compile= (=C-x p c=) in Emacs 27 and older,
• =async-shell-command= (=M-&=).

Fortunately, the problem has an easy fix provided by the [[https://github.com/purcell/inheritenv][inheritenv]] package, which see.

Alternatively, if you speak Elisp and want to keep your configuration lean, you can just copy the function below and apply it as an =:around= advice to any affected commands.

+begin_src emacs-lisp

(eval-when-compile (require 'cl-lib)) (defun buffer-env-inherit (fn &rest args) "Call FN with ARGS using the buffer-local process environment. Intended as an advice around commands that start a process after switching buffers." (cl-letf (((default-value 'process-environment) process-environment) ((default-value 'exec-path) exec-path)) (apply fn args)))

+end_src

** Related packages This package is essentially a knockoff of the [[https://github.com/purcell/envrc][envrc]] package by Steve Purcell. The main difference is that envrc depends on and tightly integrates with the [[https://direnv.net/][direnv]] program, while buffer-env is minimalist and has no extra dependencies.

For a comparison of the buffer-local approach to environment variables with the global approach used by most of the similar packages, see [[https://github.com/purcell/envrc#design-notes][envrc's design notes]].

There is a large number of Emacs packages interfacing with the Python virtualenv system. They all seem to take the global approach and, therefore, the comparisons and caveats in the envrc design notes also apply, mutatis mutandis.

** Contributing Discussions, suggestions and code contributions are welcome! Since this package is part of GNU ELPA, contributions require a copyright assignment to the FSF.