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.
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 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.
The usual way to activate this package in Emacs is by including the following in your init file:
(add-hook 'hack-local-variables-hook #'buffer-env-update)
(add-hook 'comint-mode-hook #'buffer-env-update)In this way, any buffer potentially affected by 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.
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 ofVARIABLE=valuepairs. They are still executed as shell scripts (which dictates when and how quotes are to be used, for instance), but noexportstatements are needed.guix.scmandmanifest.scm: The development environment of the Guix package is loaded and exported to Emacs. Make sure you have enteredguix shellat least once before to install the dependencies, otherwise you may block Emacs for a long time.flake.nixandshell.nix: These files are used by the Nix package manager and are handled similarly to Guix.pyproject.toml: If you are using Poetry, Hatch or 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.
In most cases, the easiest way to interface with Python virtualenvs is
to create an .envrc file with the following contents:
source path-to-virtualenv/bin/activateYou 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
(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"))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.
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.
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:
(with-eval-after-load 'buffer-env
(add-to-list 'buffer-env-command-alist '("/\\.envrc\\'" . "direnv exec . env -0")))If you need tighter integration with direnv, you may want to check out the envrc package.
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:
compileandproject-compile(C-x p c) in Emacs 27 and older,async-shell-command(M-&).
Fortunately, the problem has an easy fix provided by the 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.
(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)))This package is essentially a knockoff of the envrc package by Steve Purcell. The main difference is that envrc depends on and tightly integrates with the 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 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.
Discussions, suggestions and code contributions are welcome! Since this package is part of GNU ELPA, contributions require a copyright assignment to the FSF.
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