Envfs

A fuse filesystem that dynamically populates contents of /bin and /usr/bin/ so that it contains all executables from the PATH of the requesting process. This allows executing FHS based programs on a non-FHS system. For example, this is useful to execute shebangs on NixOS that assume hard coded locations like /bin or /usr/bin etc.

Demo

Mount envfs on /usr/bin sudo envfs /usr/bin

Programs are magically available, based on the PATH of the calling process. $ /usr/bin/env --version

env (GNU coreutils) 9.1
Packaged by https://nixos.org
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Richard Mlynarik, David MacKenzie, and Assaf Gordon.

$ echo $PATH

If the caller doesn't have the binary in the PATH, it will fail. $ PATH= /usr/bin/env --version 2>&1 || true

As you can see, /usr/bin is empty. $ ls -la /usr/bin

total 1
drwxr-xr-x 3345300086 root root 0 Jan  1  1970 .
drwxr-xr-x          3 root root 3 Sep  5 13:04 ..

By default, binaries are only available whenever the calling process executes or open a program, not when using stat or listing the directory: $ ls -la /usr/bin/env 2>&1 || true

ls: cannot access '/usr/bin/env': No such file or directory

This behaviour can be overridden by setting ENVFS_RESOLVE_ALWAYS=1. $ ENVFS_RESOLVE_ALWAYS=1 ls -la /usr/bin/env

lr----x--t 1 root root 0 Jan  1  1970 /usr/bin/env -> /nix/store/4vjigg3pr8bns6id4af51mza5p73l9lx-coreutils-9.1/bin/env

In conclusion, combined with the usual Nix wrappers or nix-shells, it makes things magically work!

Installation in NixOS

Choose one of the following methods:

In NixOS starting with 23.05 (Current recommendation)

Since NixOS 23.05 you can enable envfs with a single line:

{
  services.envfs.enable = true;
}

Flakes

If you use nix flakes support:

{
  inputs.envfs.url = "github:Mic92/envfs";
  inputs.envfs.inputs.nixpkgs.follows = "nixpkgs";
  
  outputs = { self, nixpkgs, envfs }: {
    # change `yourhostname` to your actual hostname
    nixosConfigurations.yourhostname = nixpkgs.lib.nixosSystem {
      # change to your system:
      system = "x86_64-linux";
      modules = [
        ./configuration.nix
        envfs.nixosModules.envfs
      ];
    };
  };
}

niv

First add it to niv:

$ niv add Mic92/envfs

Then add the following to your configuration.nix in the imports list:

{
  imports = [ "${(import ./nix/sources.nix).envfs}/modules/envfs.nix" ];
}

nix-channel

As root run:

$ nix-channel --add https://github.com/Mic92/envfs/archive/main.tar.gz envfs
$ nix-channel --update

Then add the following to your configuration.nix in the imports list:

{
  imports = [ <envfs/modules/envfs.nix> ];
}

fetchTarball

Add the following to your configuration.nix:

{
  imports = [ "${builtins.fetchTarball "https://github.com/Mic92/envfs/archive/main.tar.gz"}/modules/envfs.nix" ];
}

or with pinning:

{
  imports = let
    # replace this with an actual commit id or tag
    commit = "f2783a8ef91624b375a3cf665c3af4ac60b7c278";
  in [ 
    "${builtins.fetchTarball {
      url = "https://github.com/Mic92/envfs/archive/${commit}.tar.gz";
      # replace this with an actual hash
      sha256 = "0000000000000000000000000000000000000000000000000000";
    }}/modules/envfs.nix"
  ];
}

Build and run from source

$ nix-build
$ sudo ./result/bin/envfs -o bind-mount=/bin /usr/bin