Envfs

Fuse filesystem that returns symlinks to executables based on the PATH of the requesting process. This is useful to execute shebangs on NixOS that assume hard coded locations in 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:

Flakes (Current recommendation)

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 /usr/bin
$ mount --bind /usr/bin /bin