nix-shell --command doesn't work for FHS environments

[ekmecic]

I'm scripting a tedious process I have to do to get a build for a project.The project lives in a FHS built by buildFHSUserEnv. It would be very convenient if I could pass commands to be executed in the chroot like it works with regular nix-shell environments, e.g.

nix-shell default.nix --command "make -j4 && cp some_file other_place"

Unfortunately this doesn't seem to be possible.

[nioncode]

A nix-shell with a buildFHSUserEnv does not behave like a regular nix-shell at all, unfortunately.
Things like --command and --run are simply broken, even simple things like using an interactive bash are not possible.
Putting bashInteractive into the targetPkgs doesn't change anything.

[cprussin]

Hi folks, just wondering if there's any workaround/solution to this yet?

[JohnAZoidberg]

If you set runScript = "bash", you can run nix-shell and you get a shell in the environment.
Still doesn't work with --run but it'll give you a shell.
If you only ever want to run one command, you can put that as your runScript.

[steshaw]

The way I solve this problem is to make runScript an argument, then you can supply your command to that argument. e.g.

# fhs.nix
{
  pkgs ? import <nixpkgs> {},
  # This allows us to provide a command to run via `--argstr run COMMAND`.
  run ? "bash"
}:
with pkgs; (buildFHSUserEnv {
  name = "fhs";
  targetPkgs = pkgs: with pkgs; [
    curl
    git
    gnumake
  ];
  runScript = "${run}";
  profile = ''
    export SSL_CERT_FILE=${cacert}/etc/ssl/certs/ca-bundle.crt
    export GIT_SSL_CAINFO="$SSL_CERT_FILE"
    export LANG=C.UTF-8
  '';
}).env

You can execute your command with:

$ nix-shell --pure --argstr run "sh -c 'make -j4 && cp some_file other_place'" fhs.nix

I tend to wrap it up in a little script, fhs:

#!/usr/bin/env bash

set -euo pipefail

fhs="./fhs.nix"

function run {
  nix-shell --pure --argstr run "$*" "${fhs}"
}

if [[ $# -eq 0 ]]; then
  run bash
else
  run "$@"
fi

Usage is then:

$ ./fhs # for a bash shell in the FHS
$ ./fhs echo hi # do execute command in FHS