Nix on non-NixOS -- path to e.g. bash when PATH is unset?

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1

Hello,

After a year or so of halfhearted investigation, I’m finally giving a real shot to using nix, nix-darwin, and home-manager on my MacOS and Arch Linux systems (with NixOS on a VM and a Pi as well).

One issue I’ve run into a few times is adapting my current launchd / systemd / Quicksilver / Hammerspoon scripts to use nix-installed programs (e.g. bash). These are usually run without my PATH being fully configured. Previously, for example, if I wanted to use bash 5 via homebrew (instead of system-installed bash 3 on MacOS), I could just prefix PATH=/opt/homebrew/bin:${PATH}, or directly use /opt/homebrew/bin/bash myscript.sh (if no other homebrew binaries were required).

I’ve now brew unlinked bash from /opt/homebrew/bin, so all my scripts are broken for the moment.

I have nixpkgs#bashInteractive installed, so using /usr/bin/env bash as my shebang works from a login shell, but not for things getting called by launchd / root.

It looks like I have maybe a few options –

  • /run/current-system/sw/bin/bash
  • /nix/var/nix/profiles/system/sw/bin/bash

These two also look possible, though I don’t have bash there currently (just system-wide), but I’m sure it would be easy to do:

  • /nix/var/nix/profiles/per-user/n8henrie/home-manager/home-path/bin/
  • /etc/profiles/per-user/n8henrie/bin

Even if I were to change the shebang to nix-shell I would still need to specify a path to nix-shell, right?

Perhaps not:

$ which bash
/run/current-system/sw/bin/bash
$ sudo -i which bash
/bin/bash
$ sudo -i which nix-shell
/run/current-system/sw/bin/nix-shell

So perhaps nix is at least already on my PATH? So maybe I should change these paths to all use nix run nixpkgs#bash -- myscript.sh?

EDIT: systemd requires ExecStart to start with an absolute path, so just nix run won’t work – /usr/bin/env nix in that case?

2

The way to do it on NixOS is to write your systemd modules with nix, and use its string interpolation of packages to get the correct binary. Something like this: nixpkgs/emacs.nix at 47edaa313fc3767ce3026037a5b62352f22f3602 · NixOS/nixpkgs · GitHub

The documentation for that option is here: systemd.services

Ignore that, didn’t spot you were using home-manager. It has similar options for systemd units though, so also here I would recommend writing the services with nix and using string interpolation for the package: Appendix A. Configuration Options

I guess in theory running /usr/bin/env nix-shell might work, but I’m not at all sure what channels that command would be using. It’s definitely not reproducible, whatever the case.

3

My primary machine is MacOS.

Yes, I’m aware there are systemd and launchd options for home-manager, but not Quicksilver or Hammerspoon, or other applications running scripts that I’m hoping to eventually migrate to nix, but it would be nice to have a quick fix in the meantime.

Which path would you consider least bad for a service on a non-NixOS system?

  • /nix/var/nix/profiles/system/sw/bin/bash
  • /run/current-system/sw/bin/bash
  • /etc/profiles/per-user/n8henrie/bin/bash
  • /usr/bin/env nix run nixpkgs#bash

0 voters

4

Unfortunately it looks like PATH is only /usr/bin:/bin:/usr/sbin:/sbin in some of the environments, so unfortunately I don’t think that will be a good option.

5

Neither one of these quite match your case, but FWIW my /etc/shells uses the current-system path (I think nix-darwin does this):

$ cat /etc/shells
Tue Oct 18 2022 14:40:29 --> 
# List of acceptable shells for chpass(1).
# Ftpd will not allow users to connect who are not using
# one of these shells.

/bin/bash
/bin/csh
/bin/dash
/bin/ksh
/bin/sh
/bin/tcsh
/bin/zsh

# List of shells managed by nix.
/run/current-system/sw/bin/bash

It might also be acceptable to define the launchdaemon in Nix without fully nix-packaging the script, and then use the system path:

    launchd.user.agents.backup = {
        path = [ config.environment.systemPath ];
        command = "~/.config/persistence/backup.sh ${user.handle}";
        serviceConfig = {
            StartCalendarInterval = [
                { Hour = 7; }
                { Hour = 23;}
            ];
        };
    };
6

Mine has /run/current-system/sw/bin/bash there too; something in e.g. Hammerspoon must be overriding it.

7

IIRC it’s just a list of valid user shells, so you still have to use chsh to adopt it. Probably not a very safe idea to use a non-builtin shell as the root user’s shell, though.

I just mean it as evidence that at least one other Nixer has looked at this question and decided that hardcoding /run/current-system/sw/bin/bash is tolerable!

1 Like
8

Yes, I have already done so for my user as well as root.

:laughing: