How do you inspect NixOS / Home Manager configurations?

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1

When I want to debug my modules I often want to check that I have an option set to what I think it is set to.

Here are my notes on how I currently do this:

NixOS options are under .#nixosConfigurations.<config>.config. So for example:

# Show security.pam.loginLimits option
nix eval .#nixosConfigurations.chungito.config.security.pam.loginLimits

For NixOS hosts, Home Manager options are under
.#nixosConfigurations.<config>.config.home-manager.users.<user> So for example:

# Show programs.waybar.enable option
nix eval .#nixosConfigurations.chungito.config.home-manager.users.brendan.programs.waybar.enable

For hosts using Home Manager standalone, they are under
.#homeConfigurations.<config>. So for example:

nix eval '.#homeConfigurations.jackmanb@jackmanb01.config.wayland.windowManager.sway.xwayland'

This is fine for small/scalar options but doesn’t really work for inspecting larger attrsets as it just prints one big single-line Nix expression. Passing --json and then piping to jq sometimes helps but:

  1. Some values can’t be serialised as JSON at all (e.g. functions)
  2. Some values are intended to produce an error when serialised.

I think the Nix REPL would offer a way to work around this issue but it doesn’t actually seem to be usable (e.g. I can’t find a way to go through command history? Am I missing something there?)

Home Manager has an instantiate/option commands but they don’t work on Flake configurations.

So… is there a good solution here?

2

I use the repl, personally.

nix-repl> f = builtins.getFlake "git+file:${(builtins.getEnv "PWD")}"
nix-repl> f.nixosConfigurations.chungito.config.security.pam.loginLimits
…

With Lix’s repl-overlays feature, I can have f automatically defined when I enter the repl.

2 Likes
3

nixos-rebuild repl can do this just fine as well

❯ nixos-rebuild repl -f ~/src/mine/nixnix -A lamorna.system
building Nix...
Nix 2.28.5
Type :? for help.
Loading installable 'lamorna.system'...
Added 6 variables.
nix-repl>

Season to taste with flakes or whatever.

4

The repl’s :p command can handle errors gracefully by printing an error for the problematic thunk and continuing:

nix-repl> :p { foo = builtins.abort "foo"; bar = 1; }
{
  bar = 1;
  foo = «error: evaluation aborted with the following error message: 'foo'»;
}

I find it’s most valuable to put together a repl environment with my config’s module args available in scope, then exploring config (or trying snippets of code to see if they do what I think they do).

5

I wonder if there’s a way to print the whole configuration at once.

6

Yeah, this is also something I’d wondered. You can’t evaluate the whole config, since there are options etc that fail the program if you evaluate them (like the ones I mentioned breaking --json).

It would be cool if you could print the subset of the configuration that actually got evaluated when you realise your config. I have no idea if this is possible.

To the REPL users… am I missing anything or does it not support command history?

1 Like
7

Lix’s does. Idk about CppNix’s.

8

Not with an unpatched Nix, but it’s not hard to extend Nix to support this. I demoed this here: Comparing module system configurations - #8 by rhendric

It definitely does for me, both within and across REPL sessions. I don’t know why it isn’t working for you.

1 Like
9

I think you would be interested in nix-inspect.

1 Like
10

No idea about standalone hm.
But nixos-option works for nixos configs, and it’s already on your system. It handles individual options and nested options too. It would of course work for hm as a nixos module as well.
It even tells you which files affected the final value.

11

Oh, it works on my other computer, seems just a normal readline prompt. So… yeah I guess I need to debug that.

Seems useful but as you implied, I think it only works for NixOS, so no good for standalone HM. (I use non-NixOS systems too. I suspect this is not a particularly common case. But I have a separate distro for my day job).