om health

The om health command checks the health of your Nix install. Furthermore, individual projects can configure their own health checks in their flake.nix. For example, the nammayatri project checks that the cachix cache is in use.

Checks performed

Note that some checks are considered non-essential. For eg., the disk space check looks for 1TB+ disk space, but if the user is on a laptop with 256GB SSD, the check will report a warning instead of failing. This can also be configured in per-project basis from flake.nix (see below).

Usage

om health

To run use the health check configuration specified in a project flake, pass that flake as an argument. For eg., to run halth checks defined from the nammayatri project, run:

# The argument can be any flake URL (including a local path)
om health github:nammayatri/nammayatri

Configuring in flake.nix

To add project specific health checks or configure health checks, add the following flake output:

{
  outputs = inputs: {
    om.health.default = {
      # Add configuration here
      caches.required = [ "https://ourproject.cachix.org" ];
    };
  };
}

To see all available configuration options, run om health --dump-schema. This will dump the schema of the configuration in JSON format. Convert that to a Nix attrset to see what can be added under the om.health.default attrset of your flake.

$ om health --dump-schema > schema.json
$ nix eval --impure --expr 'builtins.fromJSON (builtins.readFile ./schema.json)' \
  | nix run nixpkgs#alejandra -- --quiet

This will output:

{
  caches = {required = ["https://cache.nixos.org/"];};
  direnv = {
    enable = true;
    required = false;
  };
  flake-enabled = {};
  max-jobs = {};
  nix-version = {min-required = "2.16.0";};
  rosetta = {
    enable = true;
    required = true;
  };
  shell = {
    enable = true;
    required = false;
  };
  trusted-users = {};
}

Adding devShell check

You can automatically run om health whenever your Nix dev shell starts. To do this, import the flake module in your flake and use it in your devShell:

{
  inputs = {
    omnix-flake.url = "github:juspay/omnix?dir=nix/om";
  };
  outputs = inputs:
    inputs.flake-parts.lib.mkFlake { inherit inputs; } {
      imports = [
        inputs.omnix-flake.flakeModules.default
      ];
      perSystem = { config, pkgs, ... }: {
        devShells.default = pkgs.mkShell {
          inputsFrom = [
            config.om.health.outputs.devShell
          ]
        };
      };
    };
}

Now suppose you have Nix 2.18 installed, but your project requires 2.19 or above due to the following config in its flake.nix:

flake.om.health.default = {
  nix-version.min-required = "2.19.0";
};

you can expect the devShell to print a giant message like this:

Note that you will still be dropped into the Nix dev shell (there’s no way to abrupt the launching of a dev Shell).