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Merge pull request #748 from fricklerhandwerk/add-links

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clean up detailed explanation on building VMs
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Alexander Groleau 2023-10-09 08:08:14 -07:00 committed by GitHub
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source/tutorials/nixos/nixos-configuration-on-vm.md
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@ -125,11 +125,9 @@ The complete `configuration.nix` file now looks like this:
}
```
## Creating a QEMU based virtual machine using a configuration
## Creating a QEMU based virtual machine from a NixOS configuration
A virtual machine is created with the `nix-build` command.
To select `configuration.nix` in the working directory, specify the configuration file on the command line:
A NixOS virtual machine is created with the `nix-build` command:
```shell-session
nix-build '<nixpkgs/nixos>' -A vm \
@ -141,13 +139,19 @@ This command builds the attribute `vm` from the `nixos-22.11` release of NixOS,
<details><summary> Detailed explanation </summary>
The first optional argument of [`nix-build`](https://nixos.org/manual/nix/stable/command-ref/nix-build.html) is a path to the derivation to be built.
With `'<nixpkgs>'` Nix is instructed to resolve the search path defined with the [`NIX_PATH` environment variable](https://nixos.org/manual/nix/stable/command-ref/env-common.html#env-NIX_PATH) or the [`-I` option](https://nixos.org/manual/nix/unstable/command-ref/opt-common.html#opt-I).
The virtual machine setup is provided by NixOS, which is part of the `nixpkgs` repository, therefore we use `'<nixpkgs/nixos>'`.
The [`-A` option](https://nixos.org/manual/nix/stable/command-ref/opt-common.html#opt-attr) specifies the attribute to pick from the provided [Nix expression `<nixpkgs>`](search-path-tutorial).
To build the virtual machine, you choose the `vm` attribute as defined in [`nixos/default.nix`](https://github.com/NixOS/nixpkgs/blob/7c164f4bea71d74d98780ab7be4f9105630a2eba/nixos/default.nix#L19).
The [`-I` option](https://nixos.org/manual/nix/stable/command-ref/opt-common.html#opt-I) adds search paths.
Here we set `nixpkgs` to refer to a specific version of NixOS and to set `nix-config` to the `configuration.nix` file in the current directory.
- The positional argument to [`nix-build`](https://nixos.org/manual/nix/stable/command-ref/nix-build.html) is a path to the derivation to be built.
That path can be obtained from [a Nix expression that evaluates to a derivation](derivations).
The virtual machine build helper is defined in NixOS, which is part of the [`nixpkgs` repository](https://github.com/NixOS/nixpkgs).
Therefore we use the [lookup path](search-path-tutorial) `<nixpkgs/nixos>`.
- The [`-A` option](https://nixos.org/manual/nix/stable/command-ref/opt-common.html#opt-attr) specifies the attribute to pick from the provided Nix expression `<nixpkgs/nixos>`.
To build the virtual machine, we choose the `vm` attribute as defined in [`nixos/default.nix`](https://github.com/NixOS/nixpkgs/blob/7c164f4bea71d74d98780ab7be4f9105630a2eba/nixos/default.nix#L19).
- The [`-I` option](https://nixos.org/manual/nix/stable/command-ref/opt-common.html#opt-I) prepends entries to the search path.
Here we set `nixpkgs` to refer to a [specific version of Nixpkgs](ref-pinning-nixpkgs) and set `nix-config` to the `configuration.nix` file in the current directory.
:::{admonition} NixOS
On NixOS the `$NIX_PATH` environment variable is usually set up automatically, and there is also [a convenience command for building virtual machines](https://nixos.org/manual/nixos/stable/#sec-changing-config).