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I am trying to programmatically extract one of the generated files from a NixOS install. Basically on my NixOS system I run sudo nixos-rebuild switch and as part of the build it writes to /nix/store/<hash>-package/file. I want to output just the contents of that file.

Given that nix is "just" a configuration language and store manager, I believe there should be some way to evaluate /etc/nixos/configuration.nix (and any files imported from it) and do something with the resulting expression to extract the generated file.

I currently can instantiate a new NixOS VM, copy the files in /etc/nixos/ and run sudo nixos-rebuild switch then pull /nix/store/*-package/file from the virtual drive and delete the VM. There has to be a better way?

How can I evaluate /etc/nixos/configuration.nix and dump the contents of one of its generated files even without being on NixOS or loading the configuration?

To clarify, I want to extract a generated file's content from a set of nixos configuration files without loading them as my active nixos configuration.

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    I don't understand what you're asking. At face value you seem to want to cat /nix/store/<hash>-package/file ... the question is then why don't you just do that? Commented Nov 8 at 22:56
  • I want to get the same effect without loading the config. As an example, lets say my friend hands me his nixos configuration directory and I want to copy his nginx configuration, but don't want to load his whole system. He might have many packages that interact with nginx to make arbitrarily complicated nginx config files. I just want that config. Commented Nov 8 at 23:02
  • How about you edit your question to that effect ... may help others understanding it, too, without having to read through the comment section. Commented Nov 9 at 3:44

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