How can I install binary file into NixOS - nixos

Question

I'm trying to install external binary inside NixOS, using declarative ways. Inside nix-pkg manual, I found such way of getting external binary inside NixOS
{ pkgs ? import <nixpkgs> {} }:
pkgs.stdenv.mkDerivation {
name = "goss";
src = pkgs.fetchurl {
url = "https://github.com/aelsabbahy/goss/releases/download/v0.3.13/goss-linux-amd64";
sha256 = "1q0kfdbifffszikcl0warzmqvsbx4bg19l9a3vv6yww2jvzj4dgb";
};
phases = ["installPhase"];
installPhase = ''
'';
But I'm wondering, what should I add inside InstallPhase, to make this binary being installed inside the system?

Answer 1

This seems to be an open source Go application, so it's preferable to use Nixpkgs' Go support instead, which may be more straightforward than patching a binary.
That said, installPhase is responsible creating the $out path; typically mkdir -p $out/bin followed by cp, make install or similar commands.
So that's not actually installing it into the system; after all Nix derivations are not supposed to have side effects. "Installing" it into the system is the responsibility of NixOS's derivations, as configured by you.
You could say that 'installation' is the combination of modifying the NixOS configuration + switching to the new NixOS. I tend to think about the modification to the configuration only; the build and switch feel like implementation details, even though nixos-rebuild is usually a manual operation.
Example:
installPhase = ''
install -D $src $out/bin/goss
chmod a+x $out/bin/goss
'';
Normally chmod would be done to a local file by the build phase, but we don't really need that phase here.

Answer 2

I have no idea why this was so hard to figure out. Having robust configuration systems is fine, but at the end of the day sometimes you just need to be able to download and expose a single flipping file on the $PATH.
The result of fetchurl is "the unaltered contents of the URL within the Nix store", which is being used for the src. So in installPhase, $src points to the downloaded data, and you just have to copy/install/link that into $out/…..
pkgs.stdenv.mkDerivation {
name = "hello_static";
src = pkgs.fetchurl {
name = "hello_static";
url = "https://raw.githubusercontent.com/ruanyf/simple-bash-scripts/6e837f949010e0f5e9305e629da946de12cc63e8/scripts/hello-world.sh";
sha256 = "sha256:somE27ajbm0TtWv9tyeqTWDW3gbIs6xvlcFS9QS1ZJ0=";
};
phases = [ "installPhase" ];
installPhase = ''
install -D $src $out/bin/hello_static
'';
};