How to build nix hello world - nix

Question

I'm trying to get started with a simple Hello World derivation using the Nix manual.
But it's not clear to me how to go about building it.
Is there somewhere I can download the source files from so I don't
have to copy them line-by-line?
Is there a way I can nix-build it
without having to modify anything global (eg pkgs/top-level/all-packages.nix)?
Where is pkgs/top-level/all-packages.nix?

Answer 1

One option is to clone the nixpkgs repository and then build the hello package recipe provided in that repository:
git clone https://github.com/NixOS/nixpkgs
cd nixpkgs
nix-build -A hello
Doing it this way, you don't have to modify all-packages.nix, because it already has an entry for hello. If you do want to modify all-packages.nix, you can find it in the nixpkgs repository that you cloned. Just take the path of the repository you cloned, (e.g. ~/nixpkgs) and add pkgs/top-level/all-packages.nix to get the path to all-packages.nix. You can see a copy of that file here:
https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/all-packages.nix
When you start building your own software that is not part of nixpkgs, you might chose to write your own default.nix file in your own repository and put a line like this in there to import nixpkgs, using the NIX_PATH environment variable:
let
nixpkgs = import <nixpkgs> { };
...

Answer 2

Note that I am running NixOs, not sure if my answer will valid for other (non-Linux) system.
Is there somewhere I can download the source files from so I don't have to copy them line-by-line?
You could browse nixpkgs, All source files are there.
Is there a way I can nix-build it without having to modify anything global (eg pkgs/top-level/all-packages.nix)?
David Grayson give excellent answer already.
I would love to add some information.
nix-build will looking for default.nix on the current directory and the build result will symlink named result on current working directory.
Another way to test if nix expression could build is nix-shell which also looking for default.nix or shell.nix on current directory. If the build success you will get shell prompt with your packages avaliable.
Both nix-build and nix-shell have -I argument that you can point to any nix repository including remote one.
For example, if I use nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz -p hello, nix will download binary cache if exists or build hello using current master branch expression and give me a shell which hello is avaliable.
$ nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz -p hello
downloading ‘https://github.com/NixOS/nixpkgs/archive/master.tar.gz’... [12850/0 KiB, 1404.5 KiB/s]
unpacking ‘https://github.com/NixOS/nixpkgs/archive/master.tar.gz’...
these paths will be fetched (0.04 MiB download, 0.19 MiB unpacked):
/nix/store/s3vlmp0k8b07h0r81bn7lh24q2mqcai8-hello-2.10
fetching path ‘/nix/store/s3vlmp0k8b07h0r81bn7lh24q2mqcai8-hello-2.10’...
*** Downloading ‘https://cache.nixos.org/nar/1ax9cr6qqqqrb4pdm1mpqn7whm6alwp56dvsh5hpgs5g8rrpnjxd.nar.xz’ (signed by ‘cache.nixos.org-1’) to ‘/nix/store/s3vlmp0k8b07h0r81bn7lh24q2mqcai8-hello-2.10’...
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 40364 100 40364 0 0 40364 0 0:00:01 0:00:01 --:--:-- 37099
[nix-shell:~]$ which hello
/nix/store/s3vlmp0k8b07h0r81bn7lh24q2mqcai8-hello-2.10/bin/hello
Where is pkgs/top-level/all-packages.nix?
There is NIX_PATH enviroment variable. The nixpkgs portion will point you to your current repository
$ echo $NIX_PATH
nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels
My all-packages.nix is located at /nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/top-level/all-packages.nix