Where is <nixpkgs> ?

nixpkgs.md

Where is <nixpkgs> ?

Nix looks for expressions at NIX_PATH.

NIX_PATH is very similar to PATH but used by nix expressions (an expression is search on the NIX_PATH from left to right)

$ nix-instantiate --eval -E '<nixpkgs>'
/nix/var/nix/profiles/per-user/root/channels/nixos
$ echo $NIX_PATH
nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels

<nixpkgs> is refering to a path in our filesystem specified by NIX_PATH

$ nix-instantiate --eval -E '<ping>' # Obviously not found.

# But if we add it to the `NIX_PATH`
$ NIX_PATH=$PATH nix-instantiate --eval -E '<ping>'

# NIX_PATH has this handy syntax:
$ NIX_PATH="ping=/bin/ping" nix-instantiate --eval -E '<ping>'

<ping> equivalent to ./ping but for nix expressions.

To find where is exactly on the system:

$ nix-instantiate --find-file nixpkgs
# Then, print .version and .version-suffix

About nix-env

nix-env is a little different than nix-instantiate/nix-build.

nix-env uses ~/.nix-defexpr instead of NIX_PATH (though NIX_PATH usually includes .nix-defexpr).

An error occurs while fetching sources from an url, how do I fix it ?

Try this in order:

$ nix-channel --update
# and try to rebuild

$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable  nixos-unstable
$ nix-channel --update nixos-unstable # Now you can import <nixos-unstable> {};
# and try to rebuild

# Navigate to your channel's expressions (google it) and find the package
$ vim default.nix $ alter the url and the sha256 (use `nix-prefetch-url url` to get the sha256)

On NixOS, you must run those commands as root because the channels are at root level.