niv/nix/sources.json

{
    "nixpkgs": {
        "url": "https://github.com/NixOS/nixpkgs-channels/archive/8e70d4bd7d5e1c137006fe483d8d116311694bc2.tar.gz",
        "owner": "NixOS",
        "branch": "nixos-18.09",
        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz",
        "repo": "nixpkgs-channels",
        "sha256": "14vl7qlb48v9fdwdlz0cj6h83kj6zlc7qg3n6sbiabr52y2bhmkh",
        "description": "Nixpkgs/NixOS branches that track the Nixpkgs/NixOS channels",
        "rev": "8e70d4bd7d5e1c137006fe483d8d116311694bc2"
    }
}