# Nix Tidbits
In which tidbits regarding nix can be found...

## Nix
Inherit
```
# introduces the 'hi' attribute to the following scope
# only valid inside a let block
nix-repl> let inherit ({greeting = "helloworld";}) greeting; in greeting
"helloworld"
```
System nixpkgs
```
import <nixpkgs> {};
```
Git nixpkgs
```
let
  inherit (import <nixpkgs> {}) fetchFromGitHub:
  nixpkgs = fetchFromGitHub {
    owner = "NixOS";
    repo = "nixpkgs-channels";
    rev = "00abc...12300";
    sha256 = "345oeu...nth876";
  };
in ...
```

## NixOS
Build a package from a channel
```
nix build -f channel:$CHANNEL hello
```
Install a package from a channel
```
nix-env -f https://github.com/NixOS/nixpkgs-channels/archive/${CHANNEL}.tar.gz -iA hello
```
Remove old generations
```
sudo nix-env -p /nix/var/nix/profiles/system --delete-generations old
nix-collect-garbage -d
nix-env -p /nix/var/nix/profiles/system --list-generations
## Remove entries from /boot/loader/entries:
find /boot/loader/entries ! -name nixos-generation-${CURRENT_GENERATION_NUMBER}.conf -type f -exec sudo rm {} +
```

## NixOps
Deploy to GCP
* create GCP account
* create project `foo`
* create service account `bar`
* download a JSON key for `bar` locally and note its path
* export these variables:
```
export GCE_PROJECT=foo
export GCE_SERVICE_ACCOUNT=bar@iam.gserviceaccount.com
export ACCESS_KEY_PATH=/path/to/key.json
```
* write a nix expression containing your desired network
```
let baz = qux; in { network; }
```
* `nixops create -d foonet ./file.nix ./file2.nix`
* `nixops deploy -d foonet`
* edit expressions as desired and then
    * `nixops modify -d foonet ./file.nix ./file2.nix`
    * `nixops deploy -d foonet`
* if anything gets into a wonky state
    * `nixops deploy -d foonet --check`
* if all else fails, turn it off and on again
    * `nixops destroy -d foonet --confirm`
    * `nixops deploy -d foonet`