how to look up a package in nix flake inputs

Signed-off-by: Xe <me@christine.website>

1 files changed, 136 insertions, 0 deletions

diff --git a/blog/nix-flakes-look-up-package.markdown b/blog/nix-flakes-look-up-package.markdown
new file mode 100644
index 0000000..2a6330b
--- /dev/null
+++ b/blog/nix-flakes-look-up-package.markdown
@@ -0,0 +1,136 @@
+---
+title: "How to look up a Nix package's Nix store path from flake inputs"
+date: 2022-08-06
+tags:
+ - nix
+ - nixos
+ - flakes
+series: nix-flakes
+---
+
+<xeblog-hero file="fall-archons" prompt="The fall of the Archons, colored pencil drawing, fireball spell, bright sky, digital art, lake of fire"></xeblog-hero>
+
+Sometimes God is dead and you need to figure out what the version of a package
+in your Nix flake's inputs is. With flakes, you can figure this out using `nix
+eval` on a flake reference, but what the hecc is a flake reference?
+
+Every URL has a "fragment", or the thing after the `#` that mainly tells the
+browser where to scroll to. Nix flakes uses this to allow you to sub-index on
+flakes by either name or URL. This allows you to address packages like tmux as:
+
+```
+nixpkgs#legacyPackages.x86_64-linux.tmux
+```
+
+Or my site's code as:
+
+```
+github:Xe/site#packages.x86_64-linux.default
+```
+
+Or even some [arbitrary git repo](https://tulpa.dev/cadey/printerfacts):
+
+```
+git+https://tulpa.dev/cadey/printerfacts.git?ref=main#defaultPackage.x86_64-linux
+```
+
+<xeblog-conv name="Mara" mood="hacker">The flake reference is what drills down
+into the flake so that Nix can tell the difference between part of the path to
+the repository and the path inside the flake. It usually works on the `outputs`
+of a flake that you can find out with `nix flake show`:</xeblog-conv>
+
+```
+$ nix flake show github:Xe/site
+github:Xe/site/5a50cacef85c9e3b9695a2749359b27d0530f86a
+├───devShell
+│   ├───aarch64-linux: development environment 'nix-shell'
+│   └───x86_64-linux: development environment 'nix-shell'
+├───nixosModules
+│   ├───aarch64-linux: NixOS module
+│   └───x86_64-linux: NixOS module
+└───packages
+    ├───aarch64-linux
+    │   ├───bin: package 'xesite-2.4.0'
+    │   ├───config: package 'xesite-config-2.4.0'
+    │   ├───default: package 'xesite-2.4.0'
+    │   ├───posts: package 'xesite-posts-2.4.0'
+    │   └───static: package 'xesite-static-2.4.0'
+    └───x86_64-linux
+        ├───bin: package 'xesite-2.4.0'
+        ├───config: package 'xesite-config-2.4.0'
+        ├───default: package 'xesite-2.4.0'
+        ├───posts: package 'xesite-posts-2.4.0'
+        └───static: package 'xesite-static-2.4.0'
+```
+
+<xeblog-conv name="Mara" mood="happy">Tracing through things, you can see how
+you can go from the package name `default` to get
+`github.com:Xe/site#packages.x86_64-linux.default` as the full legal name of the
+package!</xeblog-conv>
+
+You can use the `nix eval --raw` command to figure out what the nix store path
+of those packages either is or would be:
+
+```
+$ nix eval --raw nixpkgs#legacyPackages.x86_64-linux.tmux
+/nix/store/3mn362yak70vq137wnryi7whgvq2cmn2-tmux-3.3a
+
+$ nix eval --raw github:Xe/site#packages.x86_64-linux.default
+/nix/store/v3lwgh6vk03yw3aq3j398g613ff3gja0-xesite-2.4.0
+
+$ nix eval --raw git+https://tulpa.dev/cadey/printerfacts.git'?'ref=main#defaultPackage.x86_64-linux
+/nix/store/7df21bqkrx2csazc2s8ji8chaabbpqhs-printerfacts-0.3.1
+```
+
+However, this operates on the latest version by default. It won't operate on the
+current version in your Nix flake. If you need to find it out from the current
+flake, pass the `--inputs-from` flag with the current directory `.`:
+
+```
+$ nix eval --inputs-from . --raw nixpkgs#legacyPackages.x86_64-linux.tmux
+/nix/store/5sz57bwrlwhv1l8h4c4sq1bl1fhmcwfw-tmux-3.3a
+```
+
+This will use the `nixpkgs` defined in your current directory's `flake.lock`
+file. You can then share that store path with people to share in your tale of
+woe as to why a service is crashing when you try and update it.
+
+<xeblog-conv name="Mara" mood="hacker">You can also pass in an arbitrary version
+for a flake URL if you specify it in the URL parameters with `ref`. You can do
+it like this: `github:Xe/site?ref=5a50cacef85c9e3b9695a2749359b27d0530f86a` or
+by specifying the commit hash as an additional path parameter as in
+`github:Xe/site/5a50cacef85c9e3b9695a2749359b27d0530f86a`. Here's an example for
+[this xesite
+commit](https://github.com/Xe/site/commit/5a50cacef85c9e3b9695a2749359b27d0530f86a):</xeblog-conv>
+
+```
+$ nix flake show github:Xe/site/5a50cacef85c9e3b9695a2749359b27d0530f86a
+github:Xe/site/5a50cacef85c9e3b9695a2749359b27d0530f86a
+├───devShell
+│   ├───aarch64-linux: development environment 'nix-shell'
+│   └───x86_64-linux: development environment 'nix-shell'
+├───nixosModules
+│   ├───aarch64-linux: NixOS module
+│   └───x86_64-linux: NixOS module
+└───packages
+    ├───aarch64-linux
+    │   ├───bin: package 'xesite-2.4.0'
+    │   ├───config: package 'xesite-config-2.4.0'
+    │   ├───default: package 'xesite-2.4.0'
+    │   ├───posts: package 'xesite-posts-2.4.0'
+    │   └───static: package 'xesite-static-2.4.0'
+    └───x86_64-linux
+        ├───bin: package 'xesite-2.4.0'
+        ├───config: package 'xesite-config-2.4.0'
+        ├───default: package 'xesite-2.4.0'
+        ├───posts: package 'xesite-posts-2.4.0'
+        └───static: package 'xesite-static-2.4.0'
+```
+
+<xeblog-conv name="Cadey" mood="coffee">I have reached a point where googling
+for these questions gets me results on my own blog. Of course they didn't help
+me in the moment. That's why this post exists. Maybe I'll be able to fix my
+Matrix server returning that a database migration failed so that I can update my
+homelab machines because that failing makes the whole deploy rollback. I really
+hate synapse and I regret setting up my own Matrix homeserver.</xeblog-conv>
+