# Package from Source

This note covers `17-package-from-source/`, which packages a selected local source tree and excludes unrelated files from the build input.

---

## 1. Why Source Selection Matters

`src = ./.;` copies the whole example directory into the store. That is fine for small demos, but real packages often need a narrower source tree.

This example uses `lib.fileset.toSource` so the derivation only sees:

- `src/run.sh`, and
- `src/message.txt`.

That leaves `ignored.txt` out of the packaged source on purpose.

---

## 2. Why the Check Looks at the Source Path

The check does two things:

- it runs the packaged binary, and
- it asserts that the generated source path does not contain `ignored.txt`.

That keeps the example focused on the real behavior being taught: selecting build inputs, not just producing a runnable script.

---

## 3. Commands to Try

```bash
cd 17-package-from-source

nix build
./result/bin/source-greet

nix run
nix flake check
```