47 lines
1.1 KiB
Markdown
47 lines
1.1 KiB
Markdown
# Package Overrides
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This note covers `20-package-overrides/`, which customizes an existing package with `overrideAttrs`.
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---
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## 1. Why Override Instead of Rewriting
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Sometimes you want to change one part of an existing package, not replace the whole derivation. That is what package overrides are for.
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This example starts from `pkgs.hello` and changes its installed program with `overrideAttrs`.
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---
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## 2. What `overrideAttrs` Changes
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`overrideAttrs` receives the old derivation attributes and returns an updated attrset.
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In this example, the override:
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- appends `makeWrapper` to `nativeBuildInputs`, and
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- wraps `bin/hello` so the program always receives a custom `--greeting` flag.
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The package still comes from `pkgs.hello`; only one part of its install result changes.
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---
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## 3. Why the Check Runs the Program
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The check does not inspect the derivation text. It runs the overridden program and asserts that the greeting changed.
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That keeps the example focused on visible package behavior.
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---
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## 4. Commands to Try
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```bash
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cd 20-package-overrides
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nix build
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./result/bin/hello
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nix run
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nix flake check
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```
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