beans: add shell completions

pkgs/by-name/be/beans/package.nix

@@ -2,6 +2,8 @@
   lib,
   buildGoModule,
   fetchFromGitHub,
+  installShellFiles,
+  stdenv,
   versionCheckHook,
 }:
 
@@ -18,6 +20,8 @@ buildGoModule (finalAttrs: {
 
   vendorHash = "sha256-TprfPZ/clb7PLMAkxF0y78bCef4XarhgHlIhIPn1nQA=";
 
+  nativeBuildInputs = [ installShellFiles ];
+
   ldflags = [
     "-s"
     "-X github.com/hmans/beans/cmd.version=${finalAttrs.version}"
@@ -25,6 +29,13 @@ buildGoModule (finalAttrs: {
     "-X github.com/hmans/beans/cmd.date=unknown"
   ];
 
+  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    installShellCompletion --cmd beans \
+      --bash <($out/bin/beans completion bash) \
+      --fish <($out/bin/beans completion fish) \
+      --zsh <($out/bin/beans completion zsh)
+  '';
+
   doInstallCheck = true;
   nativeInstallCheckInputs = [ versionCheckHook ];
   versionCheckProgramArg = "version";