fix: update nix self-update logic

internal/appupdate/appupdate.go

@@ -26,6 +26,7 @@ func HandleSelfUpdate(
 	resultChannel := make(chan string)
 
 	isHomebrewInstall := isHomebrewInstall(logger)
+	isNixInstall := isNixInstall(logger)
 
 	currentSemVer, err := semver.NewVersion(currentVersion)
 	if err != nil {
@@ -35,7 +36,7 @@ func HandleSelfUpdate(
 	}
 
 	// Check if we have previously detected a newer version
-	updateToLatestVersion(currentSemVer, logger, fs, updater, isHomebrewInstall)
+	updateToLatestVersion(currentSemVer, logger, fs, updater, isHomebrewInstall, isNixInstall)
 
 	// Check for newer versions from remote repository
 	go fetchAndSaveLatestVersion(resultChannel, logger, fs, updater)
@@ -69,6 +70,30 @@ func isHomebrewInstall(logger *zap.Logger) bool {
 	return false
 }
 
+func isNixInstall(logger *zap.Logger) bool {
+	executable, err := executablePath()
+	if err != nil {
+		logger.Debug("failed to determine executable path for update strategy", zap.Error(err))
+		return false
+	}
+
+	resolvedExecutable := executable
+	if resolved, err := filepath.EvalSymlinks(executable); err == nil {
+		resolvedExecutable = resolved
+	} else {
+		logger.Debug("failed to resolve symlinks for executable", zap.Error(err))
+	}
+
+	nixStorePrefix := "/nix/store/"
+	if strings.HasPrefix(resolvedExecutable, nixStorePrefix) ||
+		strings.HasPrefix(executable, nixStorePrefix) {
+		logger.Info("detected Nix installation; skipping self-update")
+		return true
+	}
+
+	return false
+}
+
 func readLatestVersion(fs filesystem.FileSystem) string {
 	file, err := fs.Open(core.LatestVersionFile())
 	if err != nil {
@@ -91,6 +116,7 @@ func updateToLatestVersion(
 	fs filesystem.FileSystem,
 	updater Updater,
 	isHomebrewInstall bool,
+	isNixInstall bool,
 ) {
 	latestVersion := readLatestVersion(fs)
 	if latestVersion == "" {
@@ -113,6 +139,13 @@ func updateToLatestVersion(
 		return
 	}
 
+	// Nix installations shouldn't self-update; just notify the user.
+	if isNixInstall {
+		fmt.Printf("\nNew version of gsh available: %s (current: %s)\n", latestVersion, currentSemVer.String())
+		fmt.Println("gsh was installed via Nix. Update your flake input to install the latest version.")
+		return
+	}
+
 	// Check for major version boundary - don't auto-update across major versions
 	if latestSemVer.Major() > currentSemVer.Major() {
 		logger.Info("major version update available",

internal/appupdate/appupdate_test.go

@@ -217,6 +217,30 @@ func TestIsHomebrewInstall(t *testing.T) {
 	}
 }
 
+func TestIsNixInstall(t *testing.T) {
+	logger := zap.NewNop()
+	originalExecutablePath := executablePath
+	defer func() { executablePath = originalExecutablePath }()
+
+	testCases := []struct {
+		name     string
+		path     string
+		expected bool
+	}{
+		{"nix store path", "/nix/store/abc123-gsh-1.2.3/bin/gsh", true},
+		{"nix store path with long hash", "/nix/store/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx-gsh-1.10.0/bin/gsh", true},
+		{"non-nix path", "/usr/local/bin/gsh", false},
+		{"homebrew path not nix", filepath.Join("/opt", "homebrew", "Cellar", "gsh", "1.2.3", "bin", "gsh"), false},
+	}
+
+	for _, tc := range testCases {
+		executablePath = func() (string, error) {
+			return tc.path, nil
+		}
+		assert.Equal(t, tc.expected, isNixInstall(logger), tc.name)
+	}
+}
+
 func TestHandleSelfUpdate_HomebrewSkipsBinaryUpdate(t *testing.T) {
 	logger := zap.NewNop()
 	originalExecutablePath := executablePath
@@ -267,3 +291,53 @@ func TestHandleSelfUpdate_HomebrewSkipsBinaryUpdate(t *testing.T) {
 	mockRemoteRelease.AssertExpectations(t)
 	mockUpdater.AssertExpectations(t)
 }
+
+func TestHandleSelfUpdate_NixSkipsBinaryUpdate(t *testing.T) {
+	logger := zap.NewNop()
+	originalExecutablePath := executablePath
+	defer func() { executablePath = originalExecutablePath }()
+
+	executablePath = func() (string, error) {
+		return "/nix/store/abc123-gsh-1.0.0/bin/gsh", nil
+	}
+
+	mockFS := new(MockFileSystem)
+	mockUpdater := new(MockUpdater)
+	mockRemoteRelease := new(MockRelease)
+
+	mockFileForRead, _ := os.CreateTemp("", "test-latest-version-read")
+	defer os.Remove(mockFileForRead.Name())
+	mockFileForRead.Write([]byte("1.2.0"))
+	mockFileForRead.Seek(0, 0)
+
+	mockFileForWrite, _ := os.CreateTemp("", "test-latest-version-write")
+	defer os.Remove(mockFileForWrite.Name())
+
+	mockFS.On("Open", core.LatestVersionFile()).Return(mockFileForRead, nil)
+	mockFS.On("Create", core.LatestVersionFile()).Return(mockFileForWrite, nil)
+
+	mockRemoteRelease.On("Version").Return("1.2.0")
+	mockUpdater.On("DetectLatest", mock.Anything, "kunchenguid/gsh").Return(mockRemoteRelease, true, nil)
+
+	originalStdout := os.Stdout
+	readPipe, writePipe, _ := os.Pipe()
+	os.Stdout = writePipe
+
+	resultChannel := HandleSelfUpdate("1.0.0", logger, mockFS, mockUpdater)
+
+	remoteVersion, ok := <-resultChannel
+	writePipe.Close()
+	os.Stdout = originalStdout
+
+	outputBytes, _ := io.ReadAll(readPipe)
+	output := string(outputBytes)
+
+	assert.Equal(t, true, ok)
+	assert.Equal(t, "1.2.0", remoteVersion)
+	assert.Contains(t, output, "Nix")
+
+	mockUpdater.AssertNotCalled(t, "UpdateTo")
+	mockFS.AssertExpectations(t)
+	mockRemoteRelease.AssertExpectations(t)
+	mockUpdater.AssertExpectations(t)
+}