feat: support HELMFILE_NAMESPACE env var for default namespace (#2592)

* feat: support HELMFILE_NAMESPACE env var for default namespace

Mirrors the existing HELMFILE_ENVIRONMENT pattern: the --namespace
CLI flag takes precedence, falling back to HELMFILE_NAMESPACE when
unset.

Signed-off-by: Dominik Schmidt <dev@dominik-schmidt.de>

* docs: mention HELMFILE_NAMESPACE in cli.md and templating.md

Signed-off-by: Dominik Schmidt <dev@dominik-schmidt.de>

---------

Signed-off-by: Dominik Schmidt <dev@dominik-schmidt.de>

pkg/config/global.go

@@ -139,7 +139,17 @@ func (g *GlobalImpl) KubeContext() string {
 
 // Namespace returns the namespace to use.
 func (g *GlobalImpl) Namespace() string {
-	return g.GlobalOptions.Namespace
+	var namespace string
+
+	switch {
+	case g.GlobalOptions.Namespace != "":
+		namespace = g.GlobalOptions.Namespace
+	case os.Getenv("HELMFILE_NAMESPACE") != "":
+		namespace = os.Getenv("HELMFILE_NAMESPACE")
+	default:
+		namespace = ""
+	}
+	return namespace
 }
 
 // Chart returns the chart to use.

pkg/config/global_test.go

@@ -82,3 +82,40 @@ func TestKubeContext(t *testing.T) {
 	}
 	os.Unsetenv(envvar.KubeContext)
 }
+
+// TestNamespace tests the namespace flag and HELMFILE_NAMESPACE env var fallback
+func TestNamespace(t *testing.T) {
+	tests := []struct {
+		opts     GlobalOptions
+		env      string
+		expected string
+	}{
+		{
+			opts:     GlobalOptions{},
+			env:      "",
+			expected: "",
+		},
+		{
+			opts:     GlobalOptions{},
+			env:      "envset",
+			expected: "envset",
+		},
+		{
+			opts:     GlobalOptions{Namespace: "flagset"},
+			env:      "",
+			expected: "flagset",
+		},
+		{
+			opts:     GlobalOptions{Namespace: "flagset"},
+			env:      "envset",
+			expected: "flagset",
+		},
+	}
+
+	for _, test := range tests {
+		os.Setenv(envvar.Namespace, test.env)
+		received := NewGlobalImpl(&test.opts).Namespace()
+		require.Equalf(t, test.expected, received, "Namespace expected %s, received %s", test.expected, received)
+	}
+	os.Unsetenv(envvar.Namespace)
+}