feat: add POSIX null command : as a builtin

src/shell/commands/mod.rs

@@ -98,6 +98,12 @@ pub fn builtin_commands() -> HashMap<String, Rc<dyn ShellCommand>> {
       "false".to_string(),
       Rc::new(ExitCodeCommand(1)) as Rc<dyn ShellCommand>,
     ),
+    // POSIX null command: discards its arguments and always exits 0.
+    // Commonly seen as `"test": ":"` in package.json no-op scripts.
+    (
+      ":".to_string(),
+      Rc::new(ExitCodeCommand(0)) as Rc<dyn ShellCommand>,
+    ),
     (
       "unset".to_string(),
       Rc::new(unset::UnsetCommand) as Rc<dyn ShellCommand>,

tests/integration_test.rs

@@ -154,6 +154,24 @@ async fn boolean_logic() {
     .await;
 }
 
+#[tokio::test]
+async fn null_command() {
+  // POSIX `:` is the null command — always succeeds, discards args.
+  TestBuilder::new().command(":").run().await;
+
+  TestBuilder::new()
+    .command(": ignored args && echo ok")
+    .assert_stdout("ok\n")
+    .run()
+    .await;
+
+  TestBuilder::new()
+    .command(": && echo yes || echo no")
+    .assert_stdout("yes\n")
+    .run()
+    .await;
+}
+
 #[tokio::test]
 async fn exit() {
   TestBuilder::new()