connect stdin of external commands to the terminal

Without Stdin, commands read from /dev/null, so interactive programs
like 'sudo su -', vim or ssh got EOF right away.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

main.go

@@ -760,9 +760,13 @@ Features:
 	// Prepare the command to execute.
 	cmd := exec.Command(args[0], args[1:]...)
 
-	// Set the correct output device.
-	cmd.Stderr = os.Stderr
+	// Connect the command to the terminal. Without Stdin the command would
+	// read from /dev/null, so interactive programs (su, vim, ssh, ...) would
+	// get EOF right away. readline only reads from stdin while a line is
+	// being edited, so it does not steal input from the command.
+	cmd.Stdin = os.Stdin
 	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
 
 	// Execute the command return the error.
 	return cmd.Run()

main_test.go

@@ -1221,3 +1221,34 @@ func TestShell_SIGINTKillsOnlyChild(t *testing.T) {
 		t.Errorf("child was not interrupted (took %v)", elapsed)
 	}
 }
+
+func TestExecInput_PassesStdin(t *testing.T) {
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	origStdin := os.Stdin
+	os.Stdin = r
+	defer func() {
+		os.Stdin = origStdin
+		r.Close()
+	}()
+
+	if _, err := w.WriteString("hello\n"); err != nil {
+		t.Fatal(err)
+	}
+	w.Close()
+
+	out := filepath.Join(t.TempDir(), "out")
+	t.Setenv("GOSH_TEST_OUT", out)
+	if err := execInput(`sh -c 'read x; printf %s "$x" > "$GOSH_TEST_OUT"'`); err != nil {
+		t.Fatal(err)
+	}
+	got, err := os.ReadFile(out)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "hello" {
+		t.Errorf("command read %q from stdin, want %q", got, "hello")
+	}
+}