diff --git a/kiauh/core/backends/__init__.py b/kiauh/core/backends/__init__.py
new file mode 100644
index 00000000..db023100
--- /dev/null
+++ b/kiauh/core/backends/__init__.py
@@ -0,0 +1,182 @@
+# ======================================================================= #
+#  Copyright (C) 2020 - 2026 Dominik Willner <th33xitus@gmail.com>        #
+#                                                                         #
+#  This file is part of KIAUH - Klipper Installation And Update Helper    #
+#                                                                         #
+#  https://github.com/dw-0/kiauh                                          #
+#                                                                         #
+#  This file may be distributed under the terms of the GNU GPLv3 license  #
+# ======================================================================= #
+from __future__ import annotations
+
+import shutil
+import subprocess
+from pathlib import Path
+from typing import Any, List, Protocol, Sequence, cast, runtime_checkable
+
+# --------------------------------------------------------------------------- #
+# Singleton backends                                                          #
+# --------------------------------------------------------------------------- #
+# There is exactly ONE ``command_runner`` and ONE ``filesystem`` global in the
+# whole project, owned by this module. They are assigned (with explicit type
+# annotations) at the bottom of this file, AFTER the default implementations
+# are defined. ``utils.fs_utils`` and ``utils.sys_utils`` delegate to these
+# singletons via the wrapper functions below, so tests patch a single place —
+# ``core.backends.command_runner`` / ``core.backends.filesystem`` — instead of
+# per-module duplicates
+
+
+def run(cmd: str | List[str], **kwargs: Any) -> subprocess.CompletedProcess[str]:
+    """Run a command through the shared command runner."""
+    return command_runner.run(cmd, **kwargs)
+
+
+def check_output(cmd: str | List[str], **kwargs: Any) -> str | bytes:
+    """Run a command and return its output through the shared command runner."""
+    return command_runner.check_output(cmd, **kwargs)
+
+
+def call(cmd: str | List[str], **kwargs: Any) -> int:
+    """Run a command and return its exit code through the shared command runner."""
+    return command_runner.call(cmd, **kwargs)
+
+
+def popen(cmd: str | List[str], **kwargs: Any) -> subprocess.Popen:
+    """Start a process through the shared command runner."""
+    return command_runner.popen(cmd, **kwargs)
+
+
+@runtime_checkable
+class CommandRunner(Protocol):
+    """Pluggable backend for executing system commands."""
+
+    def run(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> subprocess.CompletedProcess: ...
+
+    def check_output(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> str | bytes: ...
+
+    def call(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> int: ...
+
+    def popen(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> subprocess.Popen: ...
+
+
+class SubprocessRunner:
+    """Default command runner backed by the standard subprocess module."""
+
+    def run(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> subprocess.CompletedProcess[str]:
+        return subprocess.run(cmd, **kwargs)
+
+    def check_output(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> str | bytes:
+        return cast("str | bytes", subprocess.check_output(cmd, **kwargs))
+
+    def call(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> int:
+        return subprocess.call(cmd, **kwargs)
+
+    def popen(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> subprocess.Popen:
+        return subprocess.Popen(cmd, **kwargs)
+
+
+@runtime_checkable
+class FilesystemBackend(Protocol):
+    """Pluggable backend for filesystem operations."""
+
+    def exists(self, path: Path) -> bool: ...
+
+    def is_dir(self, path: Path) -> bool: ...
+
+    def is_file(self, path: Path) -> bool: ...
+
+    def is_symlink(self, path: Path) -> bool: ...
+
+    def mkdir(
+        self, path: Path, *, parents: bool = False, exist_ok: bool = False
+    ) -> None: ...
+
+    def unlink(self, path: Path) -> None: ...
+
+    def rmtree(self, path: Path) -> None: ...
+
+    def read_text(self, path: Path) -> str: ...
+
+    def write_text(self, path: Path, content: str) -> None: ...
+
+    def copy(self, source: Path, target: Path) -> None: ...
+
+    def home(self) -> Path: ...
+
+
+class LocalFilesystemBackend:
+    """Default filesystem backend backed by the local filesystem."""
+
+    def exists(self, path: Path) -> bool:
+        return path.exists()
+
+    def is_dir(self, path: Path) -> bool:
+        return path.is_dir()
+
+    def is_file(self, path: Path) -> bool:
+        return path.is_file()
+
+    def is_symlink(self, path: Path) -> bool:
+        return path.is_symlink()
+
+    def mkdir(
+        self, path: Path, *, parents: bool = False, exist_ok: bool = False
+    ) -> None:
+        path.mkdir(parents=parents, exist_ok=exist_ok)
+
+    def unlink(self, path: Path) -> None:
+        path.unlink()
+
+    def rmtree(self, path: Path) -> None:
+        shutil.rmtree(path)
+
+    def read_text(self, path: Path) -> str:
+        return path.read_text()
+
+    def write_text(self, path: Path, content: str) -> None:
+        path.write_text(content)
+
+    def copy(self, source: Path, target: Path) -> None:
+        if source.is_dir():
+            shutil.copytree(source, target)
+        else:
+            shutil.copy2(source, target)
+
+    def home(self) -> Path:
+        return Path.home()
+
+
+command_runner: CommandRunner = SubprocessRunner()
+filesystem: FilesystemBackend = LocalFilesystemBackend()
diff --git a/kiauh/core/backends/tests/__init__.py b/kiauh/core/backends/tests/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/kiauh/core/backends/tests/test_backends.py b/kiauh/core/backends/tests/test_backends.py
new file mode 100644
index 00000000..153049a9
--- /dev/null
+++ b/kiauh/core/backends/tests/test_backends.py
@@ -0,0 +1,101 @@
+from __future__ import annotations
+
+import subprocess
+from pathlib import Path
+
+import pytest
+from core import backends
+from core.backends import LocalFilesystemBackend, SubprocessRunner
+from tests.helpers.fake_backends import FakeCommandRunner, FakeFilesystemBackend
+from utils import fs_utils, sys_utils
+
+
+class TestSubprocessRunner:
+    def test_run_executes_command(self) -> None:
+        runner = SubprocessRunner()
+        result = runner.run(["true"])
+        assert result.returncode == 0
+
+    def test_check_output_returns_stdout(self) -> None:
+        runner = SubprocessRunner()
+        output = runner.check_output(["echo", "hello"], text=True)
+        assert "hello" in output
+
+
+class TestCommandRunnerInjection:
+    def test_sys_utils_uses_injected_runner(self, monkeypatch) -> None:
+        fake = FakeCommandRunner({
+            ("some", "cmd"): subprocess.CompletedProcess(["some", "cmd"], 0, "", "")
+        })
+        monkeypatch.setattr(backends, "command_runner", fake)
+
+        sys_utils.run(["some", "cmd"], check=True)
+
+        assert fake.calls[0][0] == ["some", "cmd"]
+        assert fake.calls[0][1].get("check") is True
+
+    def test_cmd_sysctl_service_records_command(self, monkeypatch) -> None:
+        expected_cmd = ["sudo", "systemctl", "start", "klipper.service"]
+        fake = FakeCommandRunner({
+            tuple(expected_cmd): subprocess.CompletedProcess(expected_cmd, 0, "", "")
+        })
+        monkeypatch.setattr(backends, "command_runner", fake)
+
+        sys_utils.cmd_sysctl_service("klipper.service", "start")
+
+        assert fake.calls[0][0] == expected_cmd
+
+    @pytest.mark.parametrize("module", [sys_utils, fs_utils])
+    def test_single_shared_command_runner_registry(self, monkeypatch, module) -> None:
+        # there is only ONE ``command_runner`` global to patch.
+        # Patching ``core.backends.command_runner`` must affect every wrapper
+        # (sys_utils.run, fs_utils.run, enum helpers) — no per-module duplicates.
+        fake = FakeCommandRunner({
+            ("shared", "cmd"): subprocess.CompletedProcess(["shared", "cmd"], 0, "", "")
+        })
+        monkeypatch.setattr(backends, "command_runner", fake)
+
+        module.run(["shared", "cmd"], check=True)
+
+        assert fake.calls[0][0] == ["shared", "cmd"]
+
+
+class TestLocalFilesystemBackend:
+    def test_write_and_read_text(self, tmp_path: Path) -> None:
+        fs = LocalFilesystemBackend()
+        target = tmp_path / "test.txt"
+        fs.write_text(target, "hello")
+        assert fs.read_text(target) == "hello"
+
+    def test_mkdir_and_exists(self, tmp_path: Path) -> None:
+        fs = LocalFilesystemBackend()
+        target = tmp_path / "new_dir"
+        assert not fs.exists(target)
+        fs.mkdir(target)
+        assert fs.exists(target)
+        assert fs.is_dir(target)
+
+
+class TestFilesystemBackendInjection:
+    def test_create_folders_uses_injected_fs(self, monkeypatch) -> None:
+        fake = FakeFilesystemBackend()
+        monkeypatch.setattr(backends, "filesystem", fake)
+
+        fs_utils.create_folders([Path("/tmp/a"), Path("/tmp/b")])
+
+        assert fake.exists(Path("/tmp/a"))
+        assert fake.exists(Path("/tmp/b"))
+
+    def test_run_remove_routines_uses_injected_fs(self, monkeypatch) -> None:
+        fake = FakeFilesystemBackend()
+        fake.add_file(Path("/tmp/file.txt"), "x")
+        monkeypatch.setattr(backends, "filesystem", fake)
+
+        assert fs_utils.run_remove_routines(Path("/tmp/file.txt")) is True
+        assert not fake.exists(Path("/tmp/file.txt"))
+
+    def test_run_remove_routines_skips_missing_file(self, monkeypatch) -> None:
+        fake = FakeFilesystemBackend()
+        monkeypatch.setattr(backends, "filesystem", fake)
+
+        assert fs_utils.run_remove_routines(Path("/tmp/missing")) is False
diff --git a/kiauh/tests/__init__.py b/kiauh/tests/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/kiauh/tests/helpers/__init__.py b/kiauh/tests/helpers/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/kiauh/tests/helpers/fake_backends.py b/kiauh/tests/helpers/fake_backends.py
new file mode 100644
index 00000000..1042be80
--- /dev/null
+++ b/kiauh/tests/helpers/fake_backends.py
@@ -0,0 +1,178 @@
+# ======================================================================= #
+#  Test-only backends. Not imported by production code.                   #
+# ======================================================================= #
+from __future__ import annotations
+
+import subprocess
+from pathlib import Path
+from typing import Any, Dict, List, Sequence, Tuple
+
+
+class FakeCommandRunner:
+    """Command runner for tests. Records calls and returns scripted responses.
+
+    By default, running a command that was not explicitly scripted raises an
+    error so missing mocks are caught during development. Pass
+    ``strict=False`` to restore the legacy "default success" behavior.
+    """
+
+    def __init__(
+        self,
+        responses: Dict[Tuple[str, ...], subprocess.CompletedProcess] | None = None,
+        *,
+        strict: bool = True,
+    ) -> None:
+        self.calls: List[Tuple[str | Sequence[str], Dict[str, Any]]] = []
+        self.responses = responses or {}
+        self.strict = strict
+
+    @staticmethod
+    def _key(cmd: str | Sequence[str]) -> Tuple[str, ...]:
+        if isinstance(cmd, str):
+            return (cmd,)
+        return tuple(str(c) for c in cmd)
+
+    def _make_response(
+        self, cmd: str | Sequence[str], returncode: int = 0
+    ) -> subprocess.CompletedProcess:
+        return subprocess.CompletedProcess(
+            args=cmd,
+            returncode=returncode,
+            stdout="",
+            stderr="",
+        )
+
+    def _unscripted(self, cmd: str | Sequence[str]) -> subprocess.CompletedProcess:
+        if self.strict:
+            raise RuntimeError(f"Unscripted command: {cmd}")
+        return self._make_response(cmd)
+
+    def run(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> subprocess.CompletedProcess:
+        self.calls.append((cmd, kwargs))
+        key = self._key(cmd)
+        if key in self.responses:
+            return self.responses[key]
+        return self._unscripted(cmd)
+
+    def check_output(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> str | bytes:
+        self.calls.append((cmd, kwargs))
+        key = self._key(cmd)
+        if key in self.responses:
+            return self.responses[key].stdout  # type: ignore[no-any-return]
+        if self.strict:
+            raise RuntimeError(f"Unscripted command: {cmd}")
+        return ""
+
+    def call(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> int:
+        self.calls.append((cmd, kwargs))
+        key = self._key(cmd)
+        if key in self.responses:
+            return self.responses[key].returncode
+        if self.strict:
+            raise RuntimeError(f"Unscripted command: {cmd}")
+        return 0
+
+    def popen(
+        self,
+        cmd: str | Sequence[str],
+        **kwargs: Any,
+    ) -> subprocess.Popen:
+        raise NotImplementedError("FakeCommandRunner.popen is not implemented")
+
+
+class FakeFilesystemBackend:
+    """In-memory filesystem backend for tests."""
+
+    def __init__(self) -> None:
+        self.dirs: set[str] = set()
+        self.files: Dict[str, str] = {}
+        self.symlinks: Dict[str, str] = {}
+        self._home: Path = Path("/home/test")
+
+    def _path(self, path: Path) -> str:
+        return str(Path(path).resolve())
+
+    def exists(self, path: Path) -> bool:
+        key = self._path(path)
+        return key in self.dirs or key in self.files or key in self.symlinks
+
+    def is_dir(self, path: Path) -> bool:
+        return self._path(path) in self.dirs
+
+    def is_file(self, path: Path) -> bool:
+        return self._path(path) in self.files
+
+    def is_symlink(self, path: Path) -> bool:
+        return self._path(path) in self.symlinks
+
+    def mkdir(
+        self, path: Path, *, parents: bool = False, exist_ok: bool = False
+    ) -> None:
+        key = self._path(path)
+        if key in self.files and not exist_ok:
+            raise FileExistsError(key)
+        if key in self.dirs and not exist_ok:
+            raise FileExistsError(key)
+        if parents:
+            for parent in reversed(Path(key).parents):
+                self.dirs.add(str(parent))
+        self.dirs.add(key)
+
+    def unlink(self, path: Path) -> None:
+        key = self._path(path)
+        if key in self.files:
+            del self.files[key]
+        elif key in self.symlinks:
+            del self.symlinks[key]
+        else:
+            raise FileNotFoundError(key)
+
+    def rmtree(self, path: Path) -> None:
+        key = self._path(path)
+        if key not in self.dirs:
+            raise FileNotFoundError(key)
+        prefix = key + "/"
+        self.dirs = {d for d in self.dirs if not (d == key or d.startswith(prefix))}
+        self.files = {k: v for k, v in self.files.items() if not k.startswith(prefix)}
+        self.symlinks = {
+            k: v for k, v in self.symlinks.items() if not k.startswith(prefix)
+        }
+
+    def read_text(self, path: Path) -> str:
+        key = self._path(path)
+        if key not in self.files:
+            raise FileNotFoundError(key)
+        return self.files[key]
+
+    def write_text(self, path: Path, content: str) -> None:
+        key = self._path(path)
+        self.files[key] = content
+        self.dirs.discard(key)
+
+    def copy(self, source: Path, target: Path) -> None:
+        content = self.read_text(source)
+        self.write_text(target, content)
+
+    def home(self) -> Path:
+        return self._home
+
+    def add_dir(self, path: Path) -> None:
+        self.dirs.add(self._path(path))
+
+    def add_file(self, path: Path, content: str = "") -> None:
+        self.files[self._path(path)] = content
+
+    def add_symlink(self, path: Path, target: Path) -> None:
+        self.symlinks[self._path(path)] = str(target)
diff --git a/kiauh/tests/test_main.py b/kiauh/tests/test_main.py
new file mode 100644
index 00000000..5cc91a1a
--- /dev/null
+++ b/kiauh/tests/test_main.py
@@ -0,0 +1,98 @@
+# ======================================================================= #
+#  Copyright (C) 2020 - 2026 Dominik Willner <th33xitus@gmail.com>        #
+#                                                                         #
+#  This file is part of KIAUH - Klipper Installation And Update Helper    #
+#  https://github.com/dw-0/kiauh                                          #
+#                                                                         #
+#  This file may be distributed under the terms of the GNU GPLv3 license  #
+# ======================================================================= #
+from __future__ import annotations
+
+from typing import List
+
+import main as main_module
+import pytest
+
+
+class _FakeMainMenu:
+    """Minimal stand-in for ``core.menus.main_menu.MainMenu``."""
+
+    instances: List["_FakeMainMenu"] = []
+
+    def __init__(self) -> None:
+        self._run = False
+        type(self).instances.append(self)
+
+    def run(self) -> None:
+        self._run = True
+
+    @classmethod
+    def reset(cls) -> None:
+        cls.instances = []
+
+
+@pytest.fixture(autouse=True)
+def _reset_fake_menu() -> None:
+    _FakeMainMenu.reset()
+    yield
+    _FakeMainMenu.reset()
+
+
+def _patch_tui_seeds(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Neutralise the heavyweight side-effects triggered when launching the TUI."""
+    monkeypatch.setattr(main_module, "KiauhSettings", lambda: None)
+    monkeypatch.setattr(main_module, "ensure_encoding", lambda: None)
+    monkeypatch.setattr(main_module, "MainMenu", _FakeMainMenu)
+
+
+class TestMainDispatch:
+    def test_no_command_launches_tui(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        # rc == -1 means "fall back to the TUI": ``MainMenu().run()`` is called.
+        monkeypatch.setattr(main_module, "run_cli", lambda: -1)
+        _patch_tui_seeds(monkeypatch)
+
+        main_module.main()
+
+        assert _FakeMainMenu.instances
+        assert all(m._run for m in _FakeMainMenu.instances)
+
+    def test_cli_success_returns_cleanly(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        # rc == 0 means the CLI succeeded; the TUI must NOT start and main must
+        # NOT call sys.exit.
+        monkeypatch.setattr(main_module, "run_cli", lambda: 0)
+        _patch_tui_seeds(monkeypatch)
+
+        main_module.main()  # must not raise SystemExit
+
+        assert _FakeMainMenu.instances == []
+
+    def test_cli_failure_exits_nonzero(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        # rc > 0 means the CLI reported a failure; main must propagate via sys.exit.
+        monkeypatch.setattr(main_module, "run_cli", lambda: 2)
+        _patch_tui_seeds(monkeypatch)
+
+        with pytest.raises(SystemExit) as exc:
+            main_module.main()
+
+        assert exc.value.code == 2
+        assert _FakeMainMenu.instances == []
+
+    def test_tui_keyboard_interrupt_is_absorbed(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        # A Ctrl-C while the TUI runs must be caught and printed friendly
+        # instead of crashing with a traceback.
+        class _InterruptingMenu(_FakeMainMenu):
+            def run(self) -> None:
+                raise KeyboardInterrupt()
+
+        monkeypatch.setattr(main_module, "run_cli", lambda: -1)
+        monkeypatch.setattr(main_module, "KiauhSettings", lambda: None)
+        monkeypatch.setattr(main_module, "ensure_encoding", lambda: None)
+        monkeypatch.setattr(main_module, "MainMenu", _InterruptingMenu)
+
+        main_module.main()  # must not raise; KeyboardInterrupt is absorbed
\ No newline at end of file