diff --git a/.gitignore b/.gitignore
index ddca873..dfd323c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,9 @@
 test/
 downloads/
 config/
+__pycache__/
+*.pyc
+.pytest_cache/
 
 # Editor
 .vscode/
diff --git a/tests/test_apply_schedule.py b/tests/test_apply_schedule.py
new file mode 100644
index 0000000..49bb862
--- /dev/null
+++ b/tests/test_apply_schedule.py
@@ -0,0 +1,118 @@
+"""
+Tests for app.apply_schedule().
+
+The function rewrites the crontab file and reloads crond via SIGHUP, falling
+back to spawning a fresh crond when the recorded pid is dead or absent. These
+tests cover the six meaningful state combinations of (schedule, crontab present,
+pid file present, pid alive).
+"""
+
+import os
+import signal
+import sys
+from unittest.mock import MagicMock
+
+import pytest
+
+# Make the app module importable without installing it.
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+import app as app_module  # noqa: E402
+
+
+@pytest.fixture
+def patched(monkeypatch, tmp_path):
+    """Redirect crontab/pid paths to tmp and swap os.kill + subprocess.Popen with mocks."""
+    crontab = tmp_path / "crontabs" / "root"
+    pid_file = tmp_path / "crond.pid"
+    monkeypatch.setattr(app_module, "CRONTAB_FILE", str(crontab))
+    monkeypatch.setattr(app_module, "CROND_PID_FILE", str(pid_file))
+
+    mock_kill = MagicMock()
+    monkeypatch.setattr(app_module.os, "kill", mock_kill)
+
+    mock_popen = MagicMock()
+    mock_popen.return_value = MagicMock(pid=9999)
+    monkeypatch.setattr(app_module.subprocess, "Popen", mock_popen)
+
+    return {
+        "crontab": crontab,
+        "pid_file": pid_file,
+        "kill": mock_kill,
+        "popen": mock_popen,
+    }
+
+
+def test_manual_with_existing_crontab_and_live_pid(patched):
+    patched["crontab"].parent.mkdir(parents=True)
+    patched["crontab"].write_text("0 2 * * * /app/sync.sh\n")
+    patched["pid_file"].write_text("1234")
+
+    app_module.apply_schedule("manual")
+
+    assert not patched["crontab"].exists()
+    patched["kill"].assert_called_once_with(1234, signal.SIGHUP)
+    patched["popen"].assert_not_called()
+
+
+def test_manual_with_no_crontab_and_no_pid_file(patched):
+    assert not patched["crontab"].exists()
+    assert not patched["pid_file"].exists()
+
+    app_module.apply_schedule("manual")  # should not raise
+
+    assert not patched["crontab"].exists()
+    patched["kill"].assert_not_called()
+    patched["popen"].assert_not_called()
+
+
+def test_real_schedule_with_live_pid(patched):
+    patched["pid_file"].write_text("4321")
+
+    app_module.apply_schedule("0 4 * * *")
+
+    expected_line = "0 4 * * * /app/sync.sh >> /var/log/youtube-sync.log 2>&1\n"
+    assert patched["crontab"].read_text() == expected_line
+    patched["kill"].assert_called_once_with(4321, signal.SIGHUP)
+    patched["popen"].assert_not_called()
+
+
+def test_real_schedule_with_dead_pid(patched):
+    patched["pid_file"].write_text("4321")
+    patched["kill"].side_effect = ProcessLookupError()
+
+    app_module.apply_schedule("0 4 * * *")
+
+    expected_line = "0 4 * * * /app/sync.sh >> /var/log/youtube-sync.log 2>&1\n"
+    assert patched["crontab"].read_text() == expected_line
+    patched["kill"].assert_called_once_with(4321, signal.SIGHUP)
+    patched["popen"].assert_called_once_with(
+        ["crond", "-f", "-l", "2"],
+        stdout=app_module.subprocess.DEVNULL,
+        stderr=app_module.subprocess.DEVNULL,
+    )
+    assert patched["pid_file"].read_text() == "9999"
+
+
+def test_real_schedule_with_no_pid_file(patched):
+    assert not patched["pid_file"].exists()
+
+    app_module.apply_schedule("0 4 * * *")
+
+    expected_line = "0 4 * * * /app/sync.sh >> /var/log/youtube-sync.log 2>&1\n"
+    assert patched["crontab"].read_text() == expected_line
+    patched["kill"].assert_not_called()
+    patched["popen"].assert_called_once()
+    assert patched["pid_file"].read_text() == "9999"
+
+
+def test_real_schedule_with_garbage_pid_file(patched):
+    patched["pid_file"].write_text("not-a-number")
+
+    app_module.apply_schedule("0 4 * * *")
+
+    expected_line = "0 4 * * * /app/sync.sh >> /var/log/youtube-sync.log 2>&1\n"
+    assert patched["crontab"].read_text() == expected_line
+    patched["kill"].assert_not_called()
+    patched["popen"].assert_called_once()
+    assert patched["pid_file"].read_text() == "9999"