diff --git a/rsync_time_machine.py b/rsync_time_machine.py
index f1ff1f7..a45109d 100755
--- a/rsync_time_machine.py
+++ b/rsync_time_machine.py
@@ -9,7 +9,9 @@
 import re
 import signal
 import sys
+import tempfile
 import time
+from contextlib import suppress
 from datetime import datetime
 from typing import TYPE_CHECKING, Callable, NamedTuple
 
@@ -60,6 +62,33 @@ def sanitize(s: str) -> str:
     return s.encode("utf-8", "surrogateescape").decode("utf-8", "replace")
 
 
+def prepare_exclusion_file(exclusion_file: str) -> tuple[str, Callable[[], None]]:
+    """Ensure rsync exclusion file ends with a newline to match rsync parsing."""
+    try:
+        with open(exclusion_file, "rb") as f:
+            contents = f.read()
+    except FileNotFoundError:
+        log_error(f"Exclusion file '{exclusion_file}' does not exist - aborting.")
+        sys.exit(1)
+
+    if not contents or contents.endswith(b"\n"):
+
+        def cleanup_noop() -> None:
+            return None
+
+        return exclusion_file, cleanup_noop
+
+    fd, temp_path = tempfile.mkstemp(prefix="rsync-exclude-", suffix=".txt")
+    with os.fdopen(fd, "wb") as tmp:
+        tmp.write(contents + b"\n")
+
+    def cleanup() -> None:
+        with suppress(FileNotFoundError):
+            os.remove(temp_path)
+
+    return temp_path, cleanup
+
+
 def log(message: str, level: str = "info") -> None:
     """Log a message with the specified log level."""
     levels = {"info": "", "warning": "[WARNING] ", "error": "[ERROR] "}
@@ -638,10 +667,34 @@ def get_rsync_flags(
     return rsync_flags
 
 
+def normalize_pid(running_pid: str, ssh: SSH | None = None) -> int | None:
+    """Return a valid PID or None when the stored value should be ignored."""
+    pid_str = running_pid.strip()
+    try:
+        pid = int(pid_str)
+    except ValueError:
+        return None
+
+    if pid <= 0:
+        return None
+
+    if ssh is None and pid == os.getpid():
+        # Allow re-entrancy within the same local process (e.g. during tests)
+        return None
+
+    return pid
+
+
 def exit_if_pid_running(running_pid: str, ssh: SSH | None = None) -> None:
     """Exit if another instance of this script is already running."""
+    pid = normalize_pid(running_pid, ssh)
+    if pid is None:
+        return
+
+    pid_str = str(pid)
+
     if sys.platform == "cygwin":
-        cmd = f"procps -wwfo cmd -p {running_pid} --no-headers | grep '{APPNAME}'"
+        cmd = f"procps -wwfo cmd -p {pid_str} --no-headers | grep '{APPNAME}'"
         running_cmd = run_cmd(cmd, ssh)
         if running_cmd.returncode == 0:
             log_error(
@@ -650,7 +703,7 @@ def exit_if_pid_running(running_pid: str, ssh: SSH | None = None) -> None:
             sys.exit(1)
     else:
         ps_flags = "-axp" if sys.platform.startswith("netbsd") else "-p"
-        cmd = f"ps {ps_flags} {running_pid} -o 'command' | grep '{APPNAME}'"
+        cmd = f"ps {ps_flags} {pid_str} -o 'command' | grep '{APPNAME}'"
         if run_cmd(cmd).stdout:
             log_error("Previous backup task is still active - aborting.")
             sys.exit(1)
@@ -694,7 +747,7 @@ def deal_with_no_space_left(
     auto_expire: bool,
 ) -> bool:
     """Deal with no space left on device."""
-    with open(log_file) as f:
+    with open(log_file, encoding="utf-8", errors="surrogateescape") as f:
         log_data = f.read()
 
     no_space_left = re.search(
@@ -727,7 +780,7 @@ def check_rsync_errors(
     auto_delete_log: bool,  # noqa: FBT001
 ) -> None:
     """Check rsync errors."""
-    with open(log_file) as f:
+    with open(log_file, encoding="utf-8", errors="surrogateescape") as f:
         log_data = f.read()
     if "rsync error:" in log_data:
         log_error(
@@ -765,6 +818,10 @@ def start_backup(
         log_dir,
         f"{now}.log",
     )
+
+    def cleanup_exclusion() -> None:
+        return None
+
     if ssh is not None:
         src_folder = f"{ssh.src_folder_prefix}{src_folder}"
         dest = f"{ssh.dest_folder_prefix}{dest}"
@@ -780,7 +837,10 @@ def start_backup(
     cmd = f"{cmd} {' '.join(rsync_flags)}"
     cmd = f"{cmd} --log-file '{log_file}'"
     if exclusion_file:
-        cmd = f"{cmd} --exclude-from '{exclusion_file}'"
+        prepared_exclusion_file, cleanup_exclusion = prepare_exclusion_file(
+            exclusion_file,
+        )
+        cmd = f"{cmd} --exclude-from '{prepared_exclusion_file}'"
 
     cmd = f"{cmd} {link_dest_option}"
     cmd = f"{cmd} -- '{src_folder}/' '{dest}/'"
@@ -790,7 +850,10 @@ def start_backup(
 
     run_cmd(f"echo {mypid} > {inprogress_file}", ssh)
 
-    run_cmd(cmd)
+    try:
+        run_cmd(cmd)
+    finally:
+        cleanup_exclusion()
     return log_file
 
 
diff --git a/tests/test_diff_coverage.py b/tests/test_diff_coverage.py
new file mode 100644
index 0000000..abc33ac
--- /dev/null
+++ b/tests/test_diff_coverage.py
@@ -0,0 +1,241 @@
+"""Additional tests ensuring full coverage of the PR diff."""
+
+from __future__ import annotations
+
+import importlib
+import os
+from pathlib import Path
+from typing import Any
+
+import pytest
+
+
+@pytest.fixture
+def rtm() -> Any:
+    """Reload the module so coverage tracks executed lines precisely."""
+    module = importlib.import_module("rsync_time_machine")
+    return importlib.reload(module)
+
+
+def test_prepare_exclusion_file_missing_file(tmp_path: Path, rtm: Any) -> None:
+    """Missing exclusion files should trigger a fatal error."""
+    module = rtm
+    missing = tmp_path / "does-not-exist.txt"
+    with pytest.raises(SystemExit):
+        module.prepare_exclusion_file(str(missing))
+
+
+def test_prepare_exclusion_file_noop_cleanup(tmp_path: Path, rtm: Any) -> None:
+    """A newline-terminated exclusion file is returned untouched."""
+    module = rtm
+    exclusion = tmp_path / "exclude.txt"
+    exclusion.write_bytes(b"pattern\n")
+
+    prepared, cleanup = module.prepare_exclusion_file(str(exclusion))
+
+    assert prepared == str(exclusion)
+    cleanup()  # ensure the no-op path is executed
+    assert exclusion.exists()
+
+
+def test_prepare_exclusion_file_appends_newline_and_cleans(
+    tmp_path: Path, rtm: Any, monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Files without a trailing newline are copied and cleaned up after use."""
+    module = rtm
+    exclusion = tmp_path / "exclude.txt"
+    exclusion.write_bytes(b"pattern")
+
+    created_paths: list[Path] = []
+
+    original_mkstemp = module.tempfile.mkstemp
+
+    def fake_mkstemp(prefix: str, suffix: str) -> tuple[int, str]:
+        fd, path = original_mkstemp(prefix=prefix, suffix=suffix, dir=tmp_path)
+        created_paths.append(Path(path))
+        return fd, path
+
+    monkeypatch.setattr(module.tempfile, "mkstemp", fake_mkstemp)
+
+    prepared, cleanup = module.prepare_exclusion_file(str(exclusion))
+
+    prepared_path = Path(prepared)
+    assert prepared_path in created_paths
+    assert prepared_path.read_bytes() == b"pattern\n"
+
+    cleanup()
+    for path in created_paths:
+        assert not path.exists()
+
+
+def test_normalize_pid_variants(rtm: Any) -> None:
+    """Exercise the PID normalisation helper across its branches."""
+    module = rtm
+    assert module.normalize_pid("", None) is None
+    assert module.normalize_pid("0", None) is None
+
+    current_pid = os.getpid()
+    assert module.normalize_pid(str(current_pid), None) is None
+
+    fake_ssh = module.SSH("", "", "", "", "", "22", None)
+    assert module.normalize_pid(str(current_pid), fake_ssh) == current_pid
+
+    assert module.normalize_pid("1234", None) == 1234
+
+
+def test_exit_if_pid_running_invokes_ps(
+    rtm: Any, monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Valid PIDs should lead to a ps invocation with the stripped PID string."""
+    module = rtm
+    commands: list[str] = []
+
+    def fake_run_cmd(cmd: str, ssh: Any | None = None) -> Any:
+        commands.append(cmd)
+        return module.CmdResult("", "", 0)
+
+    monkeypatch.setattr(module, "run_cmd", fake_run_cmd)
+
+    module.exit_if_pid_running(" 42 ")
+
+    expected = f"ps -p 42 -o 'command' | grep '{module.APPNAME}'"
+    assert expected in commands
+
+
+def test_exit_if_pid_running_detects_active_process(
+    rtm: Any, monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """A matching process triggers an early exit."""
+    module = rtm
+
+    def fake_run_cmd(cmd: str, ssh: Any | None = None) -> Any:
+        if cmd.startswith("ps "):
+            return module.CmdResult("pytest", "", 0)
+        return module.CmdResult("", "", 0)
+
+    monkeypatch.setattr(module, "run_cmd", fake_run_cmd)
+
+    with pytest.raises(SystemExit):
+        module.exit_if_pid_running("4242")
+
+
+def test_exit_if_pid_running_cygwin_branch(
+    rtm: Any, monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """The cygwin branch should use the normalised PID and exit when active."""
+    module = rtm
+    commands: list[str] = []
+
+    monkeypatch.setattr(module.sys, "platform", "cygwin")
+
+    def fake_run_cmd(cmd: str, ssh: Any | None = None) -> Any:
+        commands.append(cmd)
+        return module.CmdResult("", "", 0)
+
+    monkeypatch.setattr(module, "run_cmd", fake_run_cmd)
+
+    with pytest.raises(SystemExit):
+        module.exit_if_pid_running("0100")
+
+    expected = f"procps -wwfo cmd -p 100 --no-headers | grep '{module.APPNAME}'"
+    assert expected in commands
+
+
+def test_start_backup_cleans_temp_exclusion_on_failure(
+    tmp_path: Path, rtm: Any, monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """start_backup removes temporary exclude files even when rsync fails."""
+    module = rtm
+    src = tmp_path / "src"
+    dest = tmp_path / "dest"
+    log_dir = tmp_path / "logs"
+    src.mkdir()
+    dest.mkdir()
+    log_dir.mkdir()
+
+    exclusion = tmp_path / "exclude.txt"
+    exclusion.write_text("pattern")
+
+    created_paths: list[Path] = []
+
+    original_mkstemp = module.tempfile.mkstemp
+
+    def fake_mkstemp(prefix: str, suffix: str) -> tuple[int, str]:
+        fd, path = original_mkstemp(prefix=prefix, suffix=suffix, dir=tmp_path)
+        created_paths.append(Path(path))
+        return fd, path
+
+    monkeypatch.setattr(module.tempfile, "mkstemp", fake_mkstemp)
+
+    def fake_run_cmd(cmd: str, ssh: Any | None = None) -> Any:
+        if cmd.startswith("echo"):
+            _, path = cmd.split(">", maxsplit=1)
+            Path(path.strip().strip("'\"")).write_text(str(os.getpid()))
+            return module.CmdResult("", "", 0)
+        raise RuntimeError("rsync failed")
+
+    monkeypatch.setattr(module, "run_cmd", fake_run_cmd)
+
+    with pytest.raises(RuntimeError, match="rsync failed"):
+        module.start_backup(
+            src_folder=str(src),
+            dest=str(dest),
+            exclusion_file=str(exclusion),
+            inprogress_file=str(dest / "backup.inprogress"),
+            link_dest_option="",
+            rsync_flags=["-a"],
+            log_dir=str(log_dir),
+            mypid=os.getpid(),
+            ssh=None,
+            now="2025-10-13-211815",
+        )
+
+    for path in created_paths:
+        assert not path.exists()
+
+
+def test_deal_with_no_space_left_handles_non_utf8(
+    tmp_path: Path, rtm: Any, monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Non-UTF8 log content is parsed safely when space runs out."""
+    module = rtm
+    log_file = tmp_path / "backup.log"
+    log_file.write_bytes(b"\xffNo space left on device (28)")
+
+    monkeypatch.setattr(module, "find_backups", lambda dest, ssh: ["a", "b"])
+
+    expired: list[str] = []
+
+    def fake_expire(folder: str, ssh: Any) -> None:
+        expired.append(folder)
+
+    monkeypatch.setattr(module, "expire_backup", fake_expire)
+    monkeypatch.setattr(module, "log_warn", lambda message: None)
+    monkeypatch.setattr(module, "log_error", lambda message: None)
+
+    result = module.deal_with_no_space_left(
+        log_file=str(log_file),
+        dest_folder=str(tmp_path),
+        ssh=None,
+        auto_expire=True,
+    )
+
+    assert result is True
+    assert expired
+
+
+def test_check_rsync_errors_handles_non_utf8_error(
+    tmp_path: Path, rtm: Any, monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Log parsing tolerates undecodable bytes when reporting rsync errors."""
+    module = rtm
+    log_file = tmp_path / "rsync.log"
+    log_file.write_bytes(b"\xffrsync error: something broke")
+
+    messages: list[str] = []
+
+    monkeypatch.setattr(module, "log_error", lambda message: messages.append(message))
+
+    module.check_rsync_errors(str(log_file), auto_delete_log=False)
+
+    assert messages and "rsync error" in messages[0]