Detect whether --dry-run was used and add it as a command line flag (#113)

Author: basnijholt

README.md

@@ -71,7 +71,7 @@ usage: rsync-time-machine [-h] [-p PORT] [-i ID_RSA] [--rsync-get-flags]
                           [--rsync-set-flags RSYNC_SET_FLAGS]
                           [--rsync-append-flags RSYNC_APPEND_FLAGS]
                           [--log-dir LOG_DIR] [--strategy STRATEGY]
-                          [--no-auto-expire] [--allow-host-only]
+                          [--no-auto-expire] [--allow-host-only] [--dry-run]
                           [--exclude-from EXCLUDE_FROM] [-v]
                           src_folder dest_folder [exclusion_file]
 
@@ -86,9 +86,8 @@ positional arguments:
 
 options:
   -h, --help            show this help message and exit
-  -p PORT, --port PORT  SSH port.
-  -i ID_RSA, --id_rsa ID_RSA
-                        Specify the private ssh key to use.
+  -p, --port PORT       SSH port.
+  -i, --id_rsa ID_RSA   Specify the private ssh key to use.
   --rsync-get-flags     Display the default rsync flags that are used for
                         backup. If using remote drive over SSH, --compress
                         will be added.
@@ -117,6 +116,8 @@ options:
                         the current username. Note: this option will not
                         enforce SSH usage, it only broadens the accepted input
                         formats.
+  --dry-run             Simulate the backup process without making any
+                        persistent changes.
   --exclude-from EXCLUDE_FROM
                         Path to the file containing exclude patterns.
                         Alternative to the positional `exclusion_file`. Not to

rsync_time_machine.py

@@ -145,6 +145,11 @@ def parse_arguments() -> argparse.Namespace:  # pragma: no cover
         " This is useful if you want to use configurations from the `.ssh/config` file or rely on the current username."
         " Note: this option will not enforce SSH usage, it only broadens the accepted input formats.",
     )
+    parser.add_argument(
+        "--dry-run",
+        action="store_true",
+        help="Simulate the backup process without making any persistent changes.",
+    )
 
     parser.add_argument(
         "src_folder",
@@ -803,6 +808,7 @@ def backup(
     rsync_append_flags: str,
     rsync_get_flags: bool,
     allow_host_only: bool,
+    dry_run: bool,
 ) -> None:
     """Perform backup of src_folder to dest_folder."""
     (
@@ -852,16 +858,25 @@ def backup(
         ssh,
     )
 
+    if "-n" in rsync_flags or "--dry-run" in rsync_flags:
+        dry_run = True
+        log_info(
+            f"Dry-run detected in rsync flags - setting {style('--dry-run', 'green')}.",
+        )
+    elif dry_run:
+        rsync_flags.append("--dry-run")
+    if dry_run:
+        log_info(
+            f"Dry-run mode enabled: {style('no changes will be persisted', 'orange')}.",
+        )
+
     if rsync_get_flags:
         flags = " ".join(rsync_flags)
         log_info(f"Rsync flags:\n{style(flags, 'yellow', bold=True)}")
         sys.exit(0)
 
     for _ in range(100):  # max 100 retries when no space left
-        link_dest_option = get_link_dest_option(
-            previous_dest,
-            ssh,
-        )
+        link_dest_option = get_link_dest_option(previous_dest, ssh)
 
         if not find(dest, ssh, maxdepth=0):
             _full_dest = style(f"{ssh.cmd if ssh else ''}{dest}", bold=True)
@@ -898,6 +913,14 @@ def backup(
 
     check_rsync_errors(log_file, auto_delete_log)
 
+    if dry_run:
+        # In dry-run mode, clean up any temporary artifacts
+        # and exit without updating the "latest" symlink.
+        rm_dir(dest, ssh)
+        rm_file(inprogress_file, ssh)
+        log_info("Dry run complete - no backup was saved.")
+        return
+
     rm_file(os.path.join(dest_folder, "latest"), dest_is_ssh(ssh))
     ln(
         os.path.basename(dest),
@@ -928,6 +951,7 @@ def main() -> None:
         rsync_append_flags=args.rsync_append_flags,
         rsync_get_flags=args.rsync_get_flags,
         allow_host_only=args.allow_host_only,
+        dry_run=args.dry_run,
     )
 
 

tests/test_app.py

@@ -345,7 +345,7 @@ def assert_n_backups(dest_folder: str | Path, n_expected: int) -> None:
     assert len(find_backups(str(dest_folder), None)) == n_expected
 
 
-def test_backup(tmp_path: Path, capsys: pytest.CaptureFixture) -> None:
+def test_backup(tmp_path: Path, capsys: pytest.CaptureFixture) -> None:  # noqa: PLR0915
     """Test the backup function."""
     src_folder = tmp_path / "src"
     dest_folder = tmp_path / "dest"
@@ -367,6 +367,7 @@ def test_backup(tmp_path: Path, capsys: pytest.CaptureFixture) -> None:
         "rsync_append_flags": "",
         "rsync_get_flags": False,
         "allow_host_only": False,
+        "dry_run": False,
     }
     # Tests backup with no backup.marker file
     with pytest.raises(SystemExit):
@@ -442,6 +443,25 @@ def test_backup(tmp_path: Path, capsys: pytest.CaptureFixture) -> None:
     assert (dest_folder / "latest" / "file3.txt").exists()
     assert_n_backups(dest_folder, 2)
 
+    # Now test dry-run mode:
+    # When dry_run is True, no new backup should be persisted.
+    with patch_now_str(seconds=1):
+        backup(**dict(kw, dry_run=True))  # type: ignore[arg-type]
+    # The number of backups should remain unchanged.
+    assert_n_backups(dest_folder, 2)
+    captured = capsys.readouterr()
+    assert "Dry-run detected" not in captured.out
+    assert "Dry-run mode enabled" in captured.out
+
+    # Dry-run inside `rsync_flags`
+    with patch_now_str(seconds=2):
+        backup(**dict(kw, rsync_append_flags="-n"))  # type: ignore[arg-type]
+    # The number of backups should remain unchanged.
+    assert_n_backups(dest_folder, 2)
+    captured = capsys.readouterr()
+    assert "Dry-run detected" in captured.out
+    assert "Dry-run mode enabled" in captured.out
+
 
 def test_backup_with_non_utf8_filename(tmp_path: Path) -> None:
     """Test the backup function with a non-UTF8 filename.
@@ -478,6 +498,7 @@ def test_backup_with_non_utf8_filename(tmp_path: Path) -> None:
         "rsync_append_flags": "",
         "rsync_get_flags": False,
         "allow_host_only": False,
+        "dry_run": False,
     }
 
     # Create a backup.marker file