The Filters tab of the sync settings allows to add filters to exclude and include files.

Exclude hidden files

Files or directories which start with a dot are hidden in the Finder. Some of these files are automatically created by macOS, such as .DS_Store and .localized. If the confirmation dialog lists hidden system directories like .DocumentRevisions-V100, .Spotlight-V100, .TemporaryItems or .Trashes, it usually means that you have selected an external device’s root folder as the source or destination; instead, it is recommended that you create a dedicated directory to store your backup so that these hidden directories managed by macOS don’t get in the way.

Excluding hidden files can be convenient if you know well all the important data that you want to back up and don’t care about files you can’t see in the Finder. If you are unsure, try to run a sync without any exclude filter and in the confirmation dialog check if there are any files you don’t care about. If you find that after all you need some of the hidden files, you can simply exclude the unwanted files with one of the other filters (see below). For example, if you don’t care about the .DS_Store files but want to copy other hidden files, just add .DS_Store as a filename filter (see below).

Excluded hidden files are never copied and, if the option “Remove files not on source” is enabled, not checked against the source to determine if they should be removed. They may still be removed if a parent directory is removed.

Filter files by expression

After enabling the check box “Filter files matching any of the following expressions”, you can add a list of expressions which are checked against each file in order to determine its behaviour. When clicking on the + button, you first select one of the following expression scopes:

  • Filename
  • Tags
  • Date Modified: doesn’t match directories. A directory’s modification date doesn’t reflect if any of the sub-contents have changed (for example, if a file in a subdirectory is modified, only the parent directory’s modification date is updated, but not the grandparent’s).
  • Age (since date modified): doesn’t match directories.
  • Size: doesn’t match directories. A directory’s size can be ambiguous, particularly if some of the contained subfiles are excluded.

Once added, you can edit an expression’s value by double-clicking it. You can change some further parameters by clicking on the small arrow on the right or right-clicking the expression:

  • Filenames and Tags: the textual expression can be turned into a regular expression.
  • Date modified, Age and Size: the expression can match attributes that are larger or smaller.

Moreover, each expression has one of two modes:

  • Exclude: the default. Matching excluded files are never copied and, if the option “Remove files not on source” is enabled, not checked against the source to determine if they should be removed. They may still be removed if a parent directory is removed.
  • Include: since expressions at the start of the text field have higher priority, by placing an inclusive expression before an exclusive one, you can include a subset of the excluded files again. If you only want to include files matched by the inclusive expressions, you may do so by explicitly excluding the source directory itself (see below).

Filter explicitly selected files

Add an explicitly filtered file or directory by clicking on the + button under the “Filter following files” check box. The first added file, as well as following files added to the top level, are excluded and marked with a red background. You can then select files or directories contained in an excluded directory which will be included again and marked with a green background, and vice versa. You can alternate as many excluded and included directories as you want. Excluded and included files apply to both source and destination bases. Files contained in included directories can still be excluded and files contained in excluded directories can still be included by the above filters.

Explicitly excluded files are never copied nor removed. Files contained in excluded directories may still be removed if they are included again, either as an explicitly included file or if matched by an inclusive expression (see above).

Note: as a means of grouping together many sync items with a common parent directory and the same settings, you can exclude the source directory itself and include the desired subdirectories. For example, if you only want to sync your Documents, Music and Pictures directories but ignore all other directories contained in your home folder, select your home directory as the source, then add it as an explicitly excluded file and add Documents, Music and Pictures as explicitly included subfiles.

Hint: if you want to select a file that is hidden in the Finder, you can navigate to it in the open panel by pressing the keyboard shortcut Command-Shift-G, which will open a dialog that allows you to manually enter a path to navigate to. Alternatively, you can toggle the display of hidden files with the keyboard shortcut Command-Shift-.