Welcome to Find Dupes Fast’s documentation!

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Find Dupes Fast (A.K.A. fastdupes.py) is a simple script which identifies duplicate files several orders of magnitude more quickly than fdupes by using smarter algorithms.

It was originally inspired by Dave Bolton’s dedupe.py and Reasonable Software’s NoClone and has no external dependencies beyond the Python 2.x standard library.

Full API documentation is available on ReadTheDocs and, pending proper end user documentation, the --help option is being constantly improved.


The default mode of operation is as follows:

  1. The given paths are recursively walked (subject to --exclude) to gather a list of files.
  2. Files are grouped by size (because stat() is fast compared to read()) and single-entry groups are pruned away.
  3. Groups are subdivided and pruned by hashing the first 16KiB of each file.
  4. Groups are subdivided and pruned again by hashing full contents.
  5. Any groups which remain are sets of duplicates.

Because this multi-pass approach eliminates files from consideration as early as possible, it reduces the amount of disk I/O that needs to be performed by at least an order of magnitude, greatly speeding up the process.

Here are the final status messages from a cold-cache run I did on my machine to root out cases where my manual approach to backing up things that don’t change left duplicates lying around:

$ python fastdupes.py /srv/Burned/Music /srv/Burned_todo/Music /srv/fservroot/music
Found 72052 files to be compared for duplication.
Found 7325 sets of files with identical sizes. (72042 files examined)
Found 1197 sets of files with identical header hashes. (38315 files examined)
Found 1197 sets of files with identical hashes. (2400 files examined)

Those ... files examined numbers should show its merits. The total wall clock runtime was 280.155 seconds.

Memory efficiency is also kept high by building full-content hashes incrementally in 64KiB chunks so that full files never need to be loaded into memory.

Exact Comparison Mode

If the -E switch is provided on the command line, the final full-content SHA1 hashing will be omitted. Instead, all of the files in each group will be read from the disk in parallel, comparing chunk-by-chunk and subdividing the group as differences appear.

This greatly increases the amount of disk seeking and offers no benefits in the vast majority of use cases. However, if you are storing many equally-sized files on an SSD and their headers are identical but they do vary, the incremental nature of this comparison may save you time by allowing the process to stop reading a given file as soon as it becomes obvious that it’s unique.

The other use for this (avoiding the risk of hash collisions in files that have identical sizes and do not differ in their first 16KiB of data but are different elsewhere) is such a tiny risk that very few people will need it.

(Yes, it’s a fact that, because files are longer than hashes, collisions are possible... but only an astronomically small number of the possible combinations of bytes are meaningful data that you’d find in a file on your hard drive.)

The --delete option

Like fdupes, fastdupes.py provides a --delete option which produces interactive prompts for removing duplicates.

However, unlike with fdupes, these prompts make it impossible to accidentally delete every copy of a file. (Bugs excepted, of course. A full unit test suite to ensure this behaviour is still on the TODO list.)

  • The --delete UI asks you which files you’d like to keep and won’t accept an empty response.
  • Specifying a directory more than once on the command line will not result in a file being listed as a duplicate of itself. Nor will specifying a directory and its ancestor.
  • A --symlinks option will not be added until safety can be guaranteed.

The --prefer and --noninteractive options

Often, when deduplicating with --delete, you already know that files in one directory tree should be preferred over files in another.

For example, if you have a folder named To Burn and another named Burned, then you shouldn’t have to tell your deduplicator that files in the former should be deleted.

By specifying --prefer=*/Burned on the command-line, you can skip the prompts in such a situation while still receiving prompts for other files.

Furthermore, if you’d like a fully unattended deduplication run, include the --noninteractive option and fastdupes will assume that you want to keep all copies (do nothing) when it would otherwise prompt.

Finally, a --dry-run option is provided in case you need to test the effects of a --delete setup without risk to your files.