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Sync-by-rename local directories with hard link support, and more.

Project description



lnsync provides unidirectional sync-by-rename of locally mounted directories, with support for hard links, without copying or deleting file data. To compare files, files hashes are computed and stored in a local database. Using these file hashes, lnsync also provides other functions, such as finding duplicate files.

This tool allows arbitrary file renaming and moving in the source file tree to be quickly replicated in the target without copying any file data. It may used as a preprocessing step for other sync tools, such as rsync.

Hard Link Support

On many file systems (e.g. ext3/4, NTFS, btrfs), a file may be reached via multiple file paths, which function as aliases. A new path created for an existing file is a hard link, but all such aliases are on an equal footing, so each may be called a hard link. Removing the last hardlink to a file means deleting that file.

If both hard links are created or removed in the source (without deleting files), and the target supports hard linking, then lnsync can create and removing hardlinks as required on the target when syncing. As before, this is done without copying or deleting file data.



When you rename/move in the source file tree, lnsync makes a best-effort to sync the target by only renaming files and generally creating/removing hard links on the target, without deleting or copying any file data. Empty directories on the target which do not exist on the source are also removed.

File content is compared using xxHash (a fast, non-cryptographic hash function). Hash values are stored in a single-file database at the top-level directory of each file tree. The hash value database is a single file at the top directory of processed trees, with basename matching lnsync-[0-9]+.db. (Only one such file should exist there.) These files are ignored by all lnsync operations, and care should be taken not to sync them with other tools.

File modification times are used to detect stale hash values and are not synced. File ownership and permissions are ignored: files which cannot be read are skipped.

File Searching

Using file content hashes, lnsync can also find duplicate files (like fdupes), and more generally find files according to the which file trees they appear on, compare file trees, and check for changes/bitrot.

Offline File Trees

lnsync can save the structure of a file tree under a local directory and save it in a single-file database. These offline file trees can be used in most lnsync commands in place of an local directory, even as the source in a sync command.

Alternative Sync Solutions for Linux

Some of the many tools for syncing with rename detection:

  • There are patches for rsync (see --detect-renamed) that provide renaming on the target, relying on file size and modification time, for matching files in nearby directories. rsync can preserve hard links and sync with remote rsync instances.

  • rclone provides sync-by-rename for local as well as an amazing array of remote clients. It allows caching file hashes, after some configuration. However, it does not preserve hard links.

  • unison and bsync provide network syncing with rename detection, but do not preserve hard links.

  • git itself identifies and stores files by content, and has been adapted for syncing.

  • Support in modern file systems (e.g. btrfs) for snapshots may be adapted for syncing.

In addition to syncing, lnsync allows using the file hash database to search for files according to a variety of criteria.

Installation and Quickstart


Install the latest version from the PyPI repository with pip install -U lnsync.

Or clone the repo with git clone and run python install.

Example Usage

If you have your photo archive at /home/you/Photos and your backup is at /mnt/disk/Photos, run lnsync sync -n /home/you/Photos /mnt/disk/Photos for a dry-run, to see which sync operations would be performed. To sync, ommit the -n switch.

You will notice that two database files are created, one at the source and another at the backup directory. File hashes are cmoputed as needed and then stored in these files. The database filenames includes a random suffix to avoid accidental overwriting when syncing with a tool other than lsync.

To quickly obtain an rsync command that will complete syncing, skipping lnsync database files, run lnsync rsync /home/you/Photos /mnt/disk/Photos. To also run this command, use the -x switch. Make sure the rsync options provided by this command are suitable for you.

Finally, to check the target is in-sync by recursively comparing it to source, run lnsync cmp /home/you/Photos /mnt/disk/Photos.

To find duplicate files on the Photos directory, run lnsync fdupes /home/you/Photos. Use -z to compare by size only. Use -H to count different hardlinks to the same file as distinct files. If this option is not given, for each multiple-linked with other duplicates, a path is arbitrarily picked and printed.

To find all files in Photos which are not in the backup (under any name): lnsync onfirstonly /home/you/Photos /mnt/disk/Photos.

To sync the subdir /home/you/Photos/Best to your digital picture frame, using the database hashes at /home/you/Photos: lnsync sync /home/you/Photos/Best --root=/home/you/Photos /mnt/eframe/.

Command Reference

All lnsync commands are lnsync [<global-options>] <command> [<cmd-options>] [<cmd-parameters>].


  • lnsync sync [options] <source> <target> syncs a target dir from a source dir (or offline tree).

  • First, target files are matched to source files. Each matched target file is associated to a single source file. If either file system supports hardlinks, a file may have multiple pathnames. lnsync will not complain if the match is not unique or some files are not matched on either source and/or target.

  • For each matched target file, its pathnames are made to match those of the corresponding source file, by renaming, deleting, or creating hardlinks. New intermediate subdirectories are created as needed on the target and directories which become empty on the target are removed.

  • -z Match files by size only. In this case, hash databases are not created or updated.

  • -M=<size> Excludes all files larger than <size>, which may be given in human form, e.g. 10k, 2.1M, 3G.

  • -n Dry-run, just show which operations would be performed.

  • --exclude=<glob_pattern> Exclude source files and directories by glob pattern. Patterns are interpreted similarly to rsync --exclude=<glob_pattern> source/ target, but with * matching across slashes and no ** pattern. This option may be repeated, with each --exclude option affecting all file trees. An initial slash anchors the pattern to the file tree root. A trailing slash means the pattern applies only to directories. There is a corresponding --include. Some commands accept --exclude-once=<pattern> and --include-once=<pattern>, which apply only to the next file tree following the switch and gain precedence over global patterns.

To sum up, a file or directory is excluded if it matches an exclude pattern before matching any include pattern.

  • lnsync rsync [options] <tree> <dir> [rsync-options] Prints an rsync command that would sync target dir from source, skipping lnsync database files. Source may be a dir or an offline tree. Check the default rsync options provided are what you want. To also run the rsync command, use the -x switch.

Creating, Updating, and Accessing the Hash Database

  • lnsync update <dir> Update the hash database, creating a new database if none exists, and rehashing all new files and those with a changed modification time (mtime). Accepts --exclude=<pattern> options.
  • lnsync update <dir> Update the hash database, creating a new database if none exists, and rehashing all new files and those with a changed modification time (mtime). Accepts --exclude=<pattern> options.
  • lnsync rehash <dir> [<relpath>]+ Force rehashing files specified by paths relative to the root dir.
  • lnsync subdir <dir> <relsubdir> Update the database at relsubdir using any hash value already present in the hash database for dir.
  • lnsync mkoffline <dir> <outputfile> Update database at dir and create corresponding offline database at outputfile.
  • lnsync cleandb <dir> Remove outdated entries and re-compact the database.
  • lnsync lookup <tree> [<relpath>+] Returns (either from db or by recomputing) the hash value for the files, where tree may be a a directory or an offline tree.

Finding Files

  • lnsync cmp <tree1> <tree2> Recursively compares two file trees. Compares files at each path, does not compare the hard link structure. Accepts --exclude=<pattern> options.

  • lnsync fdupes [-h] [<tree>]+ Find files duplicated anywhere on the given trees.

  • lnsync onall [<tree>]+, lnsync onfirstonly [<tree>]+, lnsync onlastonly [<tree>]+ Find files as advertised. Some options: -M prunes by maximum size; -0 prunes empty files; -1 prints each group of files in a single line, separated by spaces and with escaped backslashes and spaces, like fdupes; -s sorts output by size. Use -H to consider multiple links to the same file as distinct files; if this option is not used, print a single, arbitrarily picked path for each multiple-linked file found to satisfy the condition of the command.

  • lnsync check [<tree>] [<path>]* Recompute hashes for given files and compare to the hash stored in the database, to check for changes/bitrot.

More Information

This package started as a Python learning project. I've found it useful enough to polish for publication, but as with any work in progress, it should be used with adequate caution.

Feedback, suggestions, comments, and corrections are very welcome.

You can support this project in bitcoin at 17HS828pkQMiXZGy7UNbA49TYCz7LAQ2ze.

This program comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it under certain conditions. See the GNU General Public Licence v3 for details.

Limitations and Future Developments

Caveats and Limitations

  • Works only on locally mounted directories, no support whatsoever for remote servers.
  • Depends on mtime to detect file content changes.
  • If source files A, B, C (with pairwise distinct contents) are renamed on target in a cycle to C, A, B, sync is currently not supported.
  • Only readable files and readable+accessible directories are read. Other files and dirs, as well as symlinks, pipes, special devices are ignored.
  • Minimal support for case-insensitive but case-preserving file systems like vfat: if a target file name differs from source match in case only, target is not updated.
  • Supports Linux only.

Possible Improvements

  • Make --include and --exclude patterns more compatible with rsync.
  • Extend cmp to take hard links into account.
  • Filenames are NOT converted to Unicode. To allow using offline database across systems, conversion is required.
  • Detect renamed directories to obtain a more compact sync schedule.
  • Use multiple CPUs to hash files in parallel.
  • Support partial hashes for quicker comparison of same-size files.
  • Further optimize the sync algorithm, though it has been working well in practice.
  • Support for checking for duplicates by actual content, not just hash.
  • Update target mtimes from source.
  • Allow more output sorting options, e.g. by name or mtime.
  • Allow config files and maybe store database along with config files in some .lnsync-DDDD directory at the root.

Release Notes

  • Version 0.3.8 -- Less hashing on onfirstonly. -- Sort file search output by size. -- Adjusted user output levels.
  • Version 0.3.7 -- Bug fix on reading offline trees. -- Change output levels and some messages.
  • Version 0.3.6 -- New: --include and --include-once options.
  • Bug fix: wrong exit code.
  • Version 0.3.5
  • Bug fix: not excluding dirs in offline mode.
  • Version 0.3.3
  • Python 3 support.
  • Version 0.3.2
  • New --root option to allow reading and updating a root tree database when querying subtrees.
  • Version 0.3.0
  • Exclude files by glob pattern in sync and other commands.
  • Better terminal output.
  • Major code overhaul.
  • Version 0.1.9
  • Improved sync algorithm.
  • Remove directories left empty after sync.
  • Version 0.1
  • Initial version.

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