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Sync by content with hard link support, fast fdupes, and much more.

Project description


lnsync provides sync-by-content of local file trees with support for hard links.

Local directories are one-way synced, as much as possible, by renaming, linking and delinking only.

In this way, file renaming, moving, and linking in the source directory are quickly and easily replicated on the target.

Files are compared via hashes, which are stored.

Additional features, such as duplicates detection, are provided based on those file hashes.

lnsync may be used as a preprocessing step for other sync tools such as rsync.

Hard Link Support

On some file systems (e.g. ext3/4, NTFS, btrfs), the same file may be reached by 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 hard link to a file means deleting that file.

Arbitrary linking/delinking (but not deleting) in the source can be easily replicated in the target, so long as the target also supports linking.

Syncing and the Hash Database

lnsync makes a best-effort to sync the target to the source without copying or deleting 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 values database is a single file at the base directory, with basename matching lnsync-[0-9]+.db. Only one such file should exist there.

Hash databases are ignored by all lnsync operations.

Care should be taken not to overwrite them when syncing with other tools.

File modification times are used to detect stale hash values. Modification times are not synced to the target.

File ownership and permissions are ignored. Files which cannot be read are skipped.

Symbolic links are skipped.

Offline File Trees

lnsync can save the file tree structure (with no file content) of a local directory to a single-file database.

These offline trees can be used in most lnsync commands in place of an local directory, e.g. as the source in a sync command.

File Searching

Using file 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 file content changes/bitrot.

It is also possible to search files by pathname.


Install the latest version from the PyPI repository with pip install -U lnsync or clone the repo with git clone and run python install.

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.

Example Usage

If your photos are at /home/you/Photos and its backup is at /mnt/disk/Photos, then lnsync sync /home/you/Photos /mnt/disk/Photos will sync the target. For a dry run, use the -n switch.

After syncing, two database files are created, one at the source /home/you/Photos and another at the target /mnt/disk/Photos.

File hashes are computed, as needed, and stored in those files.

The database filenames include a random suffix, to help avoid accidental overwriting when syncing with a tool other than lsync.

To obtain an rsync command that will complete syncing, skipping lnsync database files, run lnsync rsync /home/you/Photos /mnt/disk/Photos.

If the rsync options provided are suitable, run the command again with the -x switch to execute.

Alternatively, run lnsync syncr /home/you/Photos /mnt/disk/Photos to complete those two steps in one go.

Finallyt, to compare source and target, run lnsync cmp /home/you/Photos /mnt/disk/Photos.

To find duplicate files, run lnsync fdupes /home/you/Photos.

Use -z to compare by size only.

Use -H to treat hard links to the same file as distinct. 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 find all files with jpg extension, lnsync search /home/you/Photos "*.jpg".

To have any operation on a subdir of /home/you/Photos use the hash database at /home/you/Photos, include the option root=/home/you/Photos under section /home/you/Photos/** of your config file. (See Configuration Files below.)

For example, to sync the subdir /home/you/Photos/Best to another target, using the hash database 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 hard links, 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 hard links. 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> ... <glob_pattern> Exclude source files and directories by glob patterns. There is a corresponding --include and these are interpreted as in rsync --exclude <pattern> source/ target (beware, compatability has not been fully tested).

  • A file or directory is excluded if it matches an exclude pattern before matching any include pattern.

  • Each --exclude and --include option applies to all file trees in the command.

  • 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.

  • An initial slash anchors the pattern to the corresponding file tree root.

  • A trailing slash means the pattern applies only to directories.

  • 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.

  • lnsync syncr This convenience command is like sync, but follows it by executing the command created by rsync just above.

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 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. Use -f to force overwriting the output file.
  • 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

Files vs Paths

These commands operate on files, as opposed to paths.

Two paths to the same file do not consitute by themselves a duplication and, if there are two identical file, then fdupes will output a single, arbitrarily picked path for each file when outputing the result.

To instead operate on paths, use the --hardlinks switch on these commands.

To operate on files, but print all hardlinks, instead a of picking one, use --alllinks.

  • lnsync cmp <tree1> <tree2> Recursively compares two file trees. Accepts --exclude=<pattern> .
  • 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.
  • lnsync search <tree> [<globpat>]+ Find files one of whose relative paths matches one of the given glob patterns (which are as in --exclude).

Other Commands

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

Configuration Files

Optional command-line arguments are read from any INI-style configuration files at ./lnsync.cfg, ~/lnsync.cfg, and ~/.lnsync.cfg. In its DEFAULT section, in entries of the form key = value, the key can match the short or long option name (with intermediate minus replaced by underscores, e.g. n or dry_run).

For options taking no values, the value can be either empty or be a count of how many times to apply that option. For options taking more than one value (e.g. exclude), multiple values may be separated by line breaks. values may be set there for command-line options sort, hardlinks, alllinks, sameline, dbprefix, bysize, maxsize, skipempty, dry_run.

The DEFAULT section may contain exclude and include options, applying to all trees. Sections whose name glob-matches a directory (or offline tree file) may contain exclude and include options that applied only to that location, before all other exclude/include patterns.

To specify another configuration file altogether, --config FILENAME. To avoid all config files, --no-config.

Origin, Status, and Future Development

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 with 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.

Caveats and Limitations

  • Supports Linux only.
  • Supports only for locally mounted directories.
  • 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.

Release Notes

  • v0.6.1: Thread improvements and bug fixes.
  • v0.6.0: Threaded hashing and tree scanning for much better performance. Internal refactoring.
  • v0.5.3: New syncr. Changed mkoffline syntax. More output options (--alllinks). Hard link-aware check and cmp, improved search.
  • v0.5.2: Search files by file path glob pattern. Multiple patterns on --exclude. More powerful configuration files. --root now allowed in mkoffline and rehash. Major rewrite of the command line and config file parsers. Optimize onfirstonly and sync to do less hashing. Fix bugs in --root, cmp, check, and more. Wildcards in config section names.
  • v0.4.0: Drop Python 2 compatibility. Add config files. Bug fixes.
  • v0.3.8: Less hashing on onfirstonly. Sort file search output by size. Adjust user output levels.
  • v0.3.7: Bug fix on reading offline trees. Change output levels and some messages.
  • v0.3.6: New: --include and --include-once options. Bug fix: wrong exit code.
  • v0.3.5: Bug fix: not excluding dirs in offline mode.
  • v0.3.3: Python 3 support.
  • v0.3.2: New --root option to allow reading and updating a root tree database when querying subtrees.
  • v0.3.0: Exclude files by glob pattern in sync and other commands. Better terminal output. Major code overhaul.
  • v0.1.9: Improved sync algorithm. Remove directories left empty after sync.
  • v0.1.0: Initial version.

Possible Improvements

  • argparsecomplete support
  • Make --include and --exclude patterns more compatible with rsync.
  • 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.
  • 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.
  • More output sorting options, e.g. by name or mtime.

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