Please read this entire README and https://github.com/bcpierce00/unison/wiki/Reporting-Bugs-and-Feature-Requests before creating or commenting on a github issue.
TL;DR: Do not ask questions or ask for help in issues. Upgrade to the latest release.
For compatibility information with version 2.52, see https://github.com/bcpierce00/unison/wiki/2.52-Migration-Guide
Unison is a file-synchronization tool for POSIX-compliant systems (e.g. *BSD and GNU/Linux), macOS and Windows, with the caveat that the platform must be supported by OCaml. It allows two replicas of a collection of files and directories to be stored on different hosts (or different disks on the same host), modified separately, and then brought up to date by propagating the changes in each replica to the other.
Unison has been in use for over 20 years and many people use it to synchronize data they care about.
Unison shares a number of features with tools such as configuration management packages (CVS, Subversion, git, Mercurial, etc.), distributed filesystems (Coda, etc.), uni-directional mirroring utilities (rsync, etc.), and other synchronizers. However, there are several points where it differs:
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Unison runs on almost any system with an OCaml compiler. Moreover, Unison works across platforms, allowing you to synchronize a Windows laptop with a Unix server, for example.
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Unlike simple mirroring or backup utilities, Unison can deal with updates to both replicas of a distributed directory structure. Updates that do not conflict are propagated automatically. Conflicting updates are detected and displayed.
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Unlike many network filesystems, Unison copies data so that already-synchronized data can be read and written while offline.
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Unlike most distributed filesystems, Unison is a user-level program that simply uses normal systems calls: there is no need to modify the kernel, to have superuser privileges on either host, or to have a FUSE implementation.
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Unison works between any pair of machines connected to the internet, typically communicating over ssh, but also directly over TCP. It is careful with network bandwidth, and runs well over slow links such as PPP connections. Transfers of small updates to large files are optimized using a compression protocol similar to rsync.
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Unison is resilient to failure. It is careful to leave the replicas and its own private structures in a sensible state at all times, even in case of abnormal termination or communication failures.
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Unison has a clear and precise specification.
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Unison is Free; full source code is available under the GNU Public License, Version 3.
Note that only a very small number of people are actively working on maintaining unison. An estimate is 2.5 people and 0.1 Full-Time Equivalents. This has a substantial impact on the handling of bug reports and enhancement reports; see the wiki page linked at the top. Help in terms of high-quality bug reports, fixes, and proposed changes is very welcome. Help in answering mailinglist questions is also welcome. Please do not answer questions asked in the bug tracker, which is contrary to bug tracker usage guidance.
While much of Unison activity is now at https://github.com/bcpierce00/unison/ additional information can be found at: http://www.cis.upenn.edu/~bcpierce/unison
The Unison project provides Unison as source code. Many packaging systems (including GNU/Linux distributions) provide binary packages of Unison. Results from Continuous Integration builds, while performed for the purposes of testing, are available for use on a limited set of platforms.
See the manual in doc/ for building instructions, or read the CI recipes. (Currently, this is probably less well explained than it should be.)
You may be able to find a pre-built binary for your operating system, version, and CPU type. For a list of sources, See https://github.com/bcpierce00/unison/wiki/Downloading-Unison
Generally, you should use the most recent formal release, currently 2.52. Earlier branches (e.g. 2.51 and 2.48) are no longer maintained, and bug reports are not accepted about these versions. This is true even though many packaging systems (including GNU/Linux distributions) continue to have 2.48. There are sometimes release candidates. There is always the master branch in git, which historically has been quite stable.
For Unison versions 2.52 and newer, see https://github.com/bcpierce00/unison/wiki/2.52-Migration-Guide
The information below is true for Unison versions older than 2.52.
Beware that Unison uses OCaml's built-in data marshalling, and that this facility is unstable across versions of "ocaml" (the standard implementation of the OCaml language). Additionally, Unison has incompatible changes across minor releases (e.g. 2.48 vs 2.51, but 2.51.2 and 2.51.3 are compatible). Therefore, you must use the same Unison minor version built with the same ocaml version on all systems.
There are two mailinglists: unison-users and unison-hackers. Descriptions and instructions are at https://github.com/bcpierce00/unison/wiki/Mailing-Lists
If you want to play with the internals, have a look at the file src/ROADMAP.txt for some basic orientation. Discussion of the source code, proposed changes, etc. is most appropriate on the unison-hackers mailinglist.
Proposed code changes are also welcome (as pull requests). For significant changes, an enhancement request or bug report is likely in order to provide the proposed semantics ahead of time. For changes that are likely to be widely viewed as clearly desired, that might be enough. Others should be discussed on unison-hackers.
Proposed changes should change documentation in concert with code, and should pass CI.
Unison operates under the widely-used "inbound=outbound" contribution license process. Therefore, all contributions to Unison must be licensed under the project's license, currently GPLv3 (unless a file under a different license is being modified). New files of significance must have a copyright statement and grant permission to copy under the project's license. Significant changes should include copyright statements and/or add authors. Submitting a pull request or posting a contribution on a mailinglist is an assertion that the submitter has the authority to license their changes under the project's license. (This paragraph is intended to summarize the normal conventions, and is not intended to create any new norms. See https://sfconservancy.org/blog/2014/jun/09/do-not-need-cla/ for a longer discussion.)