Unison File Synchronizer
User Manual and Reference Guide
Version 2.48.1
Copyright 1998-2012, Benjamin C. Pierce


Contents

Overview
Preface
          •People
          •Mailing Lists and Bug Reporting
          •Development Status
          •Copying
          •Acknowledgements
Installation
          •Downloading Unison
          •Running Unison
          •Upgrading
          •Building Unison from Scratch
                  Unix
                  Mac OS X
                  Windows
                  Installation Options
Tutorial
          •Preliminaries
          •Local Usage
          •Remote Usage
          •Remote Shell Method
          •Socket Method
          •Using Unison for All Your Files
          •Using Unison to Synchronize More Than Two Machines
          •Going Further
Basic Concepts
          •Roots
          •Paths
          •What is an Update?
          •What is a Conflict?
          •Reconciliation
          •Invariants
          •Caveats and Shortcomings
Reference Guide
          •Running Unison
          •The .unison Directory
          •Archive Files
          •Preferences
          •Profiles
          •Sample Profiles
                  A Minimal Profile
                  A Basic Profile
                  A Power-User Profile
          •Keeping Backups
          •Merging Conflicting Versions
          •The User Interface
          •Exit code
          •Path specification
          •Ignoring Paths
          •Symbolic Links
          •Permissions
          •Cross-Platform Synchronization
          •Slow Links
          •Making Unison Faster on Large Files
          •Fast Update Detection
          •Mount Points and Removable Media
          •Click-starting Unison
Installing Ssh
          •Unix
          •Windows
Changes in Version 2.48.1

Overview

Unison is a file-synchronization tool for Unix and Windows. 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 shares a number of features with tools such as configuration management packages (CVS, PRCS, etc.), distributed filesystems (Coda, etc.), uni-directional mirroring utilities (rsync, etc.), and other synchronizers (Intellisync, Reconcile, etc). However, there are several points where it differs:

Preface

People

Benjamin Pierce leads the Unison project. The current version of Unison was designed and implemented by Trevor Jim, Benjamin Pierce, and Jérôme Vouillon, with Alan Schmitt, Malo Denielou, Zhe Yang, Sylvain Gommier, and Matthieu Goulay. The Mac user interface was started by Trevor Jim and enormously improved by Ben Willmore. Our implementation of the rsync protocol was built by Norman Ramsey and Sylvain Gommier. It is based on Andrew Tridgell's thesis work and inspired by his rsync utility. The mirroring and merging functionality was implemented by Sylvain Roy, improved by Malo Denielou, and improved yet further by Stéphane Lescuyer. Jacques Garrigue contributed the original Gtk version of the user interface; the Gtk2 version was built by Stephen Tse. Sundar Balasubramaniam helped build a prototype implementation of an earlier synchronizer in Java. Insik Shin and Insup Lee contributed design ideas to this implementation. Cedric Fournet contributed to an even earlier prototype.

Mailing Lists and Bug Reporting

Mailing Lists:
Moderated mailing lists are available for bug reporting, announcements of new versions, discussions among users, and discussions among developers. See
http://www.cis.upenn.edu/~bcpierce/unison/lists.html
for more information.

Development Status

Unison is no longer under active development as a research project. (Our research efforts are now focused on a follow-on project called Harmony, described at http://www.cis.upenn.edu/~bcpierce/harmony.) At this point, there is no one whose job it is to maintain Unison, fix bugs, or answer questions.

However, the original developers are all still using Unison daily. It will continue to be maintained and supported for the foreseeable future, and we will occasionally release new versions with bug fixes, small improvements, and contributed patches.

Reports of bugs affecting correctness or safety are of interest to many people and will generally get high priority. Other bug reports will be looked at as time permits. Bugs should be reported to the users list at unison-users@yahoogroups.com.

Feature requests are welcome, but will probably just be added to the ever-growing todo list. They should also be sent to unison-users@yahoogroups.com.

Patches are even more welcome. They should be sent to unison-hackers@lists.seas.upenn.edu. (Since safety and robustness are Unison's most important properties, patches will be held to high standards of clear design and clean coding.) If you want to contribute to Unison, start by downloading the developer tarball from the download page. For some details on how the code is organized, etc., see the file CONTRIB.

Copying

This file is part of Unison.

Unison is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

Unison is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

The GNU Public License can be found at http://www.gnu.org/licenses. A copy is also included in the Unison source distribution in the file COPYING.

Acknowledgements

Work on Unison has been supported by the National Science Foundation under grants CCR-9701826 and ITR-0113226, Principles and Practice of Synchronization, and by University of Pennsylvania's Institute for Research in Cognitive Science (IRCS).


Installation

Unison is designed to be easy to install. The following sequence of steps should get you a fully working installation in a few minutes. If you run into trouble, you may find the suggestions on the Frequently Asked Questions page helpful. Pre-built binaries are available for a variety of platforms.

Unison can be used with either of two user interfaces:
  1. a simple textual interface, suitable for dumb terminals (and running from scripts), and
  2. a more sophisticated grapical interface, based on Gtk2 (on Linux/Windows) or the native UI framework (on OSX).
You will need to install a copy of Unison on every machine that you want to synchronize. However, you only need the version with a graphical user interface (if you want a GUI at all) on the machine where you're actually going to display the interface (the client machine). Other machines that you synchronize with can get along just fine with the textual version.

Downloading Unison

The Unison download site lives under http://www.cis.upenn.edu/~bcpierce/unison.

If a pre-built binary of Unison is available for the client machine's architecture, just download it and put it somewhere in your search path (if you're going to invoke it from the command line) or on your desktop (if you'll be click-starting it).

The executable file for the graphical version (with a name including gtkui) actually provides both interfaces: the graphical one appears by default, while the textual interface can be selected by including -ui text on the command line. The textui executable provides just the textual interface.

If you don't see a pre-built executable for your architecture, you'll need to build it yourself. See the Building Unison section. There are also a small number of contributed ports to other architectures that are not maintained by us. See the Contributed Ports page to check what's available.

Check to make sure that what you have downloaded is really executable. Either click-start it, or type unison -version at the command line.

Unison can be used in three different modes: with different directories on a single machine, with a remote machine over a direct socket connection, or with a remote machine using ssh for authentication and secure transfer. If you intend to use the last option, you may need to install ssh; see the Installing Ssh section.

Running Unison

Once you've got Unison installed on at least one system, read the Tutorial section of the user manual (or type unison -doc tutorial) for instructions on how to get started.

Upgrading

Upgrading to a new version of Unison is as simple as throwing away the old binary and installing the new one.

Before upgrading, it is a good idea to run the old version one last time, to make sure all your replicas are completely synchronized. A new version of Unison will sometimes introduce a different format for the archive files used to remember information about the previous state of the replicas. In this case, the old archive will be ignored (not deleted — if you roll back to the previous version of Unison, you will find the old archives intact), which means that any differences between the replicas will show up as conflicts that need to be resolved manually.

Building Unison from Scratch

If a pre-built image is not available, you will need to compile it from scratch; the sources are available from the same place as the binaries.

In principle, Unison should work on any platform to which OCaml has been ported and on which the Unix module is fully implemented. It has been tested on many flavors of Windows (98, NT, 2000, XP) and Unix (OS X, Solaris, Linux, FreeBSD), and on both 32- and 64-bit architectures.

Unix

You'll need the Objective Caml compiler (version 3.11.2 or later), which is available from http://caml.inria.fr. Building and installing OCaml on Unix systems is very straightforward; just follow the instructions in the distribution. You'll probably want to build the native-code compiler in addition to the bytecode compiler, as Unison runs much faster when compiled to native code, but this is not absolutely necessary. (Quick start: on many systems, the following sequence of commands will get you a working and installed compiler: first do make world opt, then su to root and do make install.)

You'll also need the GNU make utility, standard on many Unix systems. (Type make –version to check that you've got the GNU version.) Unison's build system is not parallelizable, so don't use make flags which cause it to start processes in parallel, e.g. -j for GNU make.

Once you've got OCaml installed, grab a copy of the Unison sources, unzip and untar them, change to the new unison directory, and type “make UISTYLE=text”. The result should be an executable file called unison. Type ./unison to make sure the program is executable. You should get back a usage message.

If you want to build the graphical user interface, you will need to install two additional things: Now build unison. If your search paths are set up correctly, simply typing make again should build a unison executable with a Gtk2 graphical interface. (In previous releases of Unison, it was necessary to add UISTYLE=gtk2 to the 'make' command above. This requirement has been removed: the makefile should detect automatically when lablgtk2 is present and set this flag automatically.)

Put the unison executable somewhere in your search path, either by adding the Unison directory to your PATH variable or by copying the executable to some standard directory where executables are stored.

Mac OS X

To build the text-only user interface, follow the instructions above for building on Unix systems. You should do this first, even if you are also planning on building the GUI, just to make sure it works.

To build the basic GUI version, you'll first need to download and install the XCode developer tools from Apple. Once this is done, just type make UISTYLE=macnew in the src directory, and if things go well you should get an application that you can move from uimacnew/build/Default/Unison.app to wherever you want it.

There is also an experimental GUI with a somewhat smoother look and feel. To compile this one (once you've got the basic one working), proceed as follows:
  1. Go to the uimacnew09 directory and double-click the file BWToolkit.ibplugin.
  2. Go back up to the src directory and type make UISTYLE=macnew09.
  3. You should get an application built for you at uimacnew09/build/Default/Unison.app.

Windows

Although the binary distribution should work on any version of Windows, some people may want to build Unison from scratch on those systems too.

Bytecode version:
The simpler but slower compilation option to build a Unison executable is to build a bytecode version. You need first install Windows version of the OCaml compiler (version 3.07 or later, available from http://caml.inria.fr). Then grab a copy of Unison sources and type
       make NATIVE=false
to compile the bytecode. The result should be an executable file called unison.exe.

Native version:
Building a more efficient, native version of Unison on Windows requires a little more work. See the file INSTALL.win32 in the source code distribution.

Installation Options

The Makefile in the distribution includes several switches that can be used to control how Unison is built. Here are the most useful ones:

Tutorial

Preliminaries

Unison can be used with either of two user interfaces:
  1. a straightforward textual interface and
  2. a more sophisticated graphical interface
The textual interface is more convenient for running from scripts and works on dumb terminals; the graphical interface is better for most interactive use. For this tutorial, you can use either. If you are running Unison from the command line, just typing unison will select either the text or the graphical interface, depending on which has been selected as default when the executable you are running was built. You can force the text interface even if graphical is the default by adding -ui text. The other command-line arguments to both versions are identical.

The graphical version can also be run directly by clicking on its icon, but this may require a little set-up (see the Click-starting Unison section). For this tutorial, we assume that you're starting it from the command line.

Unison can synchronize files and directories on a single machine, or between two machines on a network. (The same program runs on both machines; the only difference is which one is responsible for displaying the user interface.) If you're only interested in a single-machine setup, then let's call that machine the client. If you're synchronizing two machines, let's call them client and server.

Local Usage

Let's get the client machine set up first and see how to synchronize two directories on a single machine.

Follow the instructions in the Installation section to either download or build an executable version of Unison, and install it somewhere on your search path. (If you just want to use the textual user interface, download the appropriate textui binary. If you just want to the graphical interface—or if you will use both interfaces [the gtkui binary actually has both compiled in]—then download the gtkui binary.)

Create a small test directory a.tmp containing a couple of files and/or subdirectories, e.g.,
       mkdir a.tmp
       touch a.tmp/a a.tmp/b
       mkdir a.tmp/d
       touch a.tmp/d/f
Copy this directory to b.tmp:
       cp -r a.tmp b.tmp
Now try synchronizing a.tmp and b.tmp. (Since they are identical, synchronizing them won't propagate any changes, but Unison will remember the current state of both directories so that it will be able to tell next time what has changed.) Type:
       unison a.tmp b.tmp
(You may need to add -ui text, depending how your unison binary was built.)


Textual Interface:
Graphical Interface: Next, make some changes in a.tmp and/or b.tmp. For example:
        rm a.tmp/a
        echo "Hello" > a.tmp/b
        echo "Hello" > b.tmp/b
        date > b.tmp/c
        echo "Hi there" > a.tmp/d/h
        echo "Hello there" > b.tmp/d/h
Run Unison again:
       unison a.tmp b.tmp
This time, the user interface will display only the files that have changed. If a file has been modified in just one replica, then it will be displayed with an arrow indicating the direction that the change needs to be propagated. For example,
                 <---  new file   c  [f]
indicates that the file c has been modified only in the second replica, and that the default action is therefore to propagate the new version to the first replica. To follow Unison's recommendation, press the “f” at the prompt.

If both replicas are modified and their contents are different, then the changes are in conflict: <-?-> is displayed to indicate that Unison needs guidance on which replica should override the other.
     new file  <-?->  new file   d/h  []
By default, neither version will be propagated and both replicas will remain as they are.

If both replicas have been modified but their new contents are the same (as with the file b), then no propagation is necessary and nothing is shown. Unison simply notes that the file is up to date.

These display conventions are used by both versions of the user interface. The only difference lies in the way in which Unison's default actions are either accepted or overridden by the user.


Textual Interface:
Graphical Interface:

Remote Usage

Next, we'll get Unison set up to synchronize replicas on two different machines.

Follow the instructions in the Installation section to download or build an executable version of Unison on the server machine, and install it somewhere on your search path. (It doesn't matter whether you install the textual or graphical version, since the copy of Unison on the server doesn't need to display any user interface at all.)

It is important that the version of Unison installed on the server machine is the same as the version of Unison on the client machine. But some flexibility on the version of Unison at the client side can be achieved by using the -addversionno option; see the Preferences section.

Now there is a decision to be made. Unison provides two methods for communicating between the client and the server: Decide which of these you want to try, and continue with the Remote Shell Method section or the Socket Method section, as appropriate.

Remote Shell Method

The standard remote shell facility on Unix systems is ssh, which provides the same functionality as the older rsh but much better security. Ssh is available from http://www.openssh.org. See section A.2 for installation instructions for the Windows version.

Running ssh requires some coordination between the client and server machines to establish that the client is allowed to invoke commands on the server; please refer to the ssh documentation for information on how to set this up. The examples in this section use ssh, but you can substitute rsh for ssh if you wish.

First, test that we can invoke Unison on the server from the client. Typing
        ssh remotehostname unison -version
should print the same version information as running
        unison -version
locally on the client. If remote execution fails, then either something is wrong with your ssh setup (e.g., “permission denied”) or else the search path that's being used when executing commands on the server doesn't contain the unison executable (e.g., “command not found”).

Create a test directory a.tmp in your home directory on the client machine.

Test that the local unison client can start and connect to the remote server. Type
          unison -testServer a.tmp ssh://remotehostname/a.tmp
Now cd to your home directory and type:
          unison a.tmp ssh://remotehostname/a.tmp
The result should be that the entire directory a.tmp is propagated from the client to your home directory on the server.

After finishing the first synchronization, change a few files and try synchronizing again. You should see similar results as in the local case.

If your user name on the server is not the same as on the client, you need to specify it on the command line:
          unison a.tmp ssh://username@remotehostname/a.tmp
Notes:

Socket Method

Warning: The socket method is insecure: not only are the texts of your changes transmitted over the network in unprotected form, it is also possible for anyone in the world to connect to the server process and read out the contents of your filesystem! (Of course, to do this they must understand the protocol that Unison uses to communicate between client and server, but all they need for this is a copy of the Unison sources.) The socket method is provided only for expert users with specific needs; everyone else should use the ssh method.
To run Unison over a socket connection, you must start a Unison daemon process on the server. This process runs continuously, waiting for connections over a given socket from client machines running Unison and processing their requests in turn.

To start the daemon, type
       unison -socket NNNN
on the server machine, where NNNN is the socket number that the daemon should listen on for connections from clients. (NNNN can be any large number that is not being used by some other program; if NNNN is already in use, Unison will exit with an error message.) Note that paths specified by the client will be interpreted relative to the directory in which you start the server process; this behavior is different from the ssh case, where the path is relative to your home directory on the server.

Create a test directory a.tmp in your home directory on the client machine. Now type:
       unison a.tmp socket://remotehostname:NNNN/a.tmp
The result should be that the entire directory a.tmp is propagated from the client to the server (a.tmp will be created on the server in the directory that the server was started from). After finishing the first synchronization, change a few files and try synchronizing again. You should see similar results as in the local case.

Since the socket method is not used by many people, its functionality is rather limited. For example, the server can only deal with one client at a time.

Using Unison for All Your Files

Once you are comfortable with the basic operation of Unison, you may find yourself wanting to use it regularly to synchronize your commonly used files. There are several possible ways of going about this:
  1. Synchronize your whole home directory, using the Ignore facility (see the Ignore section) to avoid synchronizing temporary files and things that only belong on one host.
  2. Create a subdirectory called shared (or current, or whatever) in your home directory on each host, and put all the files you want to synchronize into this directory.
  3. Create a subdirectory called shared (or current, or whatever) in your home directory on each host, and put links to all the files you want to synchronize into this directory. Use the follow preference (see the Symbolic Links section) to make Unison treat these links as transparent.
  4. Make your home directory the root of the synchronization, but tell Unison to synchronize only some of the files and subdirectories within it on any given run. This can be accomplished by using the -path switch on the command line:
           unison /home/username ssh://remotehost//home/username -path shared
    
    The -path option can be used as many times as needed, to synchronize several files or subdirectories:
           unison /home/username ssh://remotehost//home/username \
              -path shared \
              -path pub \
              -path .netscape/bookmarks.html
    
    These -path arguments can also be put in your preference file. See the Preferences section for an example.
Most people find that they only need to maintain a profile (or profiles) on one of the hosts that they synchronize, since Unison is always initiated from this host. (For example, if you're synchronizing a laptop with a fileserver, you'll probably always run Unison on the laptop.) This is a bit different from the usual situation with asymmetric mirroring programs like rdist, where the mirroring operation typically needs to be initiated from the machine with the most recent changes. the Profile section covers the syntax of Unison profiles, together with some sample profiles.

Some tips on improving Unison's performance can be found on the Frequently Asked Questions page.

Using Unison to Synchronize More Than Two Machines

Unison is designed for synchronizing pairs of replicas. However, it is possible to use it to keep larger groups of machines in sync by performing multiple pairwise synchronizations.

If you need to do this, the most reliable way to set things up is to organize the machines into a “star topology,” with one machine designated as the “hub” and the rest as “spokes,” and with each spoke machine synchronizing only with the hub. The big advantage of the star topology is that it eliminates the possibility of confusing “spurious conflicts” arising from the fact that a separate archive is maintained by Unison for every pair of hosts that it synchronizes.

Going Further

On-line documentation for the various features of Unison can be obtained either by typing
        unison -doc topics
at the command line, or by selecting the Help menu in the graphical user interface. The on-line information and the printed manual are essentially identical.

If you use Unison regularly, you should subscribe to one of the mailing lists, to receive announcements of new versions. See the Mailing Lists section.


Basic Concepts

To understand how Unison works, it is necessary to discuss a few straightforward concepts. These concepts are developed more rigorously and at more length in a number of papers, available at http://www.cis.upenn.edu/~bcpierce/papers. But the informal presentation here should be enough for most users.

Roots

A replica's root tells Unison where to find a set of files to be synchronized, either on the local machine or on a remote host. For example,
      relative/path/of/root
specifies a local root relative to the directory where Unison is started, while
      /absolute/path/of/root
specifies a root relative to the top of the local filesystem, independent of where Unison is running. Remote roots can begin with ssh://, rsh:// to indicate that the remote server should be started with rsh or ssh:
      ssh://remotehost//absolute/path/of/root
      rsh://user@remotehost/relative/path/of/root
If the remote server is already running (in the socket mode), then the syntax
      socket://remotehost:portnum//absolute/path/of/root
      socket://remotehost:portnum/relative/path/of/root
is used to specify the hostname and the port that the client Unison should use to contact it.

The syntax for roots is based on that of URIs (described in RFC 2396). The full grammar is:
  replica ::= [protocol:]//[user@][host][:port][/path]
           |  path

  protocol ::= file
            |  socket
            |  ssh
            |  rsh

  user ::= [-_a-zA-Z0-9]+

  host ::= [-_a-zA-Z0-9.]+

  port ::= [0-9]+
When path is given without any protocol prefix, the protocol is assumed to be file:. Under Windows, it is possible to synchronize with a remote directory using the file: protocol over the Windows Network Neighborhood. For example,
       unison foo //host/drive/bar
synchronizes the local directory foo with the directory drive:\bar on the machine host, provided that host is accessible via Network Neighborhood. When the file: protocol is used in this way, there is no need for a Unison server to be running on the remote host. However, running Unison this way is only a good idea if the remote host is reached by a very fast network connection, since the full contents of every file in the remote replica will have to be transferred to the local machine to detect updates.

The names of roots are canonized by Unison before it uses them to compute the names of the corresponding archive files, so //saul//home/bcpierce/common and //saul.cis.upenn.edu/common will be recognized as the same replica under different names.

Paths

A path refers to a point within a set of files being synchronized; it is specified relative to the root of the replica.

Formally, a path is just a sequence of names, separated by /. Note that the path separator character is always a forward slash, no matter what operating system Unison is running on. Forward slashes are converted to backslashes as necessary when paths are converted to filenames in the local filesystem on a particular host. (For example, suppose that we run Unison on a Windows system, synchronizing the local root c:\pierce with the root ssh://saul.cis.upenn.edu/home/bcpierce on a Unix server. Then the path current/todo.txt refers to the file c:\pierce\current\todo.txt on the client and /home/bcpierce/current/todo.txt on the server.)

The empty path (i.e., the empty sequence of names) denotes the whole replica. Unison displays the empty path as “[root].”

If p is a path and q is a path beginning with p, then q is said to be a descendant of p. (Each path is also a descendant of itself.)

What is an Update?

The contents of a path p in a particular replica could be a file, a directory, a symbolic link, or absent (if p does not refer to anything at all in that replica). More specifically: Unison keeps a record of the contents of each path after each successful synchronization of that path (i.e., it remembers the contents at the last moment when they were the same in the two replicas).

We say that a path is updated (in some replica) if its current contents are different from its contents the last time it was successfully synchronized. Note that whether a path is updated has nothing to do with its last modification time—Unison considers only the contents when determining whether an update has occurred. This means that touching a file without changing its contents will not be recognized as an update. A file can even be changed several times and then changed back to its original contents; as long as Unison is only run at the end of this process, no update will be recognized.

What Unison actually calculates is a close approximation to this definition; see the Caveats and Shortcomings section.

What is a Conflict?

A path is said to be conflicting if the following conditions all hold:
  1. it has been updated in one replica,
  2. it or any of its descendants has been updated in the other replica, and
  3. its contents in the two replicas are not identical.

Reconciliation

Unison operates in several distinct stages:
  1. On each host, it compares its archive file (which records the state of each path in the replica when it was last synchronized) with the current contents of the replica, to determine which paths have been updated.
  2. It checks for “false conflicts” — paths that have been updated on both replicas, but whose current values are identical. These paths are silently marked as synchronized in the archive files in both replicas.
  3. It displays all the updated paths to the user. For updates that do not conflict, it suggests a default action (propagating the new contents from the updated replica to the other). Conflicting updates are just displayed. The user is given an opportunity to examine the current state of affairs, change the default actions for nonconflicting updates, and choose actions for conflicting updates.
  4. It performs the selected actions, one at a time. Each action is performed by first transferring the new contents to a temporary file on the receiving host, then atomically moving them into place.
  5. It updates its archive files to reflect the new state of the replicas.

Invariants

Given the importance and delicacy of the job that it performs, it is important to understand both what a synchronizer does under normal conditions and what can happen under unusual conditions such as system crashes and communication failures.

Unison is careful to protect both its internal state and the state of the replicas at every point in this process. Specifically, the following guarantees are enforced: The upshot is that it is safe to interrupt Unison at any time, either manually or accidentally. [Caveat: the above is almost true there are occasionally brief periods where it is not (and, because of shortcoming of the Posix filesystem API, cannot be); in particular, when it is copying a file onto a directory or vice versa, it must first move the original contents out of the way. If Unison gets interrupted during one of these periods, some manual cleanup may be required. In this case, a file called DANGER.README will be left in your home directory, containing information about the operation that was interrupted. The next time you try to run Unison, it will notice this file and warn you about it.]

If an interruption happens while it is propagating updates, then there may be some paths for which an update has been propagated but which have not been marked as synchronized in Unison's archives. This is no problem: the next time Unison runs, it will detect changes to these paths in both replicas, notice that the contents are now equal, and mark the paths as successfully updated when it writes back its private state at the end of this run.

If Unison is interrupted, it may sometimes leave temporary working files (with suffix .tmp) in the replicas. It is safe to delete these files. Also, if the backups flag is set, Unison will leave around old versions of files that it overwrites, with names like file.0.unison.bak. These can be deleted safely when they are no longer wanted.

Unison is not bothered by clock skew between the different hosts on which it is running. It only performs comparisons between timestamps obtained from the same host, and the only assumption it makes about them is that the clock on each system always runs forward.

If Unison finds that its archive files have been deleted (or that the archive format has changed and they cannot be read, or that they don't exist because this is the first run of Unison on these particular roots), it takes a conservative approach: it behaves as though the replicas had both been completely empty at the point of the last synchronization. The effect of this is that, on the first run, files that exist in only one replica will be propagated to the other, while files that exist in both replicas but are unequal will be marked as conflicting.

Touching a file without changing its contents should never affect whether or not Unison does an update. (When running with the fastcheck preference set to true—the default on Unix systems—Unison uses file modtimes for a quick first pass to tell which files have definitely not changed; then, for each file that might have changed, it computes a fingerprint of the file's contents and compares it against the last-synchronized contents. Also, the -times option allows you to synchronize file times, but it does not cause identical files to be changed; Unison will only modify the file times.)

It is safe to “brainwash” Unison by deleting its archive files on both replicas. The next time it runs, it will assume that all the files it sees in the replicas are new.

It is safe to modify files while Unison is working. If Unison discovers that it has propagated an out-of-date change, or that the file it is updating has changed on the target replica, it will signal a failure for that file. Run Unison again to propagate the latest change.

Changes to the ignore patterns from the user interface (e.g., using the `i' key) are immediately reflected in the current profile.

Caveats and Shortcomings

Here are some things to be careful of when using Unison.

Reference Guide

This section covers the features of Unison in detail.

Running Unison

There are several ways to start Unison.

The .unison Directory

Unison stores a variety of information in a private directory on each host. If the environment variable UNISON is defined, then its value will be used as the name of this directory. If UNISON is not defined, then the name of the directory depends on which operating system you are using. In Unix, the default is to use $HOME/.unison. In Windows, if the environment variable USERPROFILE is defined, then the directory will be $USERPROFILE\.unison; otherwise if HOME is defined, it will be $HOME\.unison; otherwise, it will be c:\.unison.

The archive file for each replica is found in the .unison directory on that replica's host. Profiles (described below) are always taken from the .unison directory on the client host.

Note that Unison maintains a completely different set of archive files for each pair of roots.

We do not recommend synchronizing the whole .unison directory, as this will involve frequent propagation of large archive files. It should be safe to do it, though, if you really want to. Synchronizing just the profile files in the .unison directory is definitely OK.

Archive Files

The name of the archive file on each replica is calculated from This method should work well for most users. However, it is occasionally useful to change the way archive names are generated. Unison provides two ways of doing this.

The function that finds the canonical hostname of the local host (which is used, for example, in calculating the name of the archive file used to remember which files have been synchronized) normally uses the gethostname operating system call. However, if the environment variable UNISONLOCALHOSTNAME is set, its value will be used instead. This makes it easier to use Unison in situations where a machine's name changes frequently (e.g., because it is a laptop and gets moved around a lot).

A more powerful way of changing archive names is provided by the rootalias preference. The preference file may contain any number of lines of the form:
    rootalias = //hostnameA//path-to-replicaA -> //hostnameB/path-to-replicaB
When calculating the name of the archive files for a given pair of roots, Unison replaces any root that matches the left-hand side of any rootalias rule by the corresponding right-hand side.

So, if you need to relocate a root on one of the hosts, you can add a rule of the form:
    rootalias = //new-hostname//new-path -> //old-hostname/old-path
Note that root aliases are case-sensitive, even on case-insensitive file systems.

Warning: The rootalias option is dangerous and should only be used if you are sure you know what you're doing. In particular, it should only be used if you are positive that either (1) both the original root and the new alias refer to the same set of files, or (2) the files have been relocated so that the original name is now invalid and will never be used again. (If the original root and the alias refer to different sets of files, Unison's update detector could get confused.) After introducing a new rootalias, it is a good idea to run Unison a few times interactively (with the batch flag off, etc.) and carefully check that things look reasonable—in particular, that update detection is working as expected.

Preferences

Many details of Unison's behavior are configurable by user-settable “preferences.”

Some preferences are boolean-valued; these are often called flags. Others take numeric or string arguments, indicated in the preferences list by n or xxx. Most of the string preferences can be given several times; the arguments are accumulated into a list internally.

There are two ways to set the values of preferences: temporarily, by providing command-line arguments to a particular run of Unison, or permanently, by adding commands to a profile in the .unison directory on the client host. The order of preferences (either on the command line or in preference files) is not significant. On the command line, preferences and other arguments (the profile name and roots) can be intermixed in any order.

To set the value of a preference p from the command line, add an argument -p (for a boolean flag) or -p n or -p xxx (for a numeric or string preference) anywhere on the command line. To set a boolean flag to false on the command line, use -p=false.

Here are all the preferences supported by Unison. This list can be obtained by typing unison -help.
Usage: unison [options]
    or unison root1 root2 [options]
    or unison profilename [options]

Basic options: 
 -auto              automatically accept default (nonconflicting) actions
 -batch             batch mode: ask no questions at all
 -doc xxx           show documentation ('-doc topics' lists topics)
 -fat               use appropriate options for FAT filesystems
 -group             synchronize group attributes
 -ignore xxx        add a pattern to the ignore list
 -ignorenot xxx     add a pattern to the ignorenot list
 -nocreation xxx    prevent file creations on one replica
 -nodeletion xxx    prevent file deletions on one replica
 -noupdate xxx      prevent file updates and deletions on one replica
 -owner             synchronize owner
 -path xxx          path to synchronize
 -perms n           part of the permissions which is synchronized
 -root xxx          root of a replica (should be used exactly twice)
 -silent            print nothing except error messages
 -terse             suppress status messages
 -testserver        exit immediately after the connection to the server
 -times             synchronize modification times
 -version           print version and exit

Advanced options: 
 -addprefsto xxx    file to add new prefs to
 -addversionno      add version number to name of unison on server
 -backup xxx        add a pattern to the backup list
 -backupcurr xxx    add a pattern to the backupcurr list
 -backupcurrnot xxx add a pattern to the backupcurrnot list
 -backupdir xxx     directory for storing centralized backups
 -backuploc xxx     where backups are stored ('local' or 'central')
 -backupnot xxx     add a pattern to the backupnot list
 -backupprefix xxx  prefix for the names of backup files
 -backups           keep backup copies of all files (see also 'backup')
 -backupsuffix xxx  a suffix to be added to names of backup files
 -clientHostName xxx set host name of client
 -confirmbigdel     ask about whole-replica (or path) deletes (default true)
 -confirmmerge      ask for confirmation before committing results of a merge
 -contactquietly    suppress the 'contacting server' message during startup
 -copymax n         maximum number of simultaneous copyprog transfers
 -copyonconflict    keep copies of conflicting files
 -copyprog xxx      external program for copying large files
 -copyprogrest xxx  variant of copyprog for resuming partial transfers
 -copyquoterem xxx  add quotes to remote file name for copyprog (true/false/default)
 -copythreshold n   use copyprog on files bigger than this (if >=0, in Kb)
 -debug xxx         debug module xxx ('all' -> everything, 'verbose' -> more)
 -diff xxx          set command for showing differences between files
 -dontchmod         when set, never use the chmod system call
 -dumbtty           do not change terminal settings in text UI (default true)
 -fastcheck xxx     do fast update detection (true/false/default)
 -fastercheckUNSAFE skip computing fingerprints for new files (experts only!)
 -follow xxx        add a pattern to the follow list
 -force xxx         force changes from this replica to the other
 -forcepartial xxx  add a pattern to the forcepartial list
 -halfduplex        force half-duplex communication with the server
 -height n          height (in lines) of main window in graphical interface
 -host xxx          bind the socket to this host name in server socket mode
 -ignorearchives    ignore existing archive files
 -ignorecase xxx    identify upper/lowercase filenames (true/false/default)
 -ignoreinodenumbers ignore inode number changes when detecting updates
 -ignorelocks       ignore locks left over from previous run (dangerous!)
 -immutable xxx     add a pattern to the immutable list
 -immutablenot xxx  add a pattern to the immutablenot list
 -key xxx           define a keyboard shortcut for this profile (in some UIs)
 -killserver        kill server when done (even when using sockets)
 -label xxx         provide a descriptive string label for this profile
 -links xxx         allow the synchronization of symbolic links (true/false/default)
 -log               record actions in logfile (default true)
 -logfile xxx       logfile name
 -maxbackups n      number of backed up versions of a file
 -maxerrors n       maximum number of errors before a directory transfer is aborted
 -maxsizethreshold n prevent transfer of files bigger than this (if >=0, in Kb)
 -maxthreads n      maximum number of simultaneous file transfers
 -merge xxx         add a pattern to the merge list
 -mountpoint xxx    abort if this path does not exist
 -nocreationpartial xxx add a pattern to the nocreationpartial list
 -nodeletionpartial xxx add a pattern to the nodeletionpartial list
 -noupdatepartial xxx add a pattern to the noupdatepartial list
 -numericids        don't map uid/gid values by user/group names
 -prefer xxx        choose this replica's version for conflicting changes
 -preferpartial xxx add a pattern to the preferpartial list
 -repeat xxx        synchronize repeatedly (text interface only)
 -retry n           re-try failed synchronizations N times (text ui only)
 -rootalias xxx     register alias for canonical root names
 -rsrc xxx          synchronize resource forks (true/false/default)
 -rsync             activate the rsync transfer mode (default true)
 -selftest          run internal tests and exit
 -servercmd xxx     name of unison executable on remote server
 -showarchive       show 'true names' (for rootalias) of roots and archive
 -socket xxx        act as a server on a socket
 -sortbysize        list changed files by size, not name
 -sortfirst xxx     add a pattern to the sortfirst list
 -sortlast xxx      add a pattern to the sortlast list
 -sortnewfirst      list new before changed files
 -sshargs xxx       other arguments (if any) for remote shell command
 -sshcmd xxx        path to the ssh executable
 -stream            use a streaming protocol for transferring file contents (default true)
 -ui xxx            select UI ('text' or 'graphic'); command-line only
 -unicode xxx       assume Unicode encoding in case insensitive mode
 -watch             when set, use a file watcher process to detect changes (default true)
 -xferbycopying     optimize transfers using local copies (default true)

Here, in more detail, is what they do. Many are discussed in greater detail in other sections of the manual.
addprefsto xxx
By default, new preferences added by Unison (e.g., new ignore clauses) will be appended to whatever preference file Unison was told to load at the beginning of the run. Setting the preference addprefsto filename makes Unison add new preferences to the file named filename instead.

addversionno
When this flag is set to true, Unison will use unison-currentversionnumber instead of just unison as the remote server command. This allows multiple binaries for different versions of unison to coexist conveniently on the same server: whichever version is run on the client, the same version will be selected on the server.

auto
When set to true, this flag causes the user interface to skip asking for confirmations on non-conflicting changes. (More precisely, when the user interface is done setting the propagation direction for one entry and is about to move to the next, it will skip over all non-conflicting entries and go directly to the next conflict.)

backup xxx
Including the preference -backup pathspec causes Unison to keep backup files for each path that matches pathspec. These backup files are kept in the directory specified by the backuplocation preference. The backups are named according to the backupprefix and backupsuffix preferences. The number of versions that are kept is determined by the maxbackups preference.

The syntax of pathspec is described in the Path Specification section.

backupcurr xxx
Including the preference -backupcurr pathspec causes Unison to keep a backup of the current version of every file matching pathspec. This file will be saved as a backup with version number 000. Such backups can be used as inputs to external merging programs, for instance. See the documentatation for the merge preference. For more details, see the Merging Conflicting Versions section.

The syntax of pathspec is described in the Path Specification section.

backupcurrnot xxx
Exceptions to backupcurr, like the ignorenot preference.

backupdir xxx
If this preference is set, Unison will use it as the name of the directory used to store backup files specified by the backup preference, when backuplocation is set to central. It is checked after the UNISONBACKUPDIR environment variable.

backuploc xxx
This preference determines whether backups should be kept locally, near the original files, or in a central directory specified by the backupdir preference. If set to local, backups will be kept in the same directory as the original files, and if set to central, backupdir will be used instead.

backupnot xxx
The values of this preference specify paths or individual files or regular expressions that should not be backed up, even if the backup preference selects them—i.e., it selectively overrides backup. The same caveats apply here as with ignore and ignorenot.

backupprefix xxx
When a backup for a file NAME is created, it is stored in a directory specified by backuplocation, in a file called backupprefixNAMEbackupsuffix. backupprefix can include a directory name (causing Unison to keep all backup files for a given directory in a subdirectory with this name), and both backupprefix and backupsuffix can contain the string$VERSION, which will be replaced by the age of the backup (1 for the most recent, 2 for the second most recent, and so on...). This keyword is ignored if it appears in a directory name in the prefix; if it does not appear anywhere in the prefix or the suffix, it will be automatically placed at the beginning of the suffix.

One thing to be careful of: If the backuploc preference is set to local, Unison will automatically ignore all files whose prefix and suffix match backupprefix and backupsuffix. So be careful to choose values for these preferences that are sufficiently different from the names of your real files.

backups
Setting this flag to true is equivalent to setting backuplocation to local and backup to Name *.

backupsuffix xxx
See backupprefix for full documentation.

batch
When this is set to true, the user interface will ask no questions at all. Non-conflicting changes will be propagated; conflicts will be skipped.

clientHostName xxx
When specified, the host name of the client will not be guessedand the provided host name will be used to find the archive.

confirmbigdel
When this is set to true, Unison will request an extra confirmation if it appears that the entire replica has been deleted, before propagating the change. If the batch flag is also set, synchronization will be aborted. When the path preference is used, the same confirmation will be requested for top-level paths. (At the moment, this flag only affects the text user interface.) See also the mountpoint preference.

confirmmerge
Setting this preference causes both the text and graphical interfaces to ask the user if the results of a merge command may be committed to the replica or not. Since the merge command works on temporary files, the user can then cancel all the effects of applying the merge if it turns out that the result is not satisfactory. In batch-mode, this preference has no effect. Default is false.

contactquietly
If this flag is set, Unison will skip displaying the `Contacting server' message (which some users find annoying) during startup.

copymax n
A number indicating how many instances of the external copying utility Unison is allowed to run simultaneously (default to 1).

copyonconflict
When this flag is set, Unison will make a copy of files that would otherwise be overwritten or deleted in case of conflicting changes, and more generally whenever the default behavior is overriden. This makes it possible to automatically resolve conflicts in a fairly safe way when synchronizing continuously, in combination with the -repeat watch and -prefer newer preferences.

copyprog xxx
A string giving the name of an external program that can be used to copy large files efficiently (plus command-line switches telling it to copy files in-place). The default setting invokes rsync with appropriate options—most users should not need to change it.

copyprogrest xxx
A variant of copyprog that names an external program that should be used to continue the transfer of a large file that has already been partially transferred. Typically, copyprogrest will just be copyprog with one extra option (e.g., –partial, for rsync). The default setting invokes rsync with appropriate options—most users should not need to change it.

copyquoterem xxx
When set to true, this flag causes Unison to add an extra layer of quotes to the remote path passed to the external copy program. This is needed by rsync, for example, which internally uses an ssh connection requiring an extra level of quoting for paths containing spaces. When this flag is set to default, extra quotes are added if the value of copyprog contains the string rsync.

copythreshold n
A number indicating above what filesize (in kilobytes) Unison should use the external copying utility specified by copyprog. Specifying 0 will cause all copies to use the external program; a negative number will prevent any files from using it. The default is -1. See the Making Unison Faster on Large Files section for more information.

debug xxx
This preference is used to make Unison print various sorts of information about what it is doing internally on the standard error stream. It can be used many times, each time with the name of a module for which debugging information should be printed. Possible arguments for debug can be found by looking for calls to Util.debug in the sources (using, e.g., grep). Setting -debug all causes information from all modules to be printed (this mode of usage is the first one to try, if you are trying to understand something that Unison seems to be doing wrong); -debug verbose turns on some additional debugging output from some modules (e.g., it will show exactly what bytes are being sent across the network).

diff xxx
This preference can be used to control the name and command-line arguments of the system utility used to generate displays of file differences. The default is `diff -u CURRENT2 CURRENT1'. If the value of this preference contains the substrings CURRENT1 and CURRENT2, these will be replaced by the names of the files to be diffed. If not, the two filenames will be appended to the command. In both cases, the filenames are suitably quoted.

doc xxx
The command-line argument -doc secname causes unison to display section secname of the manual on the standard output and then exit. Use -doc all to display the whole manual, which includes exactly the same information as the printed and HTML manuals, modulo formatting. Use -doc topics to obtain a list of the names of the various sections that can be printed.

dontchmod
By default, Unison uses the 'chmod' system call to set the permission bits of files after it has copied them. But in some circumstances (and under some operating systems), the chmod call always fails. Setting this preference completely prevents Unison from ever calling chmod.

dumbtty
When set to true, this flag makes the text mode user interface avoid trying to change any of the terminal settings. (Normally, Unison puts the terminal in `raw mode', so that it can do things like overwriting the current line.) This is useful, for example, when Unison runs in a shell inside of Emacs.

When dumbtty is set, commands to the user interface need to be followed by a carriage return before Unison will execute them. (When it is off, Unison recognizes keystrokes as soon as they are typed.)

This preference has no effect on the graphical user interface.

dumparchives
When this preference is set, Unison will create a file unison.dump on each host, containing a text summary of the archive, immediately after loading it.

fastcheck xxx
When this preference is set to true, Unison will use the modification time and length of a file as a `pseudo inode number' when scanning replicas for updates, instead of reading the full contents of every file. (This does not apply to the very first run, when Unison will always scan all files regarless of this switch). Under Windows, this may cause Unison to miss propagating an update if the modification time and length of the file are both unchanged by the update. However, Unison will never overwrite such an update with a change from the other replica, since it always does a safe check for updates just before propagating a change. Thus, it is reasonable to use this switch under Windows most of the time and occasionally run Unison once with fastcheck set to false, if you are worried that Unison may have overlooked an update. For backward compatibility, yes, no, and default can be used in place of true, false, and auto. See the Fast Checking section for more information.

fastercheckUNSAFE
THIS FEATURE IS STILL EXPERIMENTAL AND SHOULD BE USED WITH EXTREME CAUTION.

When this flag is set to true, Unison will compute a 'pseudo-fingerprint' the first time it sees a file (either because the file is new or because Unison is running for the first time). This enormously speeds update detection, but it must be used with care, as it can cause Unison to miss conflicts: If a given path in the filesystem contains files on both sides that Unison has not yet seen, and if those files have the same length but different contents, then Unison will not notice the presence of a conflict. If, later, one of the files is changed, the changed file will be propagated, overwriting the other.

Moreover, even when the files are initially identical, setting this flag can lead to potentially confusing behavior: if a newly created file is later touched without being modified, Unison will treat this conservatively as a potential change (since it has no record of the earlier contents) and show it as needing to be propagated to the other replica.

Most users should leave this flag off – the small time savings of not fingerprinting new files is not worth the cost in terms of safety. However, it can be very useful for power users with huge replicas that are known to be already synchronized (e.g., because one replica is a newly created duplicate of the other, or because they have previously been synchronized with Unison but Unison's archives need to be rebuilt). In such situations, it is recommended that this flag be set only for the initial run of Unison, so that new archives can be created quickly, and then turned off for normal use.

fat
When this is set to true, Unison will use appropriate options to synchronize efficiently and without error a replica located on a FAT filesystem on a non-Windows machine: do not synchronize permissions (perms = 0); never use chmod ( t dontchmod = true); treat filenames as case insensitive (ignorecase = true); do not attempt to synchronize symbolic links (links = false); ignore inode number changes when detecting updates (ignoreinodenumbers = true). Any of these change can be overridden by explicitly setting the corresponding preference in the profile.

follow xxx
Including the preference -follow pathspec causes Unison to treat symbolic links matching pathspec as `invisible' and behave as if the object pointed to by the link had appeared literally at this position in the replica. See the Symbolic Links section for more details. The syntax of pathspec is described in the Path Specification section.

force xxx
Including the preference -force root causes Unison to resolve all differences (even non-conflicting changes) in favor of root. This effectively changes Unison from a synchronizer into a mirroring utility.

You can also specify -force newer (or -force older) to force Unison to choose the file with the later (earlier) modtime. In this case, the -times preference must also be enabled.

This preference is overridden by the forcepartial preference.

This preference should be used only if you are sure you know what you are doing!

forcepartial xxx
Including the preference forcepartial = PATHSPEC -> root causes Unison to resolve all differences (even non-conflicting changes) in favor of root for the files in PATHSPEC (see the Path Specification section for more information). This effectively changes Unison from a synchronizer into a mirroring utility.

You can also specify forcepartial PATHSPEC -> newer (or forcepartial PATHSPEC older) to force Unison to choose the file with the later (earlier) modtime. In this case, the -times preference must also be enabled.

This preference should be used only if you are sure you know what you are doing!

group
When this flag is set to true, the group attributes of the files are synchronized. Whether the group names or the group identifiers are synchronized depends on the preference numerids.

halfduplex
When this flag is set to true, Unison network communication is forced to be half duplex (the client and the server never simultaneously emit data). If you experience unstabilities with your network link, this may help. The communication is always half-duplex when synchronizing with a Windows machine due to a limitation of Unison current implementation that could result in a deadlock.

height n
Used to set the height (in lines) of the main window in the graphical user interface.

ignore xxx
Including the preference -ignore pathspec causes Unison to completely ignore paths that match pathspec (as well as their children). This is useful for avoiding synchronizing temporary files, object files, etc. The syntax of pathspec is described in the Path Specification section, and further details on ignoring paths is found in the Ignoring Paths section.

ignorearchives
When this preference is set, Unison will ignore any existing archive files and behave as though it were being run for the first time on these replicas. It is not a good idea to set this option in a profile: it is intended for command-line use.

ignorecase xxx
When set to true, this flag causes Unison to treat filenames as case insensitive—i.e., files in the two replicas whose names differ in (upper- and lower-case) `spelling' are treated as the same file. When the flag is set to false, Unison will treat all filenames as case sensitive. Ordinarily, when the flag is set to default, filenames are automatically taken to be case-insensitive if either host is running Windows or OSX. In rare circumstances it may be useful to set the flag manually.

ignoreinodenumbers
When set to true, this preference makes Unison not take advantage of inode numbers during fast update detection. This switch should be used with care, as it is less safe than the standard update detection method, but it can be useful with filesystems which do not support inode numbers.

ignorelocks
When this preference is set, Unison will ignore any lock files that may have been left over from a previous run of Unison that was interrupted while reading or writing archive files; by default, when Unison sees these lock files it will stop and request manual intervention. This option should be set only if you are positive that no other instance of Unison might be concurrently accessing the same archive files (e.g., because there was only one instance of unison running and it has just crashed or you have just killed it). It is probably not a good idea to set this option in a profile: it is intended for command-line use.

ignorenot xxx
This preference overrides the preference ignore. It gives a list of patterns (in the same format as ignore) for paths that should definitely not be ignored, whether or not they happen to match one of the ignore patterns.

Note that the semantics of ignore and ignorenot is a little counter-intuitive. When detecting updates, Unison examines paths in depth-first order, starting from the roots of the replicas and working downwards. Before examining each path, it checks whether it matches ignore and does not match ignorenot; in this case it skips this path and all its descendants. This means that, if some parent of a given path matches an ignore pattern, then it will be skipped even if the path itself matches an ignorenot pattern. In particular, putting ignore = Path * in your profile and then using ignorenot to select particular paths to be synchronized will not work. Instead, you should use the path preference to choose particular paths to synchronize.

immutable xxx
This preference specifies paths for directories whose immediate children are all immutable files — i.e., once a file has been created, its contents never changes. When scanning for updates, Unison does not check whether these files have been modified; this can speed update detection significantly (in particular, for mail directories).

immutablenot xxx
This preference overrides immutable.

key xxx
Used in a profile to define a numeric key (0-9) that can be used in the graphical user interface to switch immediately to this profile.

killserver
When set to true, this flag causes Unison to kill the remote server process when the synchronization is finished. This behavior is the default for ssh connections, so this preference is not normally needed when running over ssh; it is provided so that socket-mode servers can be killed off after a single run of Unison, rather than waiting to accept future connections. (Some users prefer to start a remote socket server for each run of Unison, rather than leaving one running all the time.)

label xxx
Used in a profile to provide a descriptive string documenting its settings. (This is useful for users that switch between several profiles, especially using the `fast switch' feature of the graphical user interface.)

links xxx
When set to true, this flag causes Unison to synchronize symbolic links. When the flag is set to false, symbolic links will result in an error during update detection. Ordinarily, when the flag is set to default, symbolic links are synchronized except when one of the hosts is running Windows. In rare circumstances it may be useful to set the flag manually.

log
When this flag is set, Unison will log all changes to the filesystems on a file.

logfile xxx
By default, logging messages will be appended to the file unison.log in your HOME directory. Set this preference if you prefer another file.

maxbackups n
This preference specifies the number of backup versions that will be kept by unison, for each path that matches the predicate backup. The default is 2.

maxerrors n
This preference controls after how many errors Unison aborts a directory transfer. Setting it to a large number allows Unison to transfer most of a directory even when some files fail to be copied. The default is 1. If the preference is set too high, Unison may take a long time to abort in case of repeated failures (for instance, when the disk is full).

maxsizethreshold n
A number indicating above what filesize (in kilobytes) Unison should flag a conflict instead of transferring the file. This conflict remains even in the presence of force or prefer options. A negative number will allow every transfer independently of the size. The default is -1.

maxthreads n
This preference controls how much concurrency is allowed during the transport phase. Normally, it should be set reasonably high to maximize performance, but when Unison is used over a low-bandwidth link it may be helpful to set it lower (e.g. to 1) so that Unison doesn't soak up all the available bandwidth. The default is the special value 0, which mean 20 threads when file content streaming is desactivated and 1000 threads when it is activated.

merge xxx
This preference can be used to run a merge program which will create a new version for each of the files and the backup, with the last backup and the both replicas. Setting the merge preference for a path will also cause this path to be backed up, just like t backup. The syntax of pathspec>cmd is described in the Path Specification section, and further details on Merging functions are present in the Merging files section.

mountpoint xxx
Including the preference -mountpoint PATH causes Unison to double-check, at the end of update detection, that PATH exists and abort if it does not. This is useful when Unison is used to synchronize removable media. This preference can be given more than once. See the Mount Points section.

nocreation xxx
Including the preference -nocreation root prevents Unison from performing any file creation on root root.

This preference can be included twice, once for each root, if you want to prevent any creation.

nocreationpartial xxx
Including the preference nocreationpartial = PATHSPEC -> root prevents Unison from performing any file creation in PATHSPEC on root root (see the Path Specification section for more information). It is recommended to use BelowPath patterns when selecting a directory and all its contents.

nodeletion xxx
Including the preference -nodeletion root prevents Unison from performing any file deletion on root root.

This preference can be included twice, once for each root, if you want to prevent any deletion.

nodeletionpartial xxx
Including the preference nodeletionpartial = PATHSPEC -> root prevents Unison from performing any file deletion in PATHSPEC on root root (see the Path Specification section for more information). It is recommended to use BelowPath patterns when selecting a directory and all its contents.

noupdate xxx
Including the preference -noupdate root prevents Unison from performing any file update or deletion on root root.

This preference can be included twice, once for each root, if you want to prevent any update.

noupdatepartial xxx
Including the preference noupdatepartial = PATHSPEC -> root prevents Unison from performing any file update or deletion in PATHSPEC on root root (see the Path Specification section for more information). It is recommended to use BelowPath patterns when selecting a directory and all its contents.

numericids
When this flag is set to true, groups and users are synchronized numerically, rather than by name.

The special uid 0 and the special group 0 are never mapped via user/group names even if this preference is not set.

owner
When this flag is set to true, the owner attributes of the files are synchronized. Whether the owner names or the owner identifiers are synchronizeddepends on the preference numerids.

path xxx
When no path preference is given, Unison will simply synchronize the two entire replicas, beginning from the given pair of roots. If one or more path preferences are given, then Unison will synchronize only these paths and their children. (This is useful for doing a fast sync of just one directory, for example.) Note that path preferences are intepreted literally—they are not regular expressions.

perms n
The integer value of this preference is a mask indicating which permission bits should be synchronized. It is set by default to 0o1777: all bits but the set-uid and set-gid bits are synchronised (synchronizing theses latter bits can be a security hazard). If you want to synchronize all bits, you can set the value of this preference to −1. If one of the replica is on a FAT [Windows] filesystem, you should consider using the t fat preference instead of this preference. If you need Unison not to set permissions at all, set the value of this preference to 0 and set the preference t dontchmod to t true.

prefer xxx
Including the preference -prefer root causes Unison always to resolve conflicts in favor of root, rather than asking for guidance from the user. (The syntax of root is the same as for the root preference, plus the special values newer and older.)

This preference is overridden by the preferpartial preference.

This preference should be used only if you are sure you know what you are doing!

preferpartial xxx
Including the preference preferpartial = PATHSPEC -> root causes Unison always to resolve conflicts in favor of root, rather than asking for guidance from the user, for the files in PATHSPEC (see the Path Specification section for more information). (The syntax of root is the same as for the root preference, plus the special values newer and older.)

This preference should be used only if you are sure you know what you are doing!

repeat xxx
Setting this preference causes the text-mode interface to synchronize repeatedly, rather than doing it just once and stopping. If the argument is a number, Unison will pause for that many seconds before beginning again. When the argument is watch, Unison relies on an external file monitoring process to synchronize whenever a change happens.

retry n
Setting this preference causes the text-mode interface to try again to synchronize updated paths where synchronization fails. Each such path will be tried N times.

root xxx
Each use of this preference names the root of one of the replicas for Unison to synchronize. Exactly two roots are needed, so normal modes of usage are either to give two values for root in the profile, or to give no values in the profile and provide two on the command line. Details of the syntax of roots can be found in the Roots section.

The two roots can be given in either order; Unison will sort them into a canonical order before doing anything else. It also tries to `canonize' the machine names and paths that appear in the roots, so that, if Unison is invoked later with a slightly different name for the same root, it will be able to locate the correct archives.

rootalias xxx
When calculating the name of the archive files for a given pair of roots, Unison replaces any roots matching the left-hand side of any rootalias rule by the corresponding right-hand side.

rshargs xxx
The string value of this preference will be passed as additional arguments (besides the host name and the name of the Unison executable on the remote system) to the rsh command used to invoke the remote server.

rshcmd xxx
This preference can be used to explicitly set the name of the rsh executable (e.g., giving a full path name), if necessary.

rsrc xxx
When set to true, this flag causes Unison to synchronize resource forks and HFS meta-data. On filesystems that do not natively support resource forks, this data is stored in Carbon-compatible ._ AppleDouble files. When the flag is set to false, Unison will not synchronize these data. Ordinarily, the flag is set to default, and these data are automatically synchronized if either host is running OSX. In rare circumstances it is useful to set the flag manually.

rsync
Unison uses the 'rsync algorithm' for 'diffs-only' transfer of updates to large files. Setting this flag to false makes Unison use whole-file transfers instead. Under normal circumstances, there is no reason to do this, but if you are having trouble with repeated 'rsync failure' errors, setting it to false should permit you to synchronize the offending files.

selftest
Run internal tests and exit. This option is mostly for developers and must be used carefully: in particular, it will delete the contents of both roots, so that it can install its own files for testing. This flag only makes sense on the command line. When it is provided, no preference file is read: all preferences must be specified on thecommand line. Also, since the self-test procedure involves overwriting the roots and backup directory, the names of the roots and of the backupdir preference must include the string "test" or else the tests will be aborted. (If these are not given on the command line, dummy subdirectories in the current directory will be created automatically.)

servercmd xxx
This preference can be used to explicitly set the name of the Unison executable on the remote server (e.g., giving a full path name), if necessary.

showarchive
When this preference is set, Unison will print out the 'true names'of the roots, in the same form as is expected by the rootaliaspreference.

silent
When this preference is set to true, the textual user interface will print nothing at all, except in the case of errors. Setting silent to true automatically sets the batch preference to true.

sortbysize
When this flag is set, the user interface will list changed files by size (smallest first) rather than by name. This is useful, for example, for synchronizing over slow links, since it puts very large files at the end of the list where they will not prevent smaller files from being transferred quickly.

This preference (as well as the other sorting flags, but not the sorting preferences that require patterns as arguments) can be set interactively and temporarily using the 'Sort' menu in the graphical user interface.

sortfirst xxx
Each argument to sortfirst is a pattern pathspec, which describes a set of paths. Files matching any of these patterns will be listed first in the user interface. The syntax of pathspec is described in the Path Specification section.

sortlast xxx
Similar to sortfirst, except that files matching one of these patterns will be listed at the very end.

sortnewfirst
When this flag is set, the user interface will list newly created files before all others. This is useful, for example, for checking that newly created files are not `junk', i.e., ones that should be ignored or deleted rather than synchronized.

sshargs xxx
The string value of this preference will be passed as additional arguments (besides the host name and the name of the Unison executable on the remote system) to the ssh command used to invoke the remote server.

sshcmd xxx
This preference can be used to explicitly set the name of the ssh executable (e.g., giving a full path name), if necessary.

sshversion xxx
This preference can be used to control which version of ssh should be used to connect to the server. Legal values are 1 and 2, which will cause unison to try to use ssh1 orssh2 instead of just ssh to invoke ssh. The default value is empty, which will make unison use whatever version of ssh is installed as the default `ssh' command.

stream
When this preference is set, Unison will use an experimental streaming protocol for transferring file contents more efficiently. The default value is true.

terse
When this preference is set to true, the user interface will not print status messages.

testserver
Setting this flag on the command line causes Unison to attempt to connect to the remote server and, if successful, print a message and immediately exit. Useful for debugging installation problems. Should not be set in preference files.

times
When this flag is set to true, file modification times (but not directory modtimes) are propagated.

ui xxx
This preference selects either the graphical or the textual user interface. Legal values are graphic or text.

Because this option is processed specially during Unison's start-up sequence, it can only be used on the command line. In preference files it has no effect.

If the Unison executable was compiled with only a textual interface, this option has no effect. (The pre-compiled binaries are all compiled with both interfaces available.)

unicode xxx
When set to true, this flag causes Unison to perform case insensitive file comparisons assuming Unicode encoding. This is the default. When the flag is set to false, a Latin 1 encoding is assumed. When Unison runs in case sensitive mode, this flag only makes a difference if one host is running Windows or Mac OS X. Under Windows, the flag selects between using the Unicode or 8bit Windows API for accessing the filesystem. Under Mac OS X, it selects whether comparing the filenames up to decomposition, or byte-for-byte.

version
Print the current version number and exit. (This option only makes sense on the command line.)

watch
Unison uses a file watcher process, when available, to detect filesystem changes; this is used to speed up update detection, and for continuous synchronization (-repeat watch preference. Setting this flag to false disable the use of this process.

xferbycopying
When this preference is set, Unison will try to avoid transferring file contents across the network by recognizing when a file with the required contents already exists in the target replica. This usually allows file moves to be propagated very quickly. The default value istrue.

Profiles

A profile is a text file that specifies permanent settings for roots, paths, ignore patterns, and other preferences, so that they do not need to be typed at the command line every time Unison is run. Profiles should reside in the .unison directory on the client machine. If Unison is started with just one argument name on the command line, it looks for a profile called name.prf in the .unison directory. If it is started with no arguments, it scans the .unison directory for files whose names end in .prf and offers a menu (provided that the Unison executable is compiled with the graphical user interface). If a file named default.prf is found, its settings will be offered as the default choices.

To set the value of a preference p permanently, add to the appropriate profile a line of the form
        p = true
for a boolean flag or
        p = <value>
for a preference of any other type.

Whitespaces around p and xxx are ignored. A profile may also include blank lines and lines beginning with #; both are ignored.

When Unison starts, it first reads the profile and then the command line, so command-line options will override settings from the profile.

Profiles may also include lines of the form include name, which will cause the file name (or name.prf, if name does not exist in the .unison directory) to be read at the point, and included as if its contents, instead of the include line, was part of the profile. Include lines allows settings common to several profiles to be stored in one place.

A profile may include a preference `label = desc' to provide a description of the options selected in this profile. The string desc is listed along with the profile name in the profile selection dialog, and displayed in the top-right corner of the main Unison window in the graphical user interface.

The graphical user-interface also supports one-key shortcuts for commonly used profiles. If a profile contains a preference of the form `key = n', where n is a single digit, then pressing this digit key will cause Unison to immediately switch to this profile and begin synchronization again from scratch. In this case, all actions that have been selected for a set of changes currently being displayed will be discarded.

Sample Profiles

A Minimal Profile

Here is a very minimal profile file, such as might be found in .unison/default.prf:
    # Roots of the synchronization
    root = /home/bcpierce
    root = ssh://saul//home/bcpierce

    # Paths to synchronize 
    path = current
    path = common
    path = .netscape/bookmarks.html

A Basic Profile

Here is a more sophisticated profile, illustrating some other useful features.
    # Roots of the synchronization
    root = /home/bcpierce
    root = ssh://saul//home/bcpierce

    # Paths to synchronize 
    path = current
    path = common
    path = .netscape/bookmarks.html

    # Some regexps specifying names and paths to ignore
    ignore = Name temp.*
    ignore = Name *~
    ignore = Name .*~
    ignore = Path */pilot/backup/Archive_*
    ignore = Name *.o
    ignore = Name *.tmp

    # Window height
    height = 37

    # Keep a backup copy of every file in a central location
    backuplocation = central
    backupdir = /home/bcpierce/backups
    backup = Name *
    backupprefix = $VERSION.
    backupsuffix = 

    # Use this command for displaying diffs
    diff = diff -y -W 79 --suppress-common-lines

    # Log actions to the terminal
    log = true

A Power-User Profile

When Unison is used with large replicas, it is often convenient to be able to synchronize just a part of the replicas on a given run (this saves the time of detecting updates in the other parts). This can be accomplished by splitting up the profile into several parts — a common part containing most of the preference settings, plus one “top-level” file for each set of paths that need to be synchronized. (The include mechanism can also be used to allow the same set of preference settings to be used with different roots.)

The collection of profiles implementing this scheme might look as follows. The file default.prf is empty except for an include directive:
    # Include the contents of the file common
    include common
Note that the name of the common file is common, not common.prf; this prevents Unison from offering common as one of the list of profiles in the opening dialog (in the graphical UI).

The file common contains the real preferences:
    # Roots of the synchronization
    root = /home/bcpierce
    root = ssh://saul//home/bcpierce

    # (... other preferences ...)

    # If any new preferences are added by Unison (e.g. 'ignore'
    # preferences added via the graphical UI), then store them in the
    # file 'common' rathen than in the top-level preference file
    addprefsto = common

    # Names and paths to ignore:
    ignore = Name temp.*
    ignore = Name *~
    ignore = Name .*~
    ignore = Path */pilot/backup/Archive_*
    ignore = Name *.o
    ignore = Name *.tmp
Note that there are no path preferences in common. This means that, when we invoke Unison with the default profile (e.g., by typing 'unison default' or just 'unison' on the command line), the whole replicas will be synchronized. (If we never want to synchronize the whole replicas, then default.prf would instead include settings for all the paths that are usually synchronized.)

To synchronize just part of the replicas, Unison is invoked with an alternate preference file—e.g., doing 'unison workingset', where the preference file workingset.prf contains
    path = current/papers
    path = Mail/inbox
    path = Mail/drafts
    include common
causes Unison to synchronize just the listed subdirectories.

The key preference can be used in combination with the graphical UI to quickly switch between different sets of paths. For example, if the file mail.prf contains
    path = Mail
    batch = true
    key = 2
    include common
then pressing 2 will cause Unison to look for updates in the Mail subdirectory and (because the batch flag is set) immediately propagate any that it finds.

Keeping Backups

When Unison overwrites a file or directory by propagating a new version from the other replica, it can keep the old version around as a backup. There are several preferences that control precisely where these backups are stored and how they are named.

To enable backups, you must give one or more backup preferences. Each of these has the form
    backup = <pathspec>
where <pathspec> has the same form as for the ignore preference. For example,
    backup = Name *
causes Unison to keep backups of all files and directories. The backupnot preference can be used to give a few exceptions: it specifies which files and directories should not be backed up, even if they match the backup pathspec.

It is important to note that the pathspec is matched against the path that is being updated by Unison, not its descendants. For example, if you set backup = Name *.txt and then delete a whole directory named foo containing some text files, these files will not be backed up because Unison will just check that foo does not match *.txt. Similarly, if the directory itself happened to be called foo.txt, then the whole directory and all the files in it will be backed up, regardless of their names.

Backup files can be stored either centrally or locally. This behavior is controlled by the preference backuplocation, whose value must be either central or local. (The default is central.)

When backups are stored locally, they are kept in the same directory as the original.

When backups are stored centrally, the directory used to hold them is controlled by the preference backupdir and the environment variable UNISONBACKUPDIR. (The environment variable is checked first.) If neither of these are set, then the directory .unison/backup in the user's home directory is used.

The preference maxbackups controls how many previous versions of each file are kept (including the current version).

By default, backup files are named .bak.VERSION.FILENAME, where FILENAME is the original filename and VERSION is the backup number (1 for the most recent, 2 for the next most recent, etc.). This can be changed by setting the preferences backupprefix and/or backupsuffix. If desired, backupprefix may include a directory prefix; this can be used with backuplocation = local to put all backup files for each directory into a single subdirectory. For example, setting
    backuplocation = local
    backupprefix = .unison/$VERSION.
    backupsuffix = 
will put all backups in a local subdirectory named .unison. Also, note that the string $VERSION in either backupprefix or backupsuffix (it must appear in one or the other) is replaced by the version number. This can be used, for example, to ensure that backup files retain the same extension as the originals.

For backward compatibility, the backups preference is also supported. It simply means backup = Name * and backuplocation = local.

Merging Conflicting Versions

Unison can invoke external programs to merge conflicting versions of a file. The preference merge controls this process.

The merge preference may be given once or several times in a preference file (it can also be given on the command line, of course, but this tends to be awkward because of the spaces and special characters involved). Each instance of the preference looks like this:
    merge = <PATHSPEC> -> <MERGECMD>
The <PATHSPEC> here has exactly the same format as for the ignore preference (see the Path specification section). For example, using “Name *.txt” as the <PATHSPEC> tells Unison that this command should be used whenever a file with extension .txt needs to be merged.

Many external merging programs require as inputs not just the two files that need to be merged, but also a file containing the last synchronized version. You can ask Unison to keep a copy of the last synchronized version for some files using the backupcurrent preference. This preference is used in exactly the same way as backup and its meaning is similar, except that it causes backups to be kept of the current contents of each file after it has been synchronized by Unison, rather than the previous contents that Unison overwrote. These backups are kept on both replicas in the same place as ordinary backup files—i.e. according to the backuplocation and backupdir preferences. They are named like the original files if backupslocation is set to 'central' and otherwise, Unison uses the backupprefix and backupsuffix preferences and assumes a version number 000 for these backups.

The <MERGECMD> part of the preference specifies what external command should be invoked to merge files at paths matching the <PATHSPEC>. Within this string, several special substrings are recognized; these will be substituted with appropriate values before invoking a sub-shell to execute the command. To accommodate the wide variety of programs that users might want to use for merging, Unison checks for several possible situations when the merge program exits: If the confirmmerge preference is set and Unison is not run in batch mode, then Unison will always ask for confirmation before actually committing the results of the merge to the replicas.

A large number of external merging programs are available. For example, on Unix systems setting the merge preference to
    merge = Name *.txt -> diff3 -m CURRENT1 CURRENTARCH CURRENT2
                            > NEW || echo "differences detected"
will tell Unison to use the external diff3 program for merging. Alternatively, users of emacs may find the following settings convenient:
    merge = Name *.txt -> emacs -q --eval '(ediff-merge-files-with-ancestor 
                             "CURRENT1" "CURRENT2" "CURRENTARCH" nil "NEW")' 
(These commands are displayed here on two lines to avoid running off the edge of the page. In your preference file, each command should be written on a single line.)

Users running emacs under windows may find something like this useful:
   merge = Name * -> C:\Progra~1\Emacs\emacs\bin\emacs.exe -q --eval
                            "(ediff-files """CURRENT1""" """CURRENT2""")"
Users running Mac OS X (you may need the Developer Tools installed to get the opendiff utility) may prefer
    merge = Name *.txt -> opendiff CURRENT1 CURRENT2 -ancestor CURRENTARCH -merge NEW
Here is a slightly more involved hack. The opendiff program can operate either with or without an archive file. A merge command of this form
    merge = Name *.txt -> 
              if [ CURRENTARCHOPTx = x ]; 
              then opendiff CURRENT1 CURRENT2 -merge NEW; 
              else opendiff CURRENT1 CURRENT2 -ancestor CURRENTARCHOPT -merge NEW; 
              fi
(still all on one line in the preference file!) will test whether an archive file exists and use the appropriate variant of the arguments to opendiff.

Linux users may enjoy this variant:
    merge = Name * -> kdiff3 -o NEW CURRENTARCHOPT CURRENT1 CURRENT2
Ordinarily, external merge programs are only invoked when Unison is not running in batch mode. To specify an external merge program that should be used no matter the setting of the batch flag, use the mergebatch preference instead of merge.
Please post suggestions for other useful values of the merge preference to the unison-users mailing list—we'd like to give several examples here.

The User Interface

Both the textual and the graphical user interfaces are intended to be mostly self-explanatory. Here are just a few tricks:

Exit code

When running in the textual mode, Unison returns an exit status, which describes whether, and at which level, the synchronization was successful. The exit status could be useful when Unison is invoked from a script. Currently, there are four possible values for the exit status: The graphical interface does not return any useful information through the exit status.

Path specification

Several Unison preferences (e.g., ignore/ignorenot, follow, sortfirst/sortlast, backup, merge, etc.) specify individual paths or sets of paths. These preferences share a common syntax based on regular-expressions. Each preference is associated with a list of path patterns; the paths specified are those that match any one of the path pattern. Some examples of path patterns appear in the Ignoring Paths section.

Ignoring Paths

Most users of Unison will find that their replicas contain lots of files that they don't ever want to synchronize — temporary files, very large files, old stuff, architecture-specific binaries, etc. They can instruct Unison to ignore these paths using patterns introduced in the Path Patterns section.

For example, the following pattern will make Unison ignore any path containing the name CVS or a name ending in .cmo:
             ignore = Name {CVS,*.cmo}
The next pattern makes Unison ignore the path a/b:
             ignore = Path a/b
Path patterns do not skip filesnames beginning with . (as Name patterns do). For example,
             ignore = Path */tmp
will include .foo/tmp in the set of ignore directories, as it is a path, not a name, that is ignored.

The following pattern makes Unison ignore any path beginning with a/b and ending with a name ending by .ml.
             ignore = Regex a/b/.*\.ml
Note that regular expression patterns are “anchored”: they must match the whole path, not just a substring of the path.

Here are a few extra points regarding the ignore preference.

Symbolic Links

Ordinarily, Unison treats symbolic links in Unix replicas as “opaque”: it considers the contents of the link to be just the string specifying where the link points, and it will propagate changes in this string to the other replica.

It is sometimes useful to treat a symbolic link “transparently,” acting as though whatever it points to were physically in the replica at the point where the symbolic link appears. To tell Unison to treat a link in this manner, add a line of the form
             follow = pathspec
to the profile, where pathspec is a path pattern as described in the Path Patterns section.

Windows file systems do not support symbolic links; Unison will refuse to propagate an opaque symbolic link from Unix to Windows and flag the path as erroneous. When a Unix replica is to be synchronized with a Windows system, all symbolic links should match either an ignore pattern or a follow pattern.

Permissions

Synchronizing the permission bits of files is slightly tricky when two different filesytems are involved (e.g., when synchronizing a Windows client and a Unix server). In detail, here's how it works:

Cross-Platform Synchronization

If you use Unison to synchronize files between Windows and Unix systems, there are a few special issues to be aware of.

Case conflicts. In Unix, filenames are case sensitive: foo and FOO can refer to different files. In Windows, on the other hand, filenames are not case sensitive: foo and FOO can only refer to the same file. This means that a Unix foo and FOO cannot be synchronized onto a Windows system — Windows won't allow two different files to have the “same” name. Unison detects this situation for you, and reports that it cannot synchronize the files.

You can deal with a case conflict in a couple of ways. If you need to have both files on the Windows system, your only choice is to rename one of the Unix files to avoid the case conflict, and re-synchronize. If you don't need the files on the Windows system, you can simply disregard Unison's warning message, and go ahead with the synchronization; Unison won't touch those files. If you don't want to see the warning on each synchronization, you can tell Unison to ignore the files (see the Ignore section).

Illegal filenames. Unix allows some filenames that are illegal in Windows. For example, colons (`:') are not allowed in Windows filenames, but they are legal in Unix filenames. This means that a Unix file foo:bar can't be synchronized to a Windows system. As with case conflicts, Unison detects this situation for you, and you have the same options: you can either rename the Unix file and re-synchronize, or you can ignore it.

Slow Links

Unison is built to run well even over relatively slow links such as modems and DSL connections.

Unison uses the “rsync protocol” designed by Andrew Tridgell and Paul Mackerras to greatly speed up transfers of large files in which only small changes have been made. More information about the rsync protocol can be found at the rsync web site (http://samba.anu.edu.au/rsync/).

If you are using Unison with ssh, you may get some speed improvement by enabling ssh's compression feature. Do this by adding the option “-sshargs -C” to the command line or “sshargs = -C” to your profile.

Making Unison Faster on Large Files

Unison's built-in implementation of the rsync algorithm makes transferring updates to existing files pretty fast. However, for whole-file copies of newly created files, the built-in transfer method is not highly optimized. Also, if Unison is interrupted in the middle of transferring a large file, it will attempt to retransfer the whole thing on the next run.

These shortcomings can be addressed with a little extra work by telling Unison to use an external file copying utility for whole-file transfers. The recommended one is the standalone rsync tool, which is available by default on most Unix systems and can easily be installed on Windows systems using Cygwin.

If you have rsync installed on both hosts, you can make Unison use it simply by setting the copythreshold flag to something non-negative. If you set it to 0, Unison will use the external copy utility for all whole-file transfers. (This is probably slower than letting Unison copy small files by itself, but can be useful for testing.) If you set it to a larger value, Unison will use the external utility for all files larger than this size (which is given in kilobytes, so setting it to 1000 will cause the external tool to be used for all transfers larger than a megabyte).

If you want to use a different external copy utility, set both the copyprog and copyprogpartial preferences—the former is used for the first transfer of a file, while the latter is used when Unison sees a partially transferred temp file on the receiving host. Be careful here: Your external tool needs to be instructed to copy files in place (otherwise if the transfer is interrupted Unison will not notice that some of the data has already been transferred, the next time it tries). The default values are:
   copyprog      =   rsync --inplace --compress
   copyprogrest  =   rsync --partial --inplace --compress
You may also need to set the copyquoterem preference. When it is set to true, this causes Unison to add an extra layer of quotes to the remote path passed to the external copy program. This is is needed by rsync, for example, which internally uses an ssh connection, requiring an extra level of quoting for paths containing spaces. When this flag is set to default, extra quotes are added if the value of copyprog contains the string rsync. The default value is default, naturally.

If a directory transfer is interrupted, the next run of Unison will automatically skip any files that were completely transferred before the interruption. (This behavior is always on: it does not depend on the setting of the copythreshold preference.) Note, though, that the new directory will not appear in the destination filesystem until everything has been transferred—partially transferred directories are kept in a temporary location (with names like .unison.DIRNAME....) until the transfer is complete.

Fast Update Detection

If your replicas are large and at least one of them is on a Windows system, you may find that Unison's default method for detecting changes (which involves scanning the full contents of every file on every sync—the only completely safe way to do it under Windows) is too slow. Unison provides a preference fastcheck that, when set to true, causes it to use file creation times as 'pseudo inode numbers' when scanning replicas for updates, instead of reading the full contents of every file.

When fastcheck is set to no, Unison will perform slow checking—re-scanning the contents of each file on each synchronization—on all replicas. When fastcheck is set to default (which, naturally, is the default), Unison will use fast checks on Unix replicas and slow checks on Windows replicas.

This strategy may cause Unison to miss propagating an update if the modification time and length of the file are both unchanged by the update. However, Unison will never overwrite such an update with a change from the other replica, since it always does a safe check for updates just before propagating a change. Thus, it is reasonable to use this switch most of the time and occasionally run Unison once with fastcheck set to no, if you are worried that Unison may have overlooked an update.

Fastcheck is (always) automatically disabled for files with extension .xls or .mpp, to prevent Unison from being confused by the habits of certain programs (Excel, in particular) of updating files without changing their modification times.

Mount Points and Removable Media

Using Unison removable media such as USB drives can be dangerous unless you are careful. If you synchronize a directory that is stored on removable media when the media is not present, it will look to Unison as though the whole directory has been deleted, and it will proceed to delete the directory from the other replica—probably not what you want!

To prevent accidents, Unison provides a preference called mountpoint. Including a line like
             mountpoint = foo
in your preference file will cause Unison to check, after it finishes detecting updates, that something actually exists at the path foo on both replicas; if it does not, the Unison run will abort.

Click-starting Unison

On Windows NT/2k/XP systems, the graphical version of Unison can be invoked directly by clicking on its icon. On Windows 95/98 systems, click-starting also works, as long as you are not using ssh. Due to an incompatibility with ocaml and Windows 95/98 that is not under our control, you must start Unison from a DOS window in Windows 95/98 if you want to use ssh.

When you click on the Unison icon, two windows will be created: Unison's regular window, plus a console window, which is used only for giving your password to ssh (if you do not use ssh to connect, you can ignore this window). When your password is requested, you'll need to activate the console window (e.g., by clicking in it) before typing. If you start Unison from a DOS window, Unison's regular window will appear and you will type your password in the DOS window you were using.

To use Unison in this mode, you must first create a profile (see the Profile section). Use your favorite editor for this.


Installing Ssh

Warning: These instructions may be out of date. More current information can be found the Unison Wiki.

Your local host will need just an ssh client; the remote host needs an ssh server (or daemon), which is available on Unix systems. Unison is known to work with ssh version 1.2.27 (Unix) and version 1.2.14 (Windows); other versions may or may not work.

Unix

Most modern Unix installations come with ssh pre-installed.

Windows

Many Windows implementations of ssh only provide graphical interfaces, but Unison requires an ssh client that it can invoke with a command-line interface. A suitable version of ssh can be installed as follows.
  1. Download an ssh executable.

    Warning: there are many implementations and ports of ssh for Windows, and not all of them will work with Unison. We have gotten Unison to work with Cygwin's port of openssh, and we suggest you try that one first. Here's how to install it:
    1. First, create a new folder on your desktop to hold temporary installation files. It can have any name you like, but in these instructions we'll assume that you call it Foo.
    2. Direct your web browser to www.cygwin.com, and click on the “Install now!” link. This will download a file, setup.exe; save it in the directory Foo. The file setup.exe is a small program that will download the actual install files from the Internet when you run it.
    3. Start setup.exe (by double-clicking). This brings up a series of dialogs that you will have to go through. Select “Install from Internet.” For “Local Package Directory” select the directory Foo. For “Select install root directory” we recommend that you use the default, C:\cygwin. The next dialog asks you to select the way that you want to connect to the network to download the installation files; we have used “Use IE5 Settings” successfully, but you may need to make a different selection depending on your networking setup. The next dialog gives a list of mirrors; select one close to you.

      Next you are asked to select which packages to install. The default settings in this dialog download a lot of packages that are not strictly necessary to run Unison with ssh. If you don't want to install a package, click on it until “skip” is shown. For a minimum installation, select only the packages “cygwin” and “openssh,” which come to about 1900KB; the full installation is much larger.
      Note that you are plan to build unison using the free CygWin GNU C compiler, you need to install essential development packages such as “gcc”, “make”, “fileutil”, etc; we refer to the file “INSTALL.win32-cygwin-gnuc” in the source distribution for further details.
      After the packages are downloaded and installed, the next dialog allows you to choose whether to “Create Desktop Icon” and “Add to Start Menu.” You make the call.
    4. You can now delete the directory Foo and its contents.
    Some people have reported problems using Cygwin's ssh with Unison. If you have trouble, you might try other ones instead:
      http://linuxmafia.com/ssh/win32.html
    


  2. You must set the environment variables HOME and PATH. Ssh will create a directory .ssh in the directory given by HOME, so that it has a place to keep data like your public and private keys. PATH must be set to include the Cygwin bin directory, so that Unison can find the ssh executable.
    • On Windows 95/98, add the lines
          set PATH=%PATH%;<SSHDIR>
          set HOME=<HOMEDIR>
      
      to the file C:\AUTOEXEC.BAT, where <HOMEDIR> is the directory where you want ssh to create its .ssh directory, and <SSHDIR> is the directory where the executable ssh.exe is stored; if you've installed Cygwin in the default location, this is C:\cygwin\bin. You will have to reboot your computer to take the changes into account.
    • On Windows NT/2k/XP, open the environment variables dialog box:
      • Windows NT: My Computer/Properties/Environment
      • Windows 2k: My Computer/Properties/Advanced/Environment variables
      then select Path and edit its value by appending ;<SSHDIR> to it, where <SSHDIR> is the full name of the directory that includes the ssh executable; if you've installed Cygwin in the default location, this is C:\cygwin\bin.
  3. Test ssh from a DOS shell by typing
          ssh <remote host> -l <login name>
    
    You should get a prompt for your password on <remote host>, followed by a working connection.
  4. Note that ssh-keygen may not work (fails with “gethostname: no such file or directory”) on some systems. This is OK: you can use ssh with your regular password for the remote system.
  5. You should now be able to use Unison with an ssh connection. If you are logged in with a different user name on the local and remote hosts, provide your remote user name when providing the remote root (i.e., //username@host/path...).

Changes in Version 2.48.1

Changes since 2.45: Changes since 2.40.63: Changes since 2.40.1: Changes since 2.32: Changes since 2.31: Changes since 2.27: Changes since 2.17: Changes since 2.13.0: Changes since 2.12.0: Changes since 2.10.2: Changes since 2.9.20: Changes since 2.9.1: Changes since 2.8.1: Changes since 2.7.78: Changes since 2.7.39: Changes since 2.7.7: Changes since 2.7.4: Changes since 2.7.1: Changes since 2.6.59: Changes since 2.6.38: Changes since 2.6.11: Changes since 2.6.1: Changes since 2.5.31: Changes since 2.5.25: Changes since 2.5.1: Changes since 2.4.1: Changes since 2.3.12: Changes since 2.3.1: Changes since 2.2: Changes since 2.1: Changes since 1.292: Changes since 1.231: Changes since 1.219: Changes since 1.200: Changes since 1.190: Changes since 1.180: Changes since 1.169: Changes since 1.146: Changes since 1.142: Changes since 1.139: Changes since 1.111:



This document was translated from LATEX by HEVEA.