Unison

Unison is a bidirectional file synchronization tool that runs on Unix-like operating systems (including Linux, macOS, and Solaris) 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.

Installation

Install the unison package, which provides CLI, GTK and GTK 2.0 interfaces.

Configuration

In order to use Unison, you need to create a profile.

GUI

To configure Unison with the GUI run unison-gtk2.

Manual

Alternatively, manually create a profile in ~/.unison and add the following lines to the default configuration file, ~/.unison/profilename.prf.

Define the root directory to be synchronized.

root=/home/user/

Define the remote directory where the files should be sychronized to.

root=ssh://example.com//path/to/server/storage

Optionally, provide arguments to SSH.

sshargs=-p 4000

Define which directories and files should be synchronized:

# dirs
path=Documents
path=Photos
path=Study
# files
path=.bashrc
path=.vimrc

You can also define which files to ignore:

ignore=Name temp.*
ignore=Name .*~
ignore=Name *.tmp

Note: For more information see the Sample profiles in the User Manual and Reference Guide.

Usage

Once your profile is set up, you can start syncing:

$ unison profilename

or using the GUI tool:

$ unison-gtk2

and select the profile. Unison has a nice interface where you can view the progress and changes.

Version incompatibility

For Unison to function properly, both ends must have installed the same Unison version compiled with the same version of OCaml.

When synchronizing between Arch and another distribution you will most likely have to manually compile OCaml and Unison on one end.

The AUR contains unofficial PKGBUILDs for versions 2.32 (unison-232-compat^AUR) and 2.40 (unison-240-compat^AUR).

Tips and tricks

Save human time and keystrokes

If one runs unison within a terminal emulator capable of maintaining a suitable scrollback buffer, there is no purpose in having to confirm every non-conflicting change; set the auto option to true to avoid these prompts.

More helpful diff output

The unison default diff command is diff -u CURRENT2 CURRENT1. When looking at the output of this command, it can be difficult to remember which changes will be kept when propagating from left to right ('>'), versus right to left ('<'). The following configuration makes it easy to remember: '>' keeps lines which start with '>':

diff = diff -u CURRENT2 CURRENT1 | perl -pe 's/^\+/>/; s/^\-/</'

Merging in Emacs

Unison has the capability to assist users in merging two conflicting files using an external merge program, but it does not configure such a program by default. The manual suggests

merge = Name *.txt -> emacs -q --eval '(ediff-merge-files-with-ancestor "CURRENT1" "CURRENT2" "CURRENTARCH" nil "NEW")'

This assumes that you are running Unison in X, because the merge command cannot be run in the terminal (Emacs: "standard input is not a tty"). Note also that Unison replaces the CURRENT1, etc., variables with single-quoted filenames. Thus, the above works, but using double quotes throughout, as in "(ediff-merge-files... \"CURRENT1\" ...)", would not work.

Using the variable CURRENTARCH tells Unison that you expect to do 3-way merges with a common ancestor, which is only possible if the "backupcurrent" preference has been set previously to the last sync. To perform an ordinary 2-way merge in a terminal, one could use the following configuration instead. This also uses emerge.el, which some find preferable to ediff.el:

merge = Name {*,.*} -> urxvt -e emacs -nw -q --eval '(emerge-files nil "CURRENT1" "CURRENT2" "NEW")'

If the variable CURRENTARCHOPT is used instead of CURRENTARCH, then Unison will provide a common ancestor when it is available, and otherwise fall back to requesting a 2-way merge (by setting the variable to the empty string). This can be detected in a shell script. For example:

merge = Name {*,.*} -> unison-merge-files CURRENT1 CURRENT2 NEW CURRENTARCHOPT

with unison-merge-files defined as follows:

#!/bin/sh
CURRENT1=$1
CURRENT2=$2
NEW=$3
CURRENTARCHOPT=$4
EMACS="urxvt -e emacs -nw"
if [ x$CURRENTARCHOPT = x ]; then
    $EMACS --eval "(emerge-files nil \"$CURRENT1\" \"$CURRENT2\" \"$NEW\")";
else
    $EMACS --eval "(emerge-files-with-ancestor nil \"$CURRENT1\" \"$CURRENT2\" \"$CURRENTARCHOPT\" \"$NEW\")";
fi

Common configuration synchronization

When syncing configuration files which would vary (e.g., due to endemic applications, security-sensitive configuration) among systems (servers, workstations, laptops, smartphones, etc.) but nevertheless contain common constructs (e.g., key bindings, basic shell aliases), it would be apt to separate such content into separate configuration files (e.g., .bashrc_common), and sync only these.

Warning: Bidirectional syncing of configuration files can lend itself to become an avenue for an attack, by enabling the peer syncing system to receive malicious changes to configuration files (and perhaps even other peers the system syncs with). This is an attractive option for adversaries, especially when the conceptual security levels of the two systems differ (e.g., public shell server vs. personal workstation), since it would likely be simpler to compromise a system of lower security. Always use the noupdate option when bidirectional syncing between two particular systems is deemed unnecessary; when necessary, verify each change when syncing. Automatic bidirectional syncs should be done with extreme caution.