Unison is a great file-synchronization tool, but it’s highly dependent on both the unison version and the OCAML version being the same between client and server to enable syncing. I’ve wasted much time over the years trying to backport versions to ubuntu as I upgrade a laptop but not the server it backs up to or vice-versa.

This seems like a great problem to solve with containers, and fortunately it seems like it’s quite easy.

Create a new unison container on the server – Dockerfile should look like:

FROM ubuntu:22.04
RUN apt update && apt -y install unison

Then, build it:

docker build --network host -t unison .

And create /usr/bin/unison looking like:

#!/bin/bash
EXTRA=""
for g in $(id -G); do
    EXTRA="$EXTRA --group-add $g"
done
exec docker run --rm \
    -v /home/:/home/ \
    -e USER=$USER -e HOME=$HOME \
    -u "$(id -u):$(id -g)" \
    $EXTRA \
    -v /etc/passwd:/etc/passwd:ro \
    -v /etc/group:/etc/group:ro \
    --network host \
    -i \
    unison \
    unison $@

It’s necessary to run on host network so it picks up the server’s hostname as unison is highly dependent on this, and the environment variables that are passed in.

Then it seems to work just fine. Happy days!