Install Unison 2.48.4 on Mac OS X with Homebrew

I use Unison to sync code between my local machine and my dev servers. To sync between two servers, it requires that the same version of Unison be installed on both servers.

Now, this isn’t usually a big deal, because once you get Unison set up, it’s set up. But, I usually get a bit frustrated when setting up a new development machine and ensuring that it has the same Unison version as my remote server.

Most recently, I needed to get Unison 2.48.4 on my local Mac so that it matched my remote server. BUT, homebrew didn’t support Unison 2.48.4.

So, after getting some feedback from one of my coworkers, we came up with the following. Maybe you’ll find it helpful.

# Get rid of existing Unison
brew uninstall --force unison

# Checkout version of homebrew with Unison 2.48.4
cd /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core
git checkout 05460e0bf3ae5f1a15ae40315940b2d39dd6ac52 Formula/unison.rb

# Install
brew install --force-bottle unison

# Set homebrew-core back to normal
git checkout master
git reset HEAD .
git checkout -- .

NOTE: If you get error: fatal: reference is not a tree: 05460e0bf3ae5f1a15ae40315940b2d39dd6ac52 after running git checkout 05460e0bf3ae5f1a15ae40315940b2d39dd6ac52 Formula/unison.rb, we’ve been able to fix the issue by recloning homebrew-core. If you get the same error, you’ll want to add these steps before retrying starting at the git checkout 05460e0bf3ae5f1a15ae40315940b2d39dd6ac52 Formula/unison.rb command above.

cd /usr/local/Homebrew/Library/Taps/homebrew
rm -rf homebrew-core
git clone
cd homebrew-core

28 responses to “Install Unison 2.48.4 on Mac OS X with Homebrew”

  1. This post has helped at least to automatticians to fix their unison environment. Thank you! It would have helped three probably, but I lost my faith in unison long ago.

      1. Miguel Lezama Avatar

        make it three!

  2. Woohoo! Thank you!

  3. Were there any additional commands that needed to be run after git checkout master command?

    1. From memory, I don’t believe so. Are you running into an issue?

      1. Thanks for the reply. If I try to run git checkout 05460e0bf3ae5f1a15ae40315940b2d39dd6ac52 Formula/unison.rb
        I’m getting the error: fatal: reference is not a tree: 05460e0bf3ae5f1a15ae40315940b2d39dd6ac52

        1. What does it look like if you do the following commands in /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core?

          • git status
          • git remote -v
          1. Says that I’m up to date with master and that the remote origins are pointing to the github repo for fetch and push.

          2. I’m experiencing this same exact problem as well.

        2. Hi Steven –

          Working with Jeff, who commented below, we were able to replicate the issue and come up with a fix. I’ve updated the post with some updated commands.

          Hope this helps.

          1. THANKS ERIC YOU’RE THE BEST! Works per the updated post.

  4. Thanks Eric!

    FYI: My git checkout silently failed to overwrite the existing Formula/unison.rb file so I had to remove that before checking it out. If I didn’t, homebrew kept installing the latest version.

  5. A less alarming way to fully clone homebrew-core is:

    cd /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core

    git fetch –unshallow

  6. Thanks, Eric!

  7. A couple enhancements I’ve found —

    export HOMEBREW_NO_INSTALL_CLEANUP=1 in my .bashrc file ensures that homebrew keeps the old versions of stuff around instead of deleting them.

    Also, brew uninstall --force unison can be swapped out for brew unlink unison — which will remove the unison alias but leave the other version in place so the old version can be installed.

    After this is all done, you should have two versions of the unison binary available, if you need to be able to connect to multiple servers running different unisons.

    … actually now that I think about it, the following may just work for everything:

    cd /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core
    git fetch --unshallow
    brew list unison --versions
    brew switch unison 2.48.4_1

    After the brew list unison --versions the version 2.48 may be available again? If not then do the git checkout stuff above

  8. This has been super helpful, thanks a lot for posting this!

  9. This comment also at
    Example below shows general method to find and install commits for other older or newer versions of unison, in this case, 2.48.15 …
    It follows advice from

    Note: this is ok for simple binary bottles, like unison, with few, if any, dependencies. But dependencies quickly break things if you try such tricks to try to have something like both python versions 3.69 and 3.74: pip3 breaks for example as shown at in python.rb per comments at

    cd /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core

    git log master -- Formula/unison.rb
    commit f5755fe4ccecb63c669d633f804428267d3bf3d1
    Author: BrewTestBot <>
    Date: Tue Feb 20 12:46:06 2018 +0000

    unison: update 2.48.15 bottle.

    $ brew unlink unison
    $ brew install
    ==> Consider using brew extract unison ...!
    This will extract your desired unison version to a stable tap instead of
    installing from an unstable URL!

    ######################################################################## 100.0%

    $ brew switch unison
    unison installed versions: 2.48.4_1, 2.48.15, 2.51.2

  10. Thank you for writing this up Eric! Worked a treat when I set up my new machine. ????

  11. Thank you Eric! This is still relevant in 2020… ran into the ‘not a tree’ issue, and the workaround steps worked perfectly.

    1. Wonderful! I’m glad it was helpful for you. Thank you for sharing. ?

  12. The work around didn’t work for me but installing it with this did:

    brew install

    I got the same git errors but after fixing them by cloning again it still installed the latest version of Unison. No idea why but glad something worked. Thanks Eric and everyone for contributing. 🙂

  13. Thank you, Eric. I’ve installed unison 2.48.4 on macOS 11.1 by your guide.

  14. It seems that 2.48 can’t be fetched anymore.
    The install command ends up trying to download from, which returns a 404.
    Is there a way to get over this? Can I force Homebrew to use another url?
    Thanks for your help.

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