Unable to install flox on Mac OS (Apple Silicon)

Help
1

I’ve tried both downloading the .pkg file directly and running brew install flox, but both methods consistently fail for me.

I had a similar issue earlier with flox-1.3.17, and I was hoping the newer version flox-1.4.0 would resolve it — unfortunately, I’m still stuck at the installation step.

Just for context, I was able to use Flox successfully back when it was in beta, so I’m wondering if this issue might be related to leftover artifacts or a previous install.

Here’s the error I get via Homebrew:

♪ brew install flox

==> Downloading https://downloads.flox.dev/by-env/stable/osx/flox-1.4.0.aarch64-darwin.pkg
Already downloaded: /Users/[snip]/Library/Caches/Homebrew/downloads/47d2370d1a1177dfac509fe889420a9ad4d5930fa36f6eb50195d2087e6153fe--flox-1.4.0.aarch64-darwin.pkg
==> Installing Cask flox
==> Running installer for flox with sudo; the password may be necessary.
Password:
installer: Package name is Flox
installer: Upgrading at base path /
installer: The upgrade failed. (The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance. An error occurred while running scripts from the package “flox-1.4.0.aarch64-darwin.pkg”.)
==> Purging files for version 1.4.0 of Cask flox
Error: Failure while executing; `/usr/bin/sudo -u root -E LOGNAME=[snip] USER=[snip] USERNAME=[snip] -- /usr/sbin/installer -pkg /opt/homebrew/Caskroom/flox/1.4.0/flox-1.4.0.aarch64-darwin.pkg -target /` exited with 1. Here's the output:
installer: Package name is Flox
installer: Upgrading at base path /
installer: The upgrade failed. (The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance. An error occurred while running scripts from the package “flox-1.4.0.aarch64-darwin.pkg”.)

Would appreciate any help in figuring this out.

2

It might be the case that there are still some artifacts leftover from a previous installation.

You can try zapping the installation, rebooting, and doing a clean installation.

$ brew uninstall --force --zap flox

Note that this will remove the installed software under /nix/store as well, and remove the volume that mounts /nix/store.

If that doesn’t help, would you mind providing me with the installer logs? I believe they’re located under /var/log/install.log on macOS.

1 Like
3

I was able to get this to work.

TL;DR: Version

Tried brew uninstall --zap flox, but it failed to unmount /nix and couldn’t remove everything cleanly.
Install attempts kept failing — turns out the postinstall script choked because _nixbld8 already existed with UID 308.
Manually deleted all _nixbld* users.
After that, brew install flox worked perfectly.

Big thanks @bryanhonof — tailing /var/log/install.log helped isolate the issue!

Detailed Version

Tried brew uninstall --force --zap flox, it asked to unmount the APFS volume Nix Store - /nix.

♪ brew uninstall --force --zap flox
==> Implied `brew uninstall --cask flox`
==> Removing launchctl service org.nixos.darwin-store
Password:

==> Removing launchctl service org.nixos.nix-daemon
==> Running uninstall script /usr/local/share/flox/scripts/uninstall
/usr/local/share/flox/scripts/uninstall: line 16: /usr/local/bin/nix: No such file or directory
==> Uninstalling packages with sudo; the password may be necessary:
com.floxdev.flox
==> Dispatching zap stanza
==> Running uninstall script /usr/local/share/flox/scripts/uninstall_zap
   7:                APFS Volume Nix Store               6.6 GB     disk3s7
Unmount failed for /nix
Load failed: 5: Input/output error
Try running `launchctl bootstrap` as root for richer errors.
Load failed: 5: Input/output error
Try running `launchctl bootstrap` as root for richer errors.
Could not unmount /nix; make sure no processes using Nix or Flox are running, and then re-run this uninstaller.
Error: Failure while executing; `/usr/bin/sudo -E -- /usr/local/share/flox/scripts/uninstall_zap --zap` exited with 1. Here's the output:
   7:                APFS Volume Nix Store               6.6 GB     disk3s7
Unmount failed for /nix
Load failed: 5: Input/output error
Try running `launchctl bootstrap` as root for richer errors.
Load failed: 5: Input/output error
Try running `launchctl bootstrap` as root for richer errors.
Could not unmount /nix; make sure no processes using Nix or Flox are running, and then re-run this uninstaller.

Tried unmounting, but that didn’t help, so proceeded with deleting the volume Nix Store.

♪ sudo diskutil apfs deleteVolume disk3s7
Started APFS operation
Deleting APFS Volume from its APFS Container
Unmounting disk3s7
Erasing any xART session referenced by 858D0DD9-0AF9-406D-AAC9-0C242EDA03CD
Deleting Volume
Removing any Preboot and Recovery Directories
Finished APFS operation

Even then the installation was failing, so tail-ed /var/log/install.log as you suggested and that helped figure out the issue.

The installer was failing during the postinstall step because a system user named _nixbld8 already existed with UID 308

[snip]

2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: ++ echo 358
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + uid=358
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + task 'Setting up the build user _nixbld8'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + echo ''
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + ok '~~> Setting up the build user _nixbld8'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + _textout '\033[32m' '~~> Setting up the build user _nixbld8'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + echo -en '\033[32m'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + shift
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + '[' '~~> Setting up the build user _nixbld8' = '' ']'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + echo '~~> Setting up the build user _nixbld8'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: ~~> Setting up the build user _nixbld8
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + echo -en '\033[0m'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + poly_user_exists _nixbld8
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + /usr/bin/dscl . -read /Users/_nixbld8
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: ++ poly_user_id_get _nixbld8
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: ++ dsclattr /Users/_nixbld8 UniqueID
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: ++ /usr/bin/dscl . -read /Users/_nixbld8
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: ++ /usr/bin/awk '/UniqueID/ { print $2 }'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + actual_uid=308
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + '[' 308 '!=' 358 ']'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + failure
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + header 'oh no!'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + follow=---------------------------------------------------------
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: ++ echo '---- oh no! ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: ++ head -c 80
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + header='---- oh no! --------------------------------------------------------------------'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + echo ''
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + _textout '\033[34m' '---- oh no! --------------------------------------------------------------------'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + echo -en '\033[34m'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + shift
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + '[' '---- oh no! --------------------------------------------------------------------' = '' ']'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + echo '---- oh no! --------------------------------------------------------------------'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: ---- oh no! --------------------------------------------------------------------
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + echo -en '\033[0m'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + _textout '\033[31m'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + echo -en '\033[31m'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + shift
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + '[' '' = '' ']'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + cat
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: It seems the build user _nixbld8 already exists, but with the UID
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: with the UID '308'. This script can't really handle that right
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: now, so I'm going to give up.
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: If you already created the users and you know they start from
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: 308 and go up from there, you can edit this script and change
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: NIX_FIRST_BUILD_UID near the top of the file to 308 and try
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: again.
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + echo -en '\033[0m'
2025-04-18 00:43:44+05:30 [snip] package_script_service[4279]: ./postinstall: + echo ''

[snip]

Previous installs had created these users…

♪ dscl . -list /Users | grep nixbld
_nixbld1
_nixbld10
_nixbld11

[snip]

_nixbld32
_nixbld4
_nixbld5
_nixbld6
_nixbld7
_nixbld8
_nixbld9

I cleared these using the following command.

♪ sudo dscl . -delete /Users/_nixbld1
[snip]
♪ sudo dscl . -delete /Users/_nixbld32

This time the install worked fine. :partying_face:

 ♪ brew install flox
==> Downloading https://formulae.brew.sh/api/formula.jws.json
==> Downloading https://formulae.brew.sh/api/cask.jws.json
==> Downloading https://downloads.flox.dev/by-env/stable/osx/flox-1.4.0.aarch64-darwin.pkg
Already downloaded: /Users/[snip]/Library/Caches/Homebrew/downloads/47d2370d1a1177dfac509fe889420a9ad4d5930fa36f6eb50195d2087e6153fe--flox-1.4.0.aarch64-darwin.pkg
==> Installing Cask flox
==> Running installer for flox with sudo; the password may be necessary.
Password:

installer: Package name is Flox
installer: Installing at base path /
installer: The install was successful.
🍺  flox was successfully installed!
==> `brew cleanup` has not been run in the last 30 days, running now...
Disable this behaviour by setting HOMEBREW_NO_INSTALL_CLEANUP.
Hide these hints with HOMEBREW_NO_ENV_HINTS (see `man brew`).
==> Autoremoving 1 unneeded formula:
libgit2@1.7
Uninstalling /opt/homebrew/Cellar/libgit2@1.7/1.7.2... (105 files, 4.5MB)
Removing: /Users/[snip]/Library/Caches/Homebrew/Cask/keycastr--0.10.2.zip... (2.4MB)
Pruned 0 symbolic links and 3 directories from /opt/homebrew

♪ flox --help
Flox is a virtual environment and package manager all in one.

With Flox you create environments that layer and replace dependencies just where
it matters, making them portable across the full software lifecycle.

[snip]
1 Like