NixOS Zsh Shell Bug On MacOS After Upgrade

Experiencing issues with your Zsh shell on macOS after upgrading your NixOS configuration can be a real headache, especially when essential customizations like aliases and prompt settings disappear. This article delves into a common bug encountered when moving from Nixpkgs version 24.11 to 25.11, specifically impacting Zsh and the overall shell experience on macOS systems managed by Home Manager. We'll explore the symptoms, the potential causes, and how to troubleshoot and resolve this frustrating issue, ensuring your Zsh environment is back to its functional best.

The Zsh Shell Conundrum After a NixOS Upgrade

A critical bug has surfaced when upgrading NixOS configurations, particularly when migrating from version 24.11 to 25.11. This issue prominently affects users on macOS who manage their system with Home Manager. The primary symptom is a complete breakdown of Zsh functionality – no aliases work, prompt configurations are lost, and the shell behaves erratically. A peculiar manifestation of this bug is the creation of a .zshenv file in the user's home directory. This file, intended to provide environment variables, instead contains a recursive source command, leading to errors like /Users/amon/.zshenv:1: job table full or recursion limit exceeded. This error is a clear indicator that the shell is caught in an infinite loop, unable to properly load its configuration.

Furthermore, the issue can lead to a cascade of other problems. You might notice that other configuration files, like .bashrc, are also affected, displaying unexpected content or becoming irrelevant to your shell setup. The file system might show a confusing array of symlinks and backup files, such as .zshrc.bak, .zshenv.hmbackup, and numerous .zcompdump files, all pointing towards a disorganized or corrupted configuration state. These artifacts are not just cosmetic; they represent a deeper problem with how Home Manager is interacting with Zsh and the user's environment after the upgrade.

The core of the problem often lies in how Home Manager generates and manages shell configuration files. In previous versions, it might have handled these files differently, but the transition to 25.11 appears to have introduced a conflict or a change in behavior that breaks the expected Zsh loading sequence. The recursive source command within .zshenv is particularly problematic because it causes Zsh to repeatedly try to load itself, consuming system resources and preventing any actual shell commands from executing. This creates a situation where the terminal is effectively unusable for interactive sessions.

Troubleshooting this requires a systematic approach. First, it's essential to identify the exact point of failure. Examining the contents of the generated .zshenv file is crucial, as it directly points to the recursive sourcing issue. Next, checking other shell configuration files like .zshrc and .bashrc can provide clues about how Home Manager is attempting to integrate with your system. The presence of numerous backup files also suggests that Home Manager might be creating new configurations without properly cleaning up or replacing old ones, leading to conflicts.

The version mismatch between Nixpkgs and Home Manager can also be a source of such problems. While the user in this case confirmed their versions were in sync, it's always a good practice to double-check this aspect. Ensuring that both Nixpkgs and Home Manager are updated to compatible versions is a prerequisite for resolving upgrade-related bugs. The journey to a stable shell environment after an upgrade can be challenging, but understanding the specific error messages and file behaviors will guide you toward a solution.

Diagnosing the Zsh Configuration Chaos

Understanding the root cause of the Zsh shell breakage after upgrading to Nixpkgs 25.11 is the first step toward a resolution. The user's report highlights a critical symptom: Home Manager creating a .zshenv file that contains a self-referential source command, leading to an infinite loop and shell errors. This recursive sourcing is the immediate culprit preventing Zsh from functioning correctly. When Zsh starts, it reads configuration files in a specific order. If .zshenv is incorrectly configured to source itself, Zsh gets stuck trying to re-execute the same configuration file repeatedly, never reaching the point where it can process user commands or load other necessary configuration files like .zshrc.

The presence of .zshenv itself is not inherently problematic. This file is designed to be sourced by all Zsh invocations, including interactive shells, login shells, and non-login shells. It's typically used for setting environment variables that should be available globally. However, when Home Manager, or any other tool, misconfigures it to source itself, it becomes a disaster. This specific error, "job table full or recursion limit exceeded," is a classic sign of such a loop. The shell runs out of resources to manage the nested execution context.

Investigating the symlinks in the home directory provides further diagnostic information. The user's ls -alh output shows that many configuration files, including .p10k.zsh, .spacemacs, and .aerospace.toml, are symlinks managed by Home Manager. This indicates that Home Manager is actively managing these files, pointing to the Nix store. However, the problematic .zshenv symlink, pointing to /nix/store/kiq3av7vbkrgz045i5hdnngnyghb7han-home-manager-files/.zshenv, is where the issue originates. The content of this file in the Nix store, when sourced by Zsh, is what causes the loop.

The bat .zshenv output is pivotal: it shows source /Users/amon/.zshenv. This means Zsh is trying to execute the file located at .zshenv in the home directory. If this .zshenv file itself contains the problematic source command, the loop is confirmed. It's crucial to understand why Home Manager generated such a faulty .zshenv. This could be due to a bug in Home Manager's 25.11 version, an incompatibility with the Nixpkgs version, or a misconfiguration in the user's home.nix or flake definition.

The ls -alh output also reveals backup files like .zshrc.bak and .zshenv.hmbackup. These backups suggest that Home Manager might have attempted to update or replace existing configurations, but the process resulted in the broken .zshenv file being generated. The presence of numerous .zcompdump files, related to Zsh's completion system, can also be a side effect of repeated Zsh startups and failures. These files are usually generated when Zsh builds its completion cache, and a broken startup can lead to their excessive regeneration or corruption.

To diagnose effectively, one should:

1. Inspect the problematic .zshenv: Verify its exact content, especially the source command.
2. Examine the Nix store path: Look at the actual file content at /nix/store/kiq3av7vbkrgz045i5hdnngnyghb7han-home-manager-files/.zshenv to see what Home Manager is trying to make Zsh execute.
3. Review Home Manager configuration: Scrutinize the Nix configuration related to Zsh and shell environments within the user's flake.
4. Check Nixpkgs and Home Manager versions: Ensure they are indeed compatible and up-to-date, or consider downgrading temporarily to pinpoint the regression.

By carefully examining these elements, the specific configuration error leading to the Zsh shell meltdown can be pinpointed. This methodical approach is key to untangling the complex interactions within a Nix-managed environment.

Fixing the Zsh Shell After the NixOS Upgrade

Resolving the Zsh shell issues after upgrading to Nixpkgs 25.11 requires a targeted approach, focusing on correcting the faulty configuration generated by Home Manager. The primary goal is to eliminate the recursive sourcing loop in .zshenv and ensure Zsh loads correctly.

The most direct fix involves correcting the Home Manager configuration that generates the problematic .zshenv file. Since Home Manager is responsible for creating these symlinks and configuration files, the error likely stems from how Zsh-related options are defined in the user's home.nix or equivalent flake configuration. Often, issues arise from how home.shell.zsh.enable or similar settings are used, or how custom Zsh configurations are integrated.

For users facing the specific .zshenv recursion issue, the first step is to identify and correct the problematic configuration in your Nix files. This might involve:

1. Disabling or correcting Zsh configuration: If you have custom Zsh configurations within your Home Manager settings, carefully review them. There might be an option or a setting that is inadvertently causing the recursive source command. For example, ensure that any custom shellHook or environment variable settings do not inadvertently try to re-source critical shell files.
2. Leveraging Home Manager's Zsh options: Home Manager provides specific options for configuring Zsh. It's generally recommended to use these options rather than manually creating or modifying files like .zshenv outside of the Nix configuration, as this can lead to conflicts and unexpected behavior. Review the Home Manager manual for the most up-to-date and recommended way to configure Zsh, including prompts, plugins, and aliases.
3. Cleaning up the Nix store and user profile: After correcting the configuration, it's often necessary to clean up any remnants of the faulty setup. This can involve running nix-collect-garbage -d to clean up old store paths and potentially removing the problematic symlinks from your home directory before rebuilding your configuration. However, it's generally better to let Home Manager manage these files entirely.

A temporary workaround, while a permanent fix is being implemented or identified, could be to manually remove the symlink pointing to the faulty .zshenv and create a minimal, working .zshenv (or even remove it entirely if not strictly necessary for your environment). However, this is not a sustainable solution, as a home-manager switch will likely recreate the problematic file.

The user's repository https://github.com/Locbac/nix/ is crucial for debugging this specific instance. By examining the flake and home.nix files within that repository, one could pinpoint the exact configuration that leads to the .zshenv problem. It's possible that a specific combination of Nixpkgs 25.11 features and Home Manager's internal handling of Zsh configurations triggers this bug.

Consider the following steps for a robust fix:

• Review home-manager's Zsh integration: Check documentation for how Zsh is supposed to be configured with Home Manager in version 25.11. Look for any deprecated options or new requirements.
• Simplify your Zsh configuration: Temporarily remove custom Zsh settings from your Nix configuration to see if the issue persists. If it resolves the problem, reintroduce your customizations one by one to find the culprit.
• Check for known issues: Search the Home Manager and Nixpkgs issue trackers for similar reports related to Zsh configuration in version 25.11.
• Report the bug: If you've identified a clear bug in Home Manager or Nixpkgs, filing a detailed bug report with a minimal reproducible example is essential for the community to address it.

By systematically addressing the Nix configuration and ensuring Home Manager is correctly set up to manage Zsh, you can restore your shell's functionality and prevent future occurrences of this disruptive bug.

Conclusion: Reclaiming Your Zsh Experience

Upgrading system configurations, especially with powerful tools like Nix and Home Manager, can sometimes introduce unexpected challenges. The bug where upgrading to Nixpkgs 25.11 breaks Zsh functionality on macOS, manifesting as a recursive .zshenv loop, is a prime example of such an issue. As we've explored, this problem arises from how Home Manager generates shell configuration files, leading to errors like "job table full or recursion limit exceeded." The diagnostic steps involve carefully inspecting the generated .zshenv file, understanding the symlinks in your home directory, and reviewing your Nix configuration.

The solution typically lies in correcting the Home Manager configuration that dictates Zsh's setup. This might involve adjusting specific Zsh-related options, ensuring compatibility with the latest Nixpkgs version, or simplifying your custom configurations to pinpoint the source of the conflict. The goal is always to let Nix and Home Manager manage your shell environment predictably and reliably.

While encountering such bugs can be frustrating, the Nix ecosystem's strength lies in its community and the transparency of its configuration. By diligently diagnosing the problem, leveraging community resources, and contributing bug reports, we can collectively improve these tools. Remember to always keep your Nixpkgs and Home Manager versions in sync and to consult the official documentation for the most current and recommended configuration practices.

For further assistance and to stay updated on Nix and Home Manager developments, consider exploring the official documentation and community forums. A great resource for understanding Nix is the NixOS Wiki, which provides extensive information on Nix, NixOS, and Home Manager.