fix-broken-precommit-hook

name	fix-broken-precommit-hook
description	Diagnose and fix a git commit hook (pre-commit, pre-push, etc.) that fails to exec even though the file exists and is executable — usually a stale /nix/store shebang path after a nix store gc.
allowed-tools	Bash

Fix Broken Pre-Commit Hook

Symptom

git commit (or push) fails with:

fatal: cannot exec '.git/hooks/pre-commit': No such file or directory

...but ls -la .git/hooks/pre-commit shows the file exists and is executable. This message is misleading — git isn't lying about the hook file, it's reporting a failed exec(), which usually means the interpreter named in the hook's shebang line no longer exists on disk.

Common root cause on Nix-based projects: pre-commit (https://pre-commit.com) generated the hook with a shebang pinned to a specific Nix store path (e.g. #!/nix/store/<hash>-bash-5.3p9/bin/bash). If that store path is later garbage-collected (nix-collect-garbage, nix store gc), the shebang points at nothing and every exec of the hook fails.

Diagnosis

1. Confirm the hook file exists and check for a configured custom hooks path:

   git config --get core.hooksPath
   ls -la .git/hooks/pre-commit
   

2. Inspect the shebang line:

   head -5 .git/hooks/pre-commit
   

3. Check whether the interpreter path in the shebang actually exists:

   ls <path-from-shebang>
   

   If this reports "No such file or directory", that's the root cause — the shebang target was gc'd.

Fix

Regenerate the hook inside the project's Nix dev shell so the new shebang points at a store path that currently exists:

nix develop --command pre-commit install --overwrite

If this errors with:

[ERROR] Cowardly refusing to install hooks with `core.hooksPath` set.
hint: `git config --unset-all core.hooksPath`

check what it's set to — git config --get core.hooksPath. If it just points at the default .git/hooks (redundant, no functional effect), it's safe to unset and retry:

git config --unset-all core.hooksPath
nix develop --command pre-commit install --overwrite

git config edits are a "confirm with the user first" class of change — don't run the unset unprompted. If core.hooksPath points somewhere non-default, stop and ask before touching it; something else may depend on that custom path.

Then retry the commit. If pre-commit isn't managing hooks for this repo (no .pre-commit-config.yaml, or a different hook manager), regenerate via whatever installed the hook originally — but the underlying fix is always "reinstall the hook from inside the environment that has a live interpreter," not to hand-edit the shebang.

Notes

• Don't delete .git/hooks/pre-commit as a shortcut — investigate the shebang first; the hook itself is very likely fine.
• Never bypass with git commit --no-verify to work around this — that skips the checks the hook exists to enforce.
• This class of bug isn't specific to pre-commit: any hook (or any script) with an absolute Nix store shebang will break the same way after a gc.