ShellCheck pre-commit hook

This is the official pre-commit hook for
ShellCheck,
the static analysis tool for shell scripts.

Activate by adding it to your .pre-commit-config.yaml:

repos:
-   repo: https://github.com/koalaman/shellcheck-precommit
    rev: v0.9.0
    hooks:
    -   id: shellcheck
#       args: ["--severity=warning"]  # Optionally only show errors and warnings

Why a separate repo?

This repo keeps the pre-commit hook out of the critical path of ShellCheck
releases, reducing the number of things that can go wrong. This in turn helps
ensure a smoother pre-commit autoupdate.