minor code style and runtime fixes to be more standard-compliant

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+### Changed
+
+- universal shebang bash interpreter to safely find bash on `$PATH`
+
+### Removed
+
+
+### Fixed
+
+- in the preflight check: Prefer our user-friendly error messages while avoiding double error message from the operating system by redirecting also `stderr` to `/dev/null`
+- potential runtime error: prefer `command -v` over `which`-command, which can have different behaviour on different Linux distributions and always use the first result in `PATH` independently of multiple defined commands