Preserve shell and userland semantics

Establish the target shell and installed utilities where they affect the command. The login shell, script interpreter, and userland can differ. macOS can run GNU utilities; Linux can use BusyBox. Do not infer the executable from the OS or a rejected --version.

Failure-prone differences

• zsh does not split unquoted $var like bash, and an unmatched glob normally errors. Use explicit arguments or arrays; quote patterns intended for tools.
• zsh can expand a word starting with = as a command path. Quote literal separators such as '==='.
• /bin/bash is not universal; macOS's system bash is 3.2. An environment shebang still needs a suitable interpreter in PATH. Avoid associative arrays, mapfile, and ${var,,} on that older target.
• set -e is context-sensitive, not universal failure handling. Handle expected nonzero statuses explicitly and preserve pipeline failures where supported. Do not add strict-mode flags without checking the script's control flow and target shell.
• Preserve arbitrary filenames with NUL-delimited processing or find ... -exec ... {} +; never parse ls. Quote expansions and protect leading-dash operands using the command's supported syntax.
• Create temporary paths securely, quote cleanup, and remove only paths the invocation owns. Replacing a file can change permissions, links, or metadata.

For sed -i, dates, stat, canonical paths, xargs, checksums, and timeouts, consult references/gnu-bsd.md for the relevant difference rather than loading every platform recipe.

Use available shell checks and exercise the affected behavior on reachable targets. Name the tested shell/userland and any untested target; one Linux run does not prove macOS compatibility.