Write a robust shell script

Shell scripts written in a hurry fail in predictable ways: an unset variable expands to an empty string and rm -rf hits the wrong directory, a failed command in a pipeline is ignored, a filename with a space splits in two, GNU-only flags break on macOS, and a second run duplicates what the first run did. The person needs a script they can run twice, read in a year, and trust in a dry run first.

Write a bash script for this goal, to run on any:

[GOAL]

1. If the goal leaves out something that decides what gets deleted, overwritten or sent (which paths, which hosts, whether it needs root), ask up to 3 questions and stop. Otherwise state your assumptions and continue.
2. Choose the strict-mode preamble for the shell:

• bash: set -Eeuo pipefail, a trap that reports the failing line on ERR, and a cleanup trap on EXIT.
• zsh: emulate -L zsh and setopt ERR_EXIT NO_UNSET PIPE_FAIL.
• posix-sh: set -eu. Do not rely on pipefail, arrays, [[ ]], local or $'...'; check pipeline stages explicitly where failure matters.
• powershell: a param() block with [CmdletBinding(SupportsShouldProcess)], Set-StrictMode -Version Latest and $ErrorActionPreference = 'Stop'; check $LASTEXITCODE after native commands.

1. Parse arguments: -h/--help (usage to stdout, exit 0), long options, required values validated up front, unknown options rejected with usage on stderr and exit 2. In bash and zsh use a while/case loop so long options work; getopts handles only short ones.
2. Add a dry-run mode (--dry-run, or -WhatIf in PowerShell) that prints every state-changing command, safely quoted, instead of running it. Route all side effects through one helper so dry run cannot miss one.
3. Make each step idempotent: test before acting, use mkdir -p and ln -sfn, check before appending to a file, and write files to a temp file on the same filesystem and then move it into place.
4. Fail clearly: check required tools with command -v at start-up, print errors to stderr with the script name and a fix, and use distinct non-zero exit codes for distinct failures.
5. Check the script against ShellCheck (or PSScriptAnalyzer) rules in your head, and fix anything they would flag.

• Quote every expansion. Use -- before user-supplied paths. Never parse ls; use find ... -print0 with while IFS= read -r -d '' (bash/zsh) or a glob loop.
• Guard destructive commands against empty variables with ${VAR:?}, and never rm -rf a path built from unchecked input.
• Portability for macOS and Linux: macOS ships bash 3.2 (no associative arrays, mapfile or ${var,,}) and BSD tools (sed -i '', no date -d, no grep -P, different stat flags). If target_os is any or macos, avoid these or branch on uname explicitly.
• No secrets in the script, arguments or logs. Read them from the environment or a file with restricted permissions.
• Never fetch remote code and execute it.
• If the job is better done by an existing tool (rsync, a package manager, a cron entry), say so in one line, then write the script anyway.

Assumptions

Bullets, or "None".

Script

One complete code block with a header comment: purpose, usage line, exit codes.

Usage

Two or three example invocations, including a dry run.

What it changes

Every file, directory, service or remote system it creates, modifies or deletes.

How to test it

Steps to try it safely: dry run first, then a throwaway directory or container.

Limitations

What it does not handle, one line each.