How would you verify a purpose-built module replacement produces the exact same end state as the shell command it replaced, in a repeatable way?

Run both versions against identical starting environments and diff the resulting state directly — snapshot a target system before either runs, apply the old shell-based task to one copy and the new module-based task to an identical copy, then compare the actual resulting filesystem/configuration state (not just "did it report success") between the two, ideally automated as a repeatable test rather than a one-time manual check.

Establish identical starting state: use disposable, reproducible environments (containers, VM snapshots, or a configuration-management test harness like Molecule) so both the old and new task versions run against genuinely identical starting conditions — comparing results is only meaningful if the inputs were the same.

Run both versions, capture actual resulting state: apply the old shell task to one environment and the new module-based task to an identical second environment, then capture the actual resulting state relevant to what the task was supposed to do — the specific file's contents and permissions, the package's installed version, the service's running configuration — not just Ansible's own "changed: true/false" report, which only reflects Ansible's own bookkeeping, not necessarily the full real-world state.

Diff the captured state directly: comparing the two environments' actual resulting state (file diffs, dpkg -l output, service config dumps — whatever's relevant to the specific task) surfaces any discrepancy directly, rather than relying on inference from Ansible's own success/failure reporting, which wouldn't catch a case where both "succeeded" but produced subtly different actual results.

Test edge cases the original shell command might have silently handled: an old shell command sometimes encodes undocumented handling of edge cases (a specific flag working around a known issue, handling for a file that might or might not already exist) that isn't obvious from reading the command alone — testing the replacement against multiple realistic starting states (not just the common case) increases confidence the replacement genuinely covers what the original did, including edge cases nobody explicitly documented.

Automate this as a repeatable test, using Molecule or equivalent: Ansible's Molecule testing framework is specifically built for this class of verification — defining test scenarios that provision a fresh environment, apply a role/playbook, and assert on the resulting state, then optionally re-apply to verify idempotency (a second run reports no changes) — turning what would otherwise be a one-time manual verification into a repeatable, CI-runnable test that also protects against future regressions if the task is touched again later.

• Compare actual resulting state (file contents, package versions, service config) between old and new task versions run against identical starting environments, not just Ansible's own success/failure reporting.
• Test against multiple realistic starting states, not just the common case, to catch edge-case handling the original shell command might have silently encoded.
• Molecule (or an equivalent test harness) turns this into a repeatable, automated, CI-runnable verification rather than a one-time manual check.
• A second re-apply as part of the same test verifies genuine idempotency (no changes reported), not just correctness of the first run.

• Molecule: Ansible testing framework
• Ansible: Testing Strategies