Write a script that is not idempotent, then make it so

short · 40 min · Objective 5.1

Task

Automation gets run twice -- by a retry, by two people, by an overlapping schedule. Write one that breaks when it happens, observe the damage, and then fix it properly rather than by adding a lock.

Steps

  1. Write lab/auto/setup-v1.sh doing three things with no existence checks: create a user entry in a file, append a configuration line, and start a container with a fixed name.
  2. Run it once and confirm the result. Run it a SECOND time and record exactly what happened to each of the three in lab/auto/rerun.md -- one will error, one will duplicate, one may silently corrupt.
  3. Write lab/auto/setup-v2.sh making each action idempotent: check before creating, ensure-a-line rather than append, and handle the container already existing.
  4. Run v2 five times in a row and confirm the end state is identical after each. Record the evidence.
  5. Add fail-loudly behaviour: check every result, exit non-zero on any failure, and prove it by breaking one step deliberately and capturing the non-zero exit in lab/auto/failure.txt.

Verify

grep -Eci 'duplicate|twice|error' lab/auto/rerun.md
bash lab/auto/setup-v2.sh >/dev/null && bash lab/auto/setup-v2.sh >/dev/null && echo "v2 re-runnable"
test -s lab/auto/failure.txt && grep -Ec 'exit|non-zero|[1-9]' lab/auto/failure.txt

v2 must run twice cleanly and the failure capture must show a non-zero exit. Automation that fails silently is worse than none, because everyone believes the task was done.

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