AI agent skill · by @aiming-lab

Idempotent Script Design

Use this skill when writing scripts, cron jobs, data pipelines, or any automated process that may be run multiple times. Design every operation to be safely re-runnable without side effects.

Archived 2026-10-23 · Source updated 2026-06-07 · License: MIT. Check the original repository for the latest version.

SKILL.md instructions

# Idempotent Script Design

An idempotent operation produces the same result whether run once or ten times.

**Techniques:**
- **Check before create:** `CREATE TABLE IF NOT EXISTS`, `mkdir -p`, check file existence before writing.
- **Upsert instead of insert:** Use `INSERT ... ON CONFLICT DO UPDATE` in SQL.
- **Atomic writes:** Write to a temp file, then rename — prevents corrupt partial output.
- **Track progress:** Write a completion marker or checkpoint so reruns skip done work.

**Testing idempotency:** Run your script twice on the same input and verify the output is identical.

**Anti-patterns:**
- Appending to a file without checking if the data is already there.
- Inserting rows without checking for duplicates.