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Hands-on Lab·Python Automation for IT

Python Locking and Single-Instance Scripts

A cron job that sometimes takes longer than its interval will eventually run twice at once. The obvious fix - a lock file - has a failure mode that is worse than the problem: a run killed while holding it locks the job out permanently. This guide shows the collision, the stale lock, and `fcntl.flock`, which the kernel releases when the holder dies no matter how it died.

OS and Process Automation Guide 22 of 39 Intermediate

Written against the versions above. `fcntl` is Unix-only and always has been; on Windows the equivalent is `msvcrt.locking`. `unlink(missing_ok=True)` needs Python 3.8.

Everything on this page runs on the control node. Any machine with Python 3 will do.
Server NameIP AddressOSRolesCPURAMHDD
RUNNER01192.168.0.27Ubuntu 26.04 LTSControl node - every script in this path runs here2 Core4 GB50 GB

Before you start

  1. Two runs at once

  2. A lock file claimed without a race

  3. And what happens when a run dies holding it

  4. A lock the kernel releases for you

  5. The file left behind is harmless

  6. Even when the holder is killed

  7. A context manager, so it cannot be forgotten

Official sources

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