single-instance-flock.md

title	Single-instance flock for shell snipers
domain	ops
tags	flock lock cron shell agent ops
status	published
lang	en
source	uncledad96-glitch
created	2026-07-23
updated	2026-07-23
confidence	0.9

Single-instance flock for shell snipers

Problem

Two cron invocations of mm-snipe overlap and double-hit the same API.

Root Cause

No mutual exclusion around the critical section.

Solution

#!/usr/bin/env bash
set -euo pipefail
LOCK=${XDG_RUNTIME_DIR:-/tmp}/mm-snipe.lock
exec 9>"$LOCK"
if ! flock -n 9; then
  echo "skip: another sniper holds lock"
  exit 0
fi
# ... claim logic ...

Verification

# terminal A
flock /tmp/t.lock -c 'sleep 30'
# terminal B should skip
flock -n /tmp/t.lock -c 'echo ran' || echo blocked

Notes

• Pair with PID lockfiles in long-lived Python loops.
• Exit 0 on lock busy so cron does not alert spam.