Prevent Overlapping Cron Job Runs
Common ErrorsThe Problem
If a cron job takes longer than its interval, the next run starts while the previous is still executing. This can cause data corruption, resource exhaustion, or deadlocks.
The Solution
Use file locking (flock) to ensure only one instance runs at a time. Combine with CronRabbit duration tracking to detect when jobs start taking too long.
Using flock
flock is the simplest solution. It acquires a file lock before running your command and skips the run if the lock is already held.
Duration Monitoring
Use CronRabbit's /start signal to track job duration. If your job suddenly takes 3x longer, you'll see it in the dashboard and can investigate before it starts overlapping.
Code Examples
Prevent overlap with flock
Bash# Crontab entry with flock
*/5 * * * * flock -n /tmp/myjob.lock /scripts/myjob.sh && curl -fsS -m 10 --retry 5 --retry-all-errors https://ping.cronrabbit.com/your-id
# Script-level locking
#!/bin/bash
LOCKFILE="/tmp/myjob.lock"
exec 200>"$LOCKFILE"
flock -n 200 || { echo "Already running"; exit 0; }
curl -fsS -m 5 --retry 3 --retry-all-errors https://ping.cronrabbit.com/your-id/start
# Your job logic here
curl -fsS -m 10 --retry 5 --retry-all-errors https://ping.cronrabbit.com/your-id