Q: You have a cron job script taking database backups at 2 AM every night. One night, the backup takes 25 hours instead of 2 hours because the DB grew. At 2 AM the next night, the cron job fires again. Now two heavy backups are running simultaneously, locking the DB. How do you prevent this in the script?

Cron jobs must be designed to be self-aware of overlapping executions. This is solved using File Locking. The script must attempt to acquire an exclusive lock before running the heavy operation. If it cannot get the lock, it immediately exits. In Linux, the standard tool is flock: 0 2 * * * /usr/bin/flock -n /var/run/backup.lock /path/to/backup_script.sh The -n (non-blocking) flag tells flock to immediately fail if the lock file is currently held by a previous, still-running instance of the script. This guarantees only one backup can ever run at a time.