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Avoiding Concurrent Crons: Easy File Locking!

Old SysAdmin tip: keep your frequent-but-long-running cron jobs from running concurrently by adding some lightweight file locking to your cron entry. For example, if you have:

* 15 * * * /usr/local/bin/db-backup.sh

On FreeBSD you could use:

* 15 * * * /usr/bin/lockf -t 0 /tmp/db-backup.lock /usr/local/bin/db-backup.sh

Or on Linux:

* 15 * * * /usr/bin/flock -w 0 /tmp/db-backup.lock /usr/local/bin/db-backup.sh

Read up on the lockf or flock man pages before you go putting this in. This can be a bit tricky because these can also be system calls. Try “man 1 lockf” or the like to nail it down to the manual for the user-executable command.