Cron

Cron accepts schedules from more than one kind of table. A user crontab belongs to one account, while system tables can name the account for each entry. Copying a line between them without adjusting its fields changes what cron interprets as the command. Identify the table format before debugging an apparently correct five-field calendar expression. ...

Cron matches calendar fields and launches commands at matching times. It does not inherently wait for an earlier invocation of the same command to finish. If a minute-level task takes seventy seconds, the next minute can launch another copy. Treat trigger policy, concurrency policy, and successful completion as separate responsibilities when operating recurring work. ...