#53 Validating User crontab Entries
One of the most helpful facilities in Unix is cron, with its ability to schedule jobs at arbitrary times in the future, recurring every minute, every few hours, monthly, or annually. Every good system administrator has a Swiss army knife of scripts running from the crontab file.
However, the format for entering cron specifications is a bit tricky, and the cron fields have numeric values, ranges, sets, and even mnemonic names for days of the week or months. What's worse is that the crontab program generates insufficient error messages when scanning in a cron file that might be incorrectly structured.
For example, specify a day of the week with a typo, and crontab reports
"/tmp/crontab.Dj7Tr4vw6R":9: bad day-of-week ...