Cron fields
| * | any value |
|---|---|
| */15 | every 15 (minutes, hours…) |
| 1-5 | a range: Monday to Friday in the weekday field |
| 1,15 | a list |
| 0 / 7 | Sunday (both work); mon, tue… also work |
One catch: if both day of month and weekday are set, cron runs when either matches, not both. Crontab does not have your shell’s environment: use full paths and redirect output to a file, or errors go to local mail. For restarts on failure, logging and dependencies use a systemd timer.