The cron seconds field: the optional sixth position

There is no flag or marker that turns on the seconds field. The parser counts the whitespace separated tokens: five means the classic layout, and six means a seconds field has been added at the front with everything else shifted right by one. Any other count is an error rather than a guess.

That makes an accidental sixth token dangerous in a specific way. * * * * * * is a perfectly valid six field expression meaning every second, and it looks almost identical to the five field every minute expression. Reading the field breakdown, rather than counting asterisks by eye, is the reliable way to tell which one you have.

The system cron daemon does not implement seconds at all. It reads five fields and treats everything after them as the command to run, so a six field line in a crontab produces a job that runs every minute and tries to execute a program whose name is whatever the sixth token was. It will fail, but it will fail once a minute rather than at install time.

Where the seconds field does exist, it behaves exactly like the minute field: range 0 to 59, no names, steps counting from 0, and the same divisor arithmetic. */15 in the seconds field gives four runs a minute, which is 5,760 a day, and that number is worth writing down before enabling it.