A user crontab contains the entry `0 6 1 * 1 /usr/local/bin/report`. On which days does the job actually run?
LPIC-1 Exam 102-500, objective 107. Administrative tasks hard
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How these questions are written — where each question comes from, what the verification ledger records, and what happens when one is found wrong.
The options
Correct On the first day of the month, and additionally on every Monday
Correct. When both the day-of-month and the day-of-week fields are restricted, cron treats them as alternatives: the command runs if either matches.
Not correct Only on the first day of the month, and only when that day is a Monday
Wrong. That is the intersection of the two fields. Cron uses the union, not the intersection, when neither field is an asterisk.
Not correct Only on the first Monday of each month
Wrong. Cron has no nth-weekday-of-month syntax; expressing that requires a daily job plus a date test inside the script.
Not correct Only on the first day of the month; the day-of-week field is ignored because a day-of-month was given
Wrong. Neither field is ignored here. A field drops out of the decision only when it is itself a lone asterisk: with `* * 1` cron runs on Mondays only, and with `1 * *` it runs on the 1st only. Because both fields are restricted in this entry, cron takes their union.
Why
The scheduling fields are minute, hour, day of month, month, day of week. Month, minute and hour are ANDed with everything else, but day of month and day of week are ORed whenever both are restricted. Because of this, `0 6 1 * 1` fires roughly five times as often as most people intend. Leave one of the two fields as * whenever you want a single, unambiguous rule.
Where this comes from
- Cited
- manual page crontab(5)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 107 and space the ones you get wrong.
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