如何解释这个 crontab 命令？

Question

我知道，如果我用crontab -e以下命令 编写，01 04 * * * somecommand那么它将somecommand在每个月的每一天的凌晨 4:01运行。

如果我写了会* * * * * somecommand怎样？它会somecommand每分钟运行一次吗？这种语法也适用吗？

它可以使用特殊字符串一样@reboot，@daily因为它是解释，等在这里。somecommand如果我写@daily somecommand命令会在一天中的什么时间执行？

Answer 1

这将每分钟运行您的命令。这是有效的语法。

以下是来自info crontab：的血腥细节：

        The first five fields  shall be integer patterns that specify the
        following:

        1. Minute [0,59]

        2. Hour [0,23]

        3. Day of the month [1,31]

        4. Month of the year [1,12]

        5. Day of the week ([0,6] with 0=Sunday)

       Each  of  these  patterns  can be either an asterisk (meaning all valid
       values), an element, or a list of elements separated by commas. An ele?
       ment  shall  be  either  a  number or two numbers separated by a hyphen
       (meaning an inclusive range). The specification of days can be made  by
       two  fields  (day  of the month and day of the week).  If month, day of
       month, and day of week are all asterisks, every day shall  be  matched.
       If either the month or day of month is specified as an element or list,
       but the day of week is an asterisk, the month and day of  month  fields
       shall  specify  the days that match. If both month and day of month are
       specified as an asterisk, but day of week is an element or  list,  then
       only the specified days of the week match. Finally, if either the month
       or day of month is specified as an element or list, and the day of week
       is  also  specified as an element or list, then any day matching either
       the month and day of month, or the day of week, shall be matched.

您链接到的文章看起来不错。它为您提供了一些很好的示例，实际上它比我在此处提供的手册页摘录更容易阅读。
您应该能够使用它谈到的语法。

根据我的 crontab，@daily 在 6:25 AM 运行。

$ grep daily /etc/crontab 
25 6    * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )