crontab 如果日期和星期冲突时,会如何执行?

Question

比如 指定

* * 22 8 3 

时,每周三执行,但是8月22日不是星期三,会如何执行?

Answer 1

crontab表达式，前面四项的关系之间为and的关系，需要同时满足才能执行；
但星期那一项与前面月份日期是or的关系，只需满足其一即执行；

所以，示例中表示8.22执行，每周三也执行

Answer 2

可以找一些在线crontab验证的工具验证一下下一次执行的日期: https://crontab.guru/

查阅了下man 5 crontab，找到了官方文档中的说法:

       Note: The day of a command's execution can be specified by two fields — day of month, and day of  week.   If  both
       fields  are  restricted (i.e., aren't *), the command will be run when either field matches the current time.  For
       example,
       ``30 4 1,15 * 5'' would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.
       One can, however, achieve the desired result by adding a test to the command (see the last example in EXAMPLE CRON
       FILE below).