下面的cron表达式是每隔45分钟吗？ [英] Is the following cron expression means every 45 minutes?

问题描述

我愿意每45分钟（而不是每小时的第45分钟）运行一个脚本

Am willing to run a script every 45 minute (not the :45th minute of every hour)

*/45 * * * *

不确定这是否是正确的表达式。

Am not sure this is the correct expression.

推荐答案

我可疑（编辑：我现在很肯定），它不做你想要的：字段是分开的， code> * / 45 分钟只不过是 0,45 。如果 * / 45 不执行此作业，我将使用以下三个条目：

I suspect (edit: I'm pretty sure by now) that it doesn't do what you want: fields are separate, and */45 for minutes is nothing more than 0,45. I would use the following three entries if */45 doesn't do the job:

0,45  0-23/3 * * *
30    1-23/3 * * *
15    2-23/3 * * *

如果您查看vixie cron源中的 entry.c 将注意到每个条目的每个字段由 get_list 解析并且表示为该字段的允许值的位图。这几乎排除了任何聪明的解释，因为 * / 45 和 0,45 阶段...但有一个 MIN_STAR 标志，设置为 * 在分钟内存在（包括 * / 45 ）。所以，我们来看看 cron.c ，一个单独的地方，其中 MIN_STAR 被检查，以了解它与我们的无关问题。现在我们确定 * / 45 表示每小时每隔45分钟：0:00，0:45，1:00 ，1:45等等。

If you take a look at entry.c file in vixie cron sources, you'll notice that each field of each entry is parsed by get_list and represented as bitmaps of allowed values for that field. That almost precludes any "smart" interpretation, as the distinction of */45 and 0,45 is lost at this stage... but there is a MIN_STAR flag, set at the presence of * in minutes (including */45). So we take a look at cron.c, a single place where MIN_STAR is examined, to learn it's unrelated to our problem. Now we know for sure that */45 means "every 45th minute of every hour": 0:00, 0:45, 1:00, 1:45 and so on.

这里有两个回答自信地说对面，引用手册中的一个不幸的段落：

There were two answers here confidently stating the opposite, quoting an unfortunate passage in the manual:

星号之后也允许执行步骤，所以如果你想每两小时都说
，只需使用* / 2

Steps are also permitted after an asterisk, so if you want to say "every two hours", just use "*/2"

我们很幸运有一个24小时的一天，包含偶数小时，使每两个小时从0:00，每天和每两小时不可区分。太糟糕了，手册没有走得足以记录不平凡的情况，使得 * * / 22 的意思是每22小时。它不是。包含步骤的星号只是使用它的字段中的值列表的缩写;它不与其他字段交互。

We are lucky to have a 24 hour day, containing even number of hours, making "every two hours from 0:00, each day" and "every two hours generally" indistinguishable. Too bad that the manual didn't go far enough to document non-trivial cases, making the impression that * */22 means every 22 hours. It does not. Star with a step is just a shorthand for a list of values in the field where it's used; it doesn't interact with other fields.

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