下面的 cron 表达式是不是每 45 分钟一次? [英] Is the following cron expression means every 45 minutes?

问题描述

我愿意每 45 分钟运行一次脚本(不是每小时的第 45 分钟)

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

例如10:00、10:45、11:30、12:15 等.

e.g. 10:00, 10:45, 11:30, 12:15, and so on.

*/45 * * * *

我不确定这是正确的表达方式.

Am not sure this is the correct expression.

推荐答案

我怀疑(我现在很确定)它没有做你想要的:字段是分开的, 和 */45 的分钟数不过是 0,45.如果 */45 不起作用，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 文件，您会注意到每个条目的每个 field 都由 get_list<解析/code> 并表示为该字段允许值的位图.这几乎排除了任何智能"解释，因为 */45 和 0,45 的区别在这个阶段丢失了......但是有一个 MIN_STAR 标志，设置在 * 的分钟内(包括 */45).所以我们看一下 cron.c，一个检查 MIN_STAR 的地方，以了解它与我们的问题无关.现在我们确定 */45 表示每小时的每 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.

这里有两个答案自信地说明了相反的情况，引用了手册中的一段不幸的话:

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

星号后也允许有步骤，所以如果你想说每两个小时"，只需使用*/2"

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

我们很幸运拥有 24 小时的一天，其中包含偶数小时，因此无法区分每天从 0:00 开始每两小时一次"和一般每两小时一次".太糟糕了，手册不足以记录非平凡的案例，给人的印象是 * */22 意味着每 22 小时一次.它不是.带有步骤的星号只是使用它的字段中的值列表的简写；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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