你不能。Cron的粒度为60秒。

* * * * * cd /srv/last_song/releases/20120308133159 && script/rails runner -e production '\''Song.insert_latest'\''
* * * * * sleep 30 && cd /srv/last_song/releases/20120308133159 && script/rails runner -e production '\''Song.insert_latest'\''

在分钟指示符中有*/30 -这意味着每分钟，但步长为30(换句话说，每半小时)。由于cron不能精确到几分钟的分辨率，您需要找到另一种方法。

有一种可能，虽然有点拼凑(a)，是有两个工作，一个抵消30秒:

# Need these to run on 30-sec boundaries, keep commands in sync.
* * * * *              /path/to/executable param1 param2
* * * * * ( sleep 30 ; /path/to/executable param1 param2 )

您将看到我添加了注释并进行了格式化，以确保它们易于保持同步。

这两个cron作业实际上每分钟运行一次，但后者在执行作业的“肉”/path/to/executable之前会等待半分钟。

关于其他(非基于cron的)选项，请参阅这里的其他答案，特别是提到fcron和systemd的答案。假设你的系统有能力使用它们(比如安装fcron或者有一个带有systemd的发行版)，这些可能是更可取的。

如果您不想使用这个笨拙的解决方案，您可以使用一个基于循环的解决方案，并进行了一个小的修改。你仍然需要管理保持你的进程以某种形式运行，但是，一旦排序，下面的脚本应该工作:

#!/bin/env bash

# Debug code to start on minute boundary and to
# gradually increase maximum payload duration to
# see what happens when the payload exceeds 30 seconds.

((maxtime = 20))
while [[ "$(date +%S)" != "00" ]]; do true; done

while true; do
    # Start a background timer BEFORE the payload runs.

    sleep 30 &

    # Execute the payload, some random duration up to the limit.
    # Extra blank line if excess payload.

    ((delay = RANDOM % maxtime + 1))
    ((maxtime += 1))
    echo "$(date) Sleeping for ${delay} seconds (max ${maxtime})."
    [[ ${delay} -gt 30 ]] && echo
    sleep ${delay}

    # Wait for timer to finish before next cycle.

    wait
done

诀窍是使用sleep 30，但在有效负载运行之前在后台启动它。然后，在有效负载完成后，只需等待背景睡眠完成。

如果有效载荷需要n秒(其中n <= 30)，那么有效载荷之后的等待时间将是30 - n秒。如果它需要超过30秒，那么下一个循环将被延迟，直到有效载荷完成，但不会更长。

您将看到其中的调试代码以一分钟为边界开始，以使输出最初更容易遵循。我还逐渐增加最大有效负载时间，因此您最终会看到有效负载超过30秒的周期时间(输出额外的空行，因此效果很明显)。

下面是一个示例运行(其中周期通常在前一个周期后30秒开始):

Tue May 26 20:56:00 AWST 2020 Sleeping for 9 seconds (max 21).
Tue May 26 20:56:30 AWST 2020 Sleeping for 19 seconds (max 22).
Tue May 26 20:57:00 AWST 2020 Sleeping for 9 seconds (max 23).
Tue May 26 20:57:30 AWST 2020 Sleeping for 7 seconds (max 24).
Tue May 26 20:58:00 AWST 2020 Sleeping for 2 seconds (max 25).
Tue May 26 20:58:30 AWST 2020 Sleeping for 8 seconds (max 26).
Tue May 26 20:59:00 AWST 2020 Sleeping for 20 seconds (max 27).
Tue May 26 20:59:30 AWST 2020 Sleeping for 25 seconds (max 28).
Tue May 26 21:00:00 AWST 2020 Sleeping for 5 seconds (max 29).
Tue May 26 21:00:30 AWST 2020 Sleeping for 6 seconds (max 30).
Tue May 26 21:01:00 AWST 2020 Sleeping for 27 seconds (max 31).
Tue May 26 21:01:30 AWST 2020 Sleeping for 25 seconds (max 32).
Tue May 26 21:02:00 AWST 2020 Sleeping for 15 seconds (max 33).
Tue May 26 21:02:30 AWST 2020 Sleeping for 10 seconds (max 34).
Tue May 26 21:03:00 AWST 2020 Sleeping for 5 seconds (max 35).
Tue May 26 21:03:30 AWST 2020 Sleeping for 35 seconds (max 36).

Tue May 26 21:04:05 AWST 2020 Sleeping for 2 seconds (max 37).
Tue May 26 21:04:35 AWST 2020 Sleeping for 20 seconds (max 38).
Tue May 26 21:05:05 AWST 2020 Sleeping for 22 seconds (max 39).
Tue May 26 21:05:35 AWST 2020 Sleeping for 18 seconds (max 40).
Tue May 26 21:06:05 AWST 2020 Sleeping for 33 seconds (max 41).

Tue May 26 21:06:38 AWST 2020 Sleeping for 31 seconds (max 42).

Tue May 26 21:07:09 AWST 2020 Sleeping for 6 seconds (max 43).

如果您想避免这种笨拙的解决方案，这可能更好。您仍然需要一个cron作业(或等效作业)来定期检测该脚本是否正在运行，如果没有，则启动它。但是脚本本身会处理计时。

(a)我的一些同事会说拼板是我的专长:-)

使用fcron (http://fcron.free.fr/) -提供以秒为单位的粒度，比cron (vixie-cron)更好，功能更丰富，也更稳定。我曾经做过一些愚蠢的事情，比如在一台机器上运行60个php脚本，设置非常愚蠢，但它仍然发挥了作用!

Have a look at frequent-cron - it's old but very stable and you can step down to micro-seconds. At this point in time, the only thing that I would say against it is that I'm still trying to work out how to install it outside of init.d but as a native systemd service, but certainly up to Ubuntu 18 it's running just fine still using init.d (distance may vary on latter versions). It has the added advantage (?) of ensuring that it won't spawn another instance of the PHP script unless a prior one has completed, which reduces potential memory leakage issues.

你不能。Cron的粒度为60秒。

* * * * * cd /srv/last_song/releases/20120308133159 && script/rails runner -e production '\''Song.insert_latest'\''
* * * * * sleep 30 && cd /srv/last_song/releases/20120308133159 && script/rails runner -e production '\''Song.insert_latest'\''

如果您正在运行带有SystemD的最新Linux操作系统，您可以使用SystemD Timer单元以您希望的任何粒度级别运行脚本(理论上可以精确到纳秒)，并且—如果您愿意—比Cron所允许的更灵活的启动规则。不需要睡眠工具

在cron文件中只设置一行代码是不够的，但是如果您需要的不是“每分钟”，那么这样做是非常值得的。

SystemD计时器模型基本上是这样的:计时器是在计时器失效时启动服务单元的单元。

因此，对于想要调度的每个脚本/命令，必须有一个服务单元和一个额外的计时器单元。单个计时器单元可以包括多个计划，因此通常不需要多个计时器和一个服务。

下面是一个简单的例子，每10秒记录一次“Hello World”:

(要创建这些文件，您可以使用sudo tee path-to-file并粘贴文件内容，然后按CTRL+D，或使用您选择的文本编辑器)

/etc/systemd/system/helloworld.service：

[Unit]
Description=Say Hello
[Service]
ExecStart=/usr/bin/logger -i Hello World

/etc/systemd/system/helloworld.timer：

[Unit]
Description=Say Hello every 10 seconds
[Timer]
OnBootSec=10
OnUnitActiveSec=10
AccuracySec=1ms
[Install]
WantedBy=timers.target

在设置这些单元之后(在/etc/systemd/system中，如上所述，用于整个系统范围的设置，或者在~/。Config /systemd/user(用户特定的设置)，你需要通过运行systemctl enable——now helloworld来启用定时器(而不是服务)。计时器(——now标志也会立即启动计时器，否则，它只会在下一次引导或用户登录后启动)。

这里使用的[Timer]部分字段如下:

OnBootSec - start the service this many seconds after each boot. OnUnitActiveSec - start the service this many seconds after the last time the service was started. This is what causes the timer to repeat itself and behave like a cron job. AccuracySec - sets the accuracy of the timer. Timers are only as accurate as this field sets, and the default is 1 minute (emulates cron). The main reason to not demand the best accuracy is to improve power consumption - if SystemD can schedule the next run to coincide with other events, it needs to wake the CPU less often. The 1ms in the example above is not ideal - I usually set accuracy to 1 (1 second) in my sub-minute scheduled jobs, but that would mean that if you look at the log showing the "Hello World" messages, you'd see that it is often late by 1 second. If you're OK with that, I suggest setting the accuracy to 1 second or more.

As you may have noticed, this timer doesn't mimic Cron all that well - in the sense that the command doesn't start at the beginning of every wall clock period (i.e. it doesn't start on the 10th second on the clock, then the 20th and so on). Instead is just happens when the timer ellapses. If the system booted at 12:05:37, then the next time the command runs will be at 12:05:47, then at 12:05:57, etc. If you are interested in actual wall clock accuracy, then you may want to replace the OnBootSec and OnUnitActiveSec fields and instead set an OnCalendar rule with the schedule that you want (which as far as I understand can't be faster than 1 second, using the calendar format). The above example can also be written as:

OnCalendar=*-*-* *:*:00,10,20,30,40,50

最后一点:正如你可能猜到的，helloworld。计时器单元启动helloworld。服务单元，因为它们具有相同的名称(减去单元类型后缀)。这是默认值，但是你可以通过设置[Timer]部分的Unit字段来覆盖它。

更多血腥的细节可以在这里找到:

Arch Linux Wiki关于SystemD定时器的页面，提供了一个很好的主题概述，并提供了示例。 男人systemd.timer 男人systemd.time 男人systemd.service 男人systemd.exec