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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (How to loop forever in bash on Linux)
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[#]: via: (https://www.networkworld.com/article/3562576/how-to-loop-forever-in-bash-on-linux.html)
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[#]: author: (Sandra Henry-Stocker https://www.networkworld.com/author/Sandra-Henry_Stocker/)
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How to loop forever in bash on Linux
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======
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[tine ivanic][1] [(CC0)][2]
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There are a number of ways to loop forever (or until you decide to stop) on Linux and you can do this on the command line or within scripts.
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The **for** and **while** commands make the job quite easy. There are only a few things to keep in mind with respect to syntax and tactics.
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**[ Also see [Invaluable tips and tricks for troubleshooting Linux][3]. ]**
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### Using while
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One of the easiest forever-loops involves using the **while** command followed by the condition "true". You don’t have to bother with logic like **while [ 1 -eq 1 ]** or similar tests. The **while true** test means the loop will run until you stop it with **CTRL-C**, close the terminal window or log out. Here's an example:
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```
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$ while true
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> do
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> echo Keep running
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> sleep 3
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> done
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Keep running
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Keep running
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Keep running
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^C
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```
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You can also do the same thing with **while :**. The key here is that the **:** always yields success so, like **while true**, this test doesn’t ever fail and the loop just keeps running.
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```
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$ while :
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> do
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> echo Keep running
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> sleep 3
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> done
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Keep running
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Keep running
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^C
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```
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If you’ve inserted an infinite loop into a script and want to remind the person who is using it how to exit the script, you can always add a hint using the **echo** command:
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```
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while :
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do
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echo Keep running
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echo "Press CTRL+C to exit"
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sleep 1
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done
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```
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### Using for
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The **for** command also provides an easy way to loop forever. While not quite as obvious as **while true**, the syntax is reasonably straightforward. You just replace the parameters in a bounded loop that would generally look something like this "start with c equal to 1 and increment it until reaches 5" specification:
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```
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$ for (( c=1; c<=5; c++ ))
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```
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with one that doesn’t specify any parameters:
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```
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$ for (( ; ; ))
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```
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With no start value, increment or exit test, this loop will run forever or until it is forcibly stopped.
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```
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$ for (( ; ; ))
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> do
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> echo Keep running
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> echo “Press CTRL+C to exit”
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> sleep 2
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> done
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Keep your spirits up
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Keep your spirits up
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Keep your spirits up
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```
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### Why loop forever?
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In real life, you’re not ever going to want to loop forever, but running until it’s time to go home, the work is done or you run into a problem is not at all unusual. Any loop that is constructed as an infinite loop can also be set up to be exited depending on various circumstances.
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This script would keep processing data until 5 p.m. or the first time it checks the time after 5 p.m.:
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```
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#!/bin/bash
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while true
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do
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if [ `date +%H` -ge 17 ]; then
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exit # exit script
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fi
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echo keep running
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~/bin/process_data # do some work
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done
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```
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If you want to exit the loop instead of exiting the script, use a **break** command instead of an **exit**.
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```
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#!/bin/bash
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while true
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do
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if [ `date +%H` -ge 17 ]; then
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break # exit loop
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fi
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echo keep running
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~/bin/process_data
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done
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… run other commands here …
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```
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#### Wrap-Up
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Looping forever is easy. Specifying the conditions under which you want to stop looping takes a little extra effort.
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Join the Network World communities on [Facebook][4] and [LinkedIn][5] to comment on topics that are top of mind.
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--------------------------------------------------------------------------------
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via: https://www.networkworld.com/article/3562576/how-to-loop-forever-in-bash-on-linux.html
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作者：[Sandra Henry-Stocker][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://www.networkworld.com/author/Sandra-Henry_Stocker/
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[b]: https://github.com/lujun9972
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[1]: https://unsplash.com/photos/u2d0BPZFXOY
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[2]: https://creativecommons.org/publicdomain/zero/1.0/
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[3]: https://www.networkworld.com/article/3242170/linux/invaluable-tips-and-tricks-for-troubleshooting-linux.html
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[4]: https://www.facebook.com/NetworkWorld/
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[5]: https://www.linkedin.com/company/network-world
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147
translated/tech/20200616 How to loop forever in bash on Linux.md
Normal file
147
translated/tech/20200616 How to loop forever in bash on Linux.md
Normal file
@ -0,0 +1,147 @@
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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (How to loop forever in bash on Linux)
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[#]: via: (https://www.networkworld.com/article/3562576/how-to-loop-forever-in-bash-on-linux.html)
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[#]: author: (Sandra Henry-Stocker https://www.networkworld.com/author/Sandra-Henry_Stocker/)
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如何在 Linux 的 bash 中永远循环
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======
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[tine ivanic][1] [(CC0)][2]
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在 Linux 中有很多永远循环（或直到你决定停止）的方法，你可以在命令行或脚本中执行此操作。
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**for** 和 **while** 命令使这件事非常容易。 关于语法和策略，只有几件事要牢记。
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**另请参见[用于排除 Linux 故障的宝贵技巧][3]。**
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### 使用 while
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最简单的永远循环之一是使用 **while** 命令，后面跟上条件 “true”。 你不必担心诸如 **while [ 1 -eq 1 ]** 之类的逻辑或类似的测试。 **while true** 测试表示循环将一直运行，直到你使用 **CTRL-C** 停止循环、关闭终端窗口或注销为止。 这是一个例子：
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```
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$ while true
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> do
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> echo Keep running
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> sleep 3
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> done
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Keep running
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Keep running
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Keep running
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^C
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```
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你也可以使用 **while :** 做同样的事情。 这里的关键是 **:** 总是返回成功，因此就像 **while true** 一样，此测试永远不会失败，并且循环会继续运行。
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```
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$ while :
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> do
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> echo Keep running
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> sleep 3
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> done
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Keep running
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Keep running
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^C
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```
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如果你在脚本中插入了无限循环，并想提醒使用它的人如何退出脚本，那么可以使用 **echo** 命令添加提示：
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```
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while :
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do
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echo Keep running
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echo "Press CTRL+C to exit"
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sleep 1
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done
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```
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### 使用 for
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**for** 命令还提供了一种永远循环的简便方法。虽然不如 **while true** 明显，但语法相当简单。你只需要在有界循环中替换参数即可，界限通常类似于 “c 从等于 1 开始递增，直到 5”：
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```
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$ for (( c=1; c<=5; c++ ))
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```
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不指定任何参数的情况下：
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```
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$ for (( ; ; ))
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```
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没有起始值、增量或退出测试，此循环将永远运行或被强制停止。
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```
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$ for (( ; ; ))
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> do
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> echo Keep running
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> echo “Press CTRL+C to exit”
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> sleep 2
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> done
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Keep your spirits up
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Keep your spirits up
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Keep your spirits up
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```
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### Why loop forever?
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在现实中，你不会想永远循环下去，但一直运行直到想要回家、工作完成或者遇到问题才退出并不罕见。任何构造为无限循环的循环都可以设置为根据各种情况退出。
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该脚本将一直处理数据直到下午 5 点。 或检查发现第一次超过 5 点的时间：
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```
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#!/bin/bash
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while true
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do
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if [ `date +%H` -ge 17 ]; then
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exit # exit script
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fi
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echo keep running
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~/bin/process_data # do some work
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done
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```
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如果要退出循环而不是退出脚本，请使用 **break** 命令而不是 **exit**。
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```
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#!/bin/bash
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while true
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do
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if [ `date +%H` -ge 17 ]; then
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break # exit loop
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fi
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echo keep running
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~/bin/process_data
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done
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… run other commands here …
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```
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#### 总结
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永远循环很容易。 指定要停止循环的条件需要花费一些额外的精力。
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加入 [Facebook][4] 和 [LinkedIn][5] 上的 Network World 社区，评论热门主题。
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--------------------------------------------------------------------------------
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via: https://www.networkworld.com/article/3562576/how-to-loop-forever-in-bash-on-linux.html
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作者：[Sandra Henry-Stocker][a]
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选题：[lujun9972][b]
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译者：[geekpi](https://github.com/geekpi)
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||||
校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://www.networkworld.com/author/Sandra-Henry_Stocker/
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[b]: https://github.com/lujun9972
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[1]: https://unsplash.com/photos/u2d0BPZFXOY
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[2]: https://creativecommons.org/publicdomain/zero/1.0/
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||||
[3]: https://www.networkworld.com/article/3242170/linux/invaluable-tips-and-tricks-for-troubleshooting-linux.html
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||||
[4]: https://www.facebook.com/NetworkWorld/
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||||
[5]: https://www.linkedin.com/company/network-world
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||||
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