Merge pull request #18772 from lujun9972/add-MjAyMDA2MTYgSG93IHRvIGxvb3AgZm9yZXZlciBpbiBiYXNoIG9uIExpbnV4Lm1kCg==

自动选题[tech]: 20200616 How to loop forever in bash on Linux

sources/tech/20200616 How to loop forever in bash on Linux.md

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+[#]: collector: (lujun9972)
+[#]: translator: ( )
+[#]: reviewer: ( )
+[#]: publisher: ( )
+[#]: url: ( )
+[#]: subject: (How to loop forever in bash on Linux)
+[#]: via: (https://www.networkworld.com/article/3562576/how-to-loop-forever-in-bash-on-linux.html)
+[#]: author: (Sandra Henry-Stocker https://www.networkworld.com/author/Sandra-Henry_Stocker/)
+
+How to loop forever in bash on Linux
+======
+
+[tine ivanic][1] [(CC0)][2]
+
+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.
+
+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.
+
+**[ Also see [Invaluable tips and tricks for troubleshooting Linux][3]. ]**
+
+### Using while
+
+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:
+
+```
+$ while true
+> do
+>   echo Keep running
+>   sleep 3
+> done
+Keep running
+Keep running
+Keep running
+^C
+```
+
+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.
+
+```
+$ while :
+> do
+>   echo Keep running
+>   sleep 3
+> done
+Keep running
+Keep running
+^C
+```
+
+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:
+
+```
+while :
+do
+        echo Keep running
+        echo "Press CTRL+C to exit"
+        sleep 1
+done
+```
+
+### Using for
+
+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:
+
+```
+$ for (( c=1; c<=5; c++ ))
+```
+
+with one that doesn’t specify any parameters:
+
+```
+$ for (( ; ; ))
+```
+
+With no start value, increment or exit test, this loop will run forever or until it is forcibly stopped.
+
+```
+$ for (( ; ; ))
+> do
+>    echo Keep running
+>   echo “Press CTRL+C to exit”
+>    sleep 2
+> done
+Keep your spirits up
+Keep your spirits up
+Keep your spirits up
+```
+
+### Why loop forever?
+
+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.
+
+This script would keep processing data until 5 p.m. or the first time it checks the time after 5 p.m.:
+
+```
+#!/bin/bash
+
+while true
+do
+  if [ `date +%H` -ge 17 ]; then
+    exit        # exit script
+  fi
+  echo keep running
+  ~/bin/process_data     # do some work
+done
+```
+
+If you want to exit the loop instead of exiting the script, use a **break** command instead of an **exit**.
+
+```
+#!/bin/bash
+
+while true
+do
+  if [ `date +%H` -ge 17 ]; then
+    break       # exit loop
+  fi
+  echo keep running
+  ~/bin/process_data
+done
+… run other commands here …
+```
+
+#### Wrap-Up
+
+Looping forever is easy. Specifying the conditions under which you want to stop looping takes a little extra effort.
+
+Join the Network World communities on [Facebook][4] and [LinkedIn][5] to comment on topics that are top of mind.
+
+--------------------------------------------------------------------------------
+
+via: https://www.networkworld.com/article/3562576/how-to-loop-forever-in-bash-on-linux.html
+
+作者：[Sandra Henry-Stocker][a]
+选题：[lujun9972][b]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]: https://www.networkworld.com/author/Sandra-Henry_Stocker/
+[b]: https://github.com/lujun9972
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+[2]: https://creativecommons.org/publicdomain/zero/1.0/
+[3]: https://www.networkworld.com/article/3242170/linux/invaluable-tips-and-tricks-for-troubleshooting-linux.html
+[4]: https://www.facebook.com/NetworkWorld/
+[5]: https://www.linkedin.com/company/network-world