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148 lines
4.4 KiB
Markdown
148 lines
4.4 KiB
Markdown
[#]: collector: (lujun9972)
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[#]: translator: ( )
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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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