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[#]: collector: (lujun9972)
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[#]: translator: (HankChow)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (And, Ampersand, and & in Linux)
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[#]: via: (https://www.linux.com/blog/learn/2019/2/and-ampersand-and-linux)
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[#]: author: (Paul Brown https://www.linux.com/users/bro66)
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And, Ampersand, and & in Linux
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======
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Take a look at the tools covered in the [three][1] [previous][2] [articles][3], and you will see that understanding the glue that joins them together is as important as recognizing the tools themselves. Indeed, tools tend to be simple, and understanding what _mkdir_ , _touch_ , and _find_ do (make a new directory, update a file, and find a file in the directory tree, respectively) in isolation is easy.
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But understanding what
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```
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mkdir test_dir 2>/dev/null || touch images.txt && find . -iname "*jpg" > backup/dir/images.txt &
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```
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does, and why we would write a command line like that is a whole different story.
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It pays to look more closely at the sign and symbols that live between the commands. It will not only help you better understand how things work, but will also make you more proficient in chaining commands together to create compound instructions that will help you work more efficiently.
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In this article and the next, we'll be looking at the the ampersand (`&`) and its close friend, the pipe (`|`), and see how they can mean different things in different contexts.
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### Behind the Scenes
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Let's start simple and see how you can use `&` as a way of pushing a command to the background. The instruction:
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```
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cp -R original/dir/ backup/dir/
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```
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Copies all the files and subdirectories in _original/dir/_ into _backup/dir/_. So far so simple. But if that turns out to be a lot of data, it could tie up your terminal for hours.
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However, using:
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```
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cp -R original/dir/ backup/dir/ &
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```
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pushes the process to the background courtesy of the final `&`. This frees you to continue working on the same terminal or even to close the terminal and still let the process finish up. Do note, however, that if the process is asked to print stuff out to the standard output (like in the case of `echo` or `ls`), it will continue to do so, even though it is being executed in the background.
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When you push a process into the background, Bash will print out a number. This number is the PID or the _Process' ID_. Every process running on your Linux system has a unique process ID and you can use this ID to pause, resume, and terminate the process it refers to. This will become useful later.
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In the meantime, there are a few tools you can use to manage your processes as long as you remain in the terminal from which you launched them:
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* `jobs` shows you the processes running in your current terminal, whether be it in the background or foreground. It also shows you a number associated with each job (different from the PID) that you can use to refer to each process:
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```
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$ jobs
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[1]- Running cp -i -R original/dir/* backup/dir/ &
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[2]+ Running find . -iname "*jpg" > backup/dir/images.txt &
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```
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* `fg` brings a job from the background to the foreground so you can interact with it. You tell `fg` which process you want to bring to the foreground with a percentage symbol (`%`) followed by the number associated with the job that `jobs` gave you:
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```
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$ fg %1 # brings the cp job to the foreground
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cp -i -R original/dir/* backup/dir/
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```
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If the job was stopped (see below), `fg` will start it again.
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* You can stop a job in the foreground by holding down [Ctrl] and pressing [Z]. This doesn't abort the action, it pauses it. When you start it again with (`fg` or `bg`) it will continue from where it left off...
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...Except for [`sleep`][4]: the time a `sleep` job is paused still counts once `sleep` is resumed. This is because `sleep` takes note of the clock time when it was started, not how long it was running. This means that if you run `sleep 30` and pause it for more than 30 seconds, once you resume, `sleep` will exit immediately.
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* The `bg` command pushes a job to the background and resumes it again if it was paused:
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```
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$ bg %1
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[1]+ cp -i -R original/dir/* backup/dir/ &
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```
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As mentioned above, you won't be able to use any of these commands if you close the terminal from which you launched the process or if you change to another terminal, even though the process will still continue working.
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To manage background processes from another terminal you need another set of tools. For example, you can tell a process to stop from a a different terminal with the [`kill`][5] command:
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```
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kill -s STOP <PID>
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```
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And you know the PID because that is the number Bash gave you when you started the process with `&`, remember? Oh! You didn't write it down? No problem. You can get the PID of any running process with the `ps` (short for _processes_ ) command. So, using
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```
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ps | grep cp
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```
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will show you all the processes containing the string " _cp_ ", including the copying job we are using for our example. It will also show you the PID:
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```
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$ ps | grep cp
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14444 pts/3 00:00:13 cp
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```
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In this case, the PID is _14444_. and it means you can stop the background copying with:
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```
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kill -s STOP 14444
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```
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Note that `STOP` here does the same thing as [Ctrl] + [Z] above, that is, it pauses the execution of the process.
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To start the paused process again, you can use the `CONT` signal:
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```
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kill -s CONT 14444
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```
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There is a good list of many of [the main signals you can send a process here][6]. According to that, if you wanted to terminate the process, not just pause it, you could do this:
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```
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kill -s TERM 14444
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```
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If the process refuses to exit, you can force it with:
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```
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kill -s KILL 14444
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```
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This is a bit dangerous, but very useful if a process has gone crazy and is eating up all your resources.
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In any case, if you are not sure you have the correct PID, add the `x` option to `ps`:
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```
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$ ps x| grep cp
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14444 pts/3 D 0:14 cp -i -R original/dir/Hols_2014.mp4
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original/dir/Hols_2015.mp4 original/dir/Hols_2016.mp4
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original/dir/Hols_2017.mp4 original/dir/Hols_2018.mp4 backup/dir/
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```
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And you should be able to see what process you need.
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Finally, there is nifty tool that combines `ps` and `grep` all into one:
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```
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$ pgrep cp
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8
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18
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19
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26
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33
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40
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47
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54
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61
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72
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88
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96
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136
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339
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6680
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13735
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14444
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```
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Lists all the PIDs of processes that contain the string " _cp_ ".
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In this case, it isn't very helpful, but this...
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```
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$ pgrep -lx cp
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14444 cp
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```
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... is much better.
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In this case, `-l` tells `pgrep` to show you the name of the process and `-x` tells `pgrep` you want an exact match for the name of the command. If you want even more details, try `pgrep -ax command`.
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### Next time
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Putting an `&` at the end of commands has helped us explain the rather useful concept of processes working in the background and foreground and how to manage them.
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One last thing before we leave: processes running in the background are what are known as _daemons_ in UNIX/Linux parlance. So, if you had heard the term before and wondered what they were, there you go.
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As usual, there are more ways to use the ampersand within a command line, many of which have nothing to do with pushing processes into the background. To see what those uses are, we'll be back next week with more on the matter.
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Read more:
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[Linux Tools: The Meaning of Dot][1]
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[Understanding Angle Brackets in Bash][2]
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[More About Angle Brackets in Bash][3]
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--------------------------------------------------------------------------------
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via: https://www.linux.com/blog/learn/2019/2/and-ampersand-and-linux
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作者：[Paul Brown][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.linux.com/users/bro66
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[b]: https://github.com/lujun9972
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[1]: https://www.linux.com/blog/learn/2019/1/linux-tools-meaning-dot
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[2]: https://www.linux.com/blog/learn/2019/1/understanding-angle-brackets-bash
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[3]: https://www.linux.com/blog/learn/2019/1/more-about-angle-brackets-bash
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[4]: https://ss64.com/bash/sleep.html
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[5]: https://bash.cyberciti.biz/guide/Sending_signal_to_Processes
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[6]: https://www.computerhope.com/unix/signals.htm
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206
translated/tech/20190206 And, Ampersand, and - in Linux.md
Normal file
206
translated/tech/20190206 And, Ampersand, and - in Linux.md
Normal file
@ -0,0 +1,206 @@
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[#]: collector: (lujun9972)
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[#]: translator: (HankChow)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (And, Ampersand, and & in Linux)
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[#]: via: (https://www.linux.com/blog/learn/2019/2/and-ampersand-and-linux)
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[#]: author: (Paul Brown https://www.linux.com/users/bro66)
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Linux 中的 &
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======
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如果阅读过我之前的[三][1][篇][2][文章][3]，你会觉得掌握连接各个命令之间的连接符号用法也是很重要的。实际上，命令的用法并不难，例如 `mkdir`、`touch` 和 `find` 也分别可以简单概括为“建立新目录”、“更新文件”和“在目录树中查找文件”而已。
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但如果要理解
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```
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mkdir test_dir 2>/dev/null || touch images.txt && find . -iname "*jpg" > backup/dir/images.txt &
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```
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这一串命令的目的，以及为什么要这样写，就没有这么简单了。
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关键之处就在于命令之间的连接符号。掌握了这些符号的用法，不仅可以让你更好理解整体的工作原理，还可以让你知道如何将不同的命令有效地结合起来，提高工作效率。
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在这一篇文章和下一篇文章中，我会介绍如何使用 `&` 号和管道符号（`|`）在不同场景下的使用方法。
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### 幕后工作
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我来举一个简单的例子，看看如何使用 `&` 号将下面这个命令放到后台运行：
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```
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cp -R original/dir/ backup/dir/
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```
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这个命令的目的是将 `original/dir/` 的内容递归地复制到 `backup/dir/` 中。虽然看起来很简单，但是如果原目录里面的文件太大，在执行过程中终端就会一直被卡住。
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所以，可以在命令的末尾加上一个 `&` 号，将这个任务放到后台去执行：
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```
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cp -R original/dir/ backup/dir/ &
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```
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任务被放到后台执行之后，就可以立即继续在同一个终端上工作了，甚至关闭终端也不影响这个任务的正常执行。需要注意的是，如果要求这个任务输出内容到标准输出中（例如 `echo` 或 `ls`），即使使用了 `&`，也会等待这些输出任务在前台运行完毕。
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当使用 `&` 将一个进程放置到后台运行的时候，Bash 会提示这个进程的进程 ID。在 Linux 系统中运行的每一个进程都有一个唯一的进程 ID，你可以使用进程 ID 来暂停、恢复或者终止对应的进程，因此进程 ID 是非常重要的。
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这个时候，只要你还停留在启动进程的终端当中，就可以使用以下几个命令来对管理后台进程：
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* `jobs` 命令可以显示当前终端正在运行的进程，包括前台运行和后台运行的进程。它对每个正在执行中的进程任务分配了一个序号（这个序号不是进程 ID），可以使用这些序号来引用各个进程任务。
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```
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$ jobs
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[1]- Running cp -i -R original/dir/* backup/dir/ &
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[2]+ Running find . -iname "*jpg" > backup/dir/images.txt &
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```
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* `fg` 命令可以将后台运行的进程任务放到前台运行，这样可以比较方便地进行交互。根据 `jobs` 命令提供的进程任务序号，再在前面加上 `%` 符号，就可以把相应的进程任务放到前台运行。
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```
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$ fg %1 # 将上面序号为 1 的 cp 任务放到前台运行
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cp -i -R original/dir/* backup/dir/
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```
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如果这个进程任务是暂停状态，`fg` 命令会将它启动起来。
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* 使用 `ctrl+z` 组合键可以将前台运行的任务暂停，仅仅是暂停，而不是将任务终止。当使用 `fg` 或者`bg` 命令将任务重新启动起来的时候，任务会从被暂停的位置开始执行。但 [`sleep`][4] 命令是一个特例，`sleep` 任务被暂停的时间会计算在 `sleep` 时间之内。因为 `sleep` 命令依据的是系统时钟的时间，而不是实际运行的时间。也就是说，如果运行了 `sleep 30`，然后将任务暂停 30 秒以上，那么任务恢复执行的时候会立即终止并退出。
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* `bg` 命令会将任务放置到后台执行，如果任务是暂停状态，也会被启动起来。
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```
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$ bg %1
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[1]+ cp -i -R original/dir/* backup/dir/ &
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```
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如上所述，以上几个命令只能在同一个终端里才能使用。如果启动进程任务的终端被关闭了，或者切换到了另一个终端，以上几个命令就无法使用了。
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如果要在另一个终端管理后台进程，就需要其它工具了。例如可以使用 [`kill`][5] 命令从另一个终端终止某个进程：
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```
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kill -s STOP <PID>
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```
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这里的 PID 就是使用 `&` 将进程放到后台时 Bash 显示的那个进程 ID。如果你当时没有把进程 ID 记录下来，也可以使用 `ps` 命令（代表 process）来获取所有正在运行的进程的进程 ID，就像这样：
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```
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ps | grep cp
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```
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执行以后会显示出包含 `cp` 字符串的所有进程，例如上面例子中的 `cp` 进程。同时还会显示出对应的进程 ID：
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```
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$ ps | grep cp
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14444 pts/3 00:00:13 cp
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```
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在这个例子中，进程 ID 是 14444，因此可以使用以下命令来暂停这个后台进程：
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|
||||||
|
```
|
||||||
|
kill -s STOP 14444
|
||||||
|
```
|
||||||
|
|
||||||
|
注意，这里的 `STOP` 等同于前面提到的 `ctrl+z` 组合键的效果，也就是仅仅把进程暂停掉。
|
||||||
|
|
||||||
|
如果想要把暂停了的进程启动起来，可以对进程发出 `CONT` 信号：
|
||||||
|
|
||||||
|
```
|
||||||
|
kill -s CONT 14444
|
||||||
|
```
|
||||||
|
|
||||||
|
这个给出一个[可以向进程发出的常用信号][6]列表。如果想要终止一个进程，可以发送 `TERM` 信号：
|
||||||
|
|
||||||
|
```
|
||||||
|
kill -s TERM 14444
|
||||||
|
```
|
||||||
|
|
||||||
|
如果进程不响应 `TERM` 信号并拒绝退出，还可以发送 `KILL` 信号强制终止进程：
|
||||||
|
|
||||||
|
```
|
||||||
|
kill -s KILL 14444
|
||||||
|
```
|
||||||
|
|
||||||
|
强制终止进程可能会有一定的风险，但如果遇到进程无节制消耗资源的情况，这样的信号还是能够派上用场的。
|
||||||
|
|
||||||
|
另外，如果你不确定进程 ID 是否正确，可以在 `ps` 命令中加上 `x` 参数：
|
||||||
|
|
||||||
|
```
|
||||||
|
$ ps x| grep cp
|
||||||
|
14444 pts/3 D 0:14 cp -i -R original/dir/Hols_2014.mp4
|
||||||
|
original/dir/Hols_2015.mp4 original/dir/Hols_2016.mp4
|
||||||
|
original/dir/Hols_2017.mp4 original/dir/Hols_2018.mp4 backup/dir/
|
||||||
|
```
|
||||||
|
|
||||||
|
这样就可以看到是不是你需要的进程 ID 了。
|
||||||
|
|
||||||
|
最后介绍一个将 `ps` 和 `grep` 结合到一起的命令：
|
||||||
|
|
||||||
|
```
|
||||||
|
$ pgrep cp
|
||||||
|
8
|
||||||
|
18
|
||||||
|
19
|
||||||
|
26
|
||||||
|
33
|
||||||
|
40
|
||||||
|
47
|
||||||
|
54
|
||||||
|
61
|
||||||
|
72
|
||||||
|
88
|
||||||
|
96
|
||||||
|
136
|
||||||
|
339
|
||||||
|
6680
|
||||||
|
13735
|
||||||
|
14444
|
||||||
|
```
|
||||||
|
|
||||||
|
`pgrep` 可以直接将带有字符串 `cp` 的进程的进程 ID 显示出来。
|
||||||
|
|
||||||
|
可以加上一些参数让它的输出更清晰：
|
||||||
|
|
||||||
|
```
|
||||||
|
$ pgrep -lx cp
|
||||||
|
14444 cp
|
||||||
|
```
|
||||||
|
|
||||||
|
在这里，`-l` 参数会让 `pgrep` 将进程的名称显示出来，`-x` 参数则是让 `pgrep` 完全匹配 `cp` 这个命令。如果还想了解这个命令的更多细节，可以尝试运行 `pgrep -ax`。
|
||||||
|
|
||||||
|
### 总结
|
||||||
|
|
||||||
|
在命令的末尾加上 `&` 可以让我们理解前台进程和后台进程的概念，以及如何管理这些进程。
|
||||||
|
|
||||||
|
在 UNIX/Linux 术语中，在后台运行的进程被称为 daemon。如果你曾经听说过这个词，那你现在应该知道它的意义了。
|
||||||
|
|
||||||
|
和其它符号一样，`&` 在命令行中还有很多别的用法。在下一篇文章中，我会更详细地介绍。
|
||||||
|
|
||||||
|
阅读更多：
|
||||||
|
|
||||||
|
[Linux Tools: The Meaning of Dot][1]
|
||||||
|
|
||||||
|
[Understanding Angle Brackets in Bash][2]
|
||||||
|
|
||||||
|
[More About Angle Brackets in Bash][3]
|
||||||
|
|
||||||
|
--------------------------------------------------------------------------------
|
||||||
|
|
||||||
|
via: https://www.linux.com/blog/learn/2019/2/and-ampersand-and-linux
|
||||||
|
|
||||||
|
作者：[Paul Brown][a]
|
||||||
|
选题：[lujun9972][b]
|
||||||
|
译者：[HankChow](https://github.com/HankChow)
|
||||||
|
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.linux.com/users/bro66
|
||||||
|
[b]: https://github.com/lujun9972
|
||||||
|
[1]: https://www.linux.com/blog/learn/2019/1/linux-tools-meaning-dot
|
||||||
|
[2]: https://www.linux.com/blog/learn/2019/1/understanding-angle-brackets-bash
|
||||||
|
[3]: https://www.linux.com/blog/learn/2019/1/more-about-angle-brackets-bash
|
||||||
|
[4]: https://ss64.com/bash/sleep.html
|
||||||
|
[5]: https://bash.cyberciti.biz/guide/Sending_signal_to_Processes
|
||||||
|
[6]: https://www.computerhope.com/unix/signals.htm
|
||||||
|
|
||||||
Loading…
Reference in New Issue
Block a user