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[Translated by nophDog] 20200630 Read and write data from anywhere with redirection in the Linux terminal.md
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[#]: collector: (lujun9972)
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[#]: translator: (nophDog)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Read and write data from anywhere with redirection in the Linux terminal)
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[#]: via: (https://opensource.com/article/20/6/redirection-bash)
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[#]: author: (Seth Kenlon https://opensource.com/users/seth)
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Read and write data from anywhere with redirection in the Linux terminal
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======
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Redirection is an efficient way to get data from one place to another
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without a lot of mouse moving and key pressing.
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![Hands programming][1]
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Redirection of input and output is a natural function of any programming or scripting language. Technically, it happens inherently whenever you interact with a computer. Input gets read from `stdin` (standard input, usually your keyboard or mouse), output goes to `stdout` (standard output, a text or data stream), and errors get sent to `stderr`. Understanding that these data streams exist enables you to control where information goes when you're using a shell, such as [Bash][2] or [Zsh][3].
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Standard in, standard out, and standard error exist as filesystem locations on Linux. You can see them in `/dev`:
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```
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$ ls /dev/std*
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/dev/stderr@ /dev/stdin@ /dev/stdout@
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```
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You can't do much with them directly, but it's sometimes useful to think of them as meta-locations where you can send data.
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The basics of redirection are simple: use some number of `>` characters to redirect output, and some number of `<` characters to redirect input.
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### Redirecting output
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To write the output of the [ls][4] command to a file:
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```
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`$ ls > list.txt`
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```
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You don't see the output of `ls` as you normally would, because the output is written to the `list.txt` file instead of your screen. This is so versatile, in fact, that you can even use it to copy the contents of one file to another. It doesn't have to be a text file, either. You can use redirection for binary data:
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```
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`$ cat image.png > picture.png`
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```
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(In case you're wondering why you'd ever want to do that, it's for a sometimes-useful repercussion on [file permissions][5].)
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### Redirecting input
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You can redirect input "into" a command, too. This is arguably less useful than redirecting output because many commands are already hard-coded to take input from an argument you provide. It can be useful, however, when a command expects a list of arguments, and you have those arguments in a file and want to quickly "copy and paste" them from the file into your terminal (except you don't actually want to copy and paste):
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```
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`$ sudo dnf install $(<package.list)`
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```
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Common uses of input redirection are the **here-document** (or just **here-doc** for short) and **here-string** techniques. This input method redirects a block of text into the standard input stream, up to a special end-of-file marker (most people use `EOF`, but it can be any string you expect to be unique). Try typing this (up to the second instance of `EOF`) into a terminal:
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```
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$ echo << EOF
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> foo
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> bar
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> baz
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> EOF
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```
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The expected result:
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```
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foo
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bar
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baz
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```
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A **here-doc** is a common trick used by [Bash][2] scripters to dump several lines of text into a file or onto the screen. As long as you don't forget to end the clause with your end-of-file marker, it's an effective way to avoid unwieldy lists of `echo` or `printf` statements.
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A **here-string** is similar to a **here-doc**, but it consists of just one string (or several strings disguised as a single string with quotation marks):
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```
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$ cat <<< "foo bar baz"
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foo bar baz
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```
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### Redirecting error messages
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Error messages go to a stream called `stderr`, designated as `2>` for the purposes of redirection. This command directs error messages to a file called `output.log`:
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```
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`$ ls /nope 2> output.log`
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```
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### Sending data to /dev/null
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Just as there are locations for standard in, standard out, and error, there's also a location for _nowhere_ on the Linux filesystem. It's called `null`, and it's located in `/dev`, so it's often pronounced "devnull" by people who use it too frequently to say "slash dev slash null."
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You can send data to `/dev/null` using redirection. For instance, the `find` command tends to be verbose, and it often reports permission conflicts while searching through your files:
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```
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$ find ~ -type f
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/home/seth/actual.file
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find: `/home/seth/foggy': Permission denied
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find: `/home/seth/groggy': Permission denied
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find: `/home/seth/soggy': Permission denied
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/home/seth/zzz.file
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```
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The `find` command processes that as an error, so you can redirect just the error messages to `/dev/null`:
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```
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$ find ~ -type f 2> /dev/null
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/home/seth/actual.file
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/home/seth/zzz.file
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```
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### Using redirection
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Redirection is an efficient way to get data from one place to another in Bash. You may not use redirection all the time, but learning to use it when you need it can save you a lot of needless opening files and copying and pasting data, all of which generally require mouse movement and lots of key presses. Don't resort to such extremes. Live the good life and use redirection.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/20/6/redirection-bash
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作者：[Seth Kenlon][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://opensource.com/users/seth
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/programming-code-keyboard-laptop.png?itok=pGfEfu2S (Hands programming)
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[2]: https://opensource.com/resources/what-bash
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[3]: https://opensource.com/article/19/9/getting-started-zsh
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[4]: https://opensource.com/article/19/7/master-ls-command
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[5]: https://opensource.com/article/19/8/linux-permissions-101
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@ -0,0 +1,139 @@
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[#]: collector: (lujun9972)
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[#]: translator: (nophDog)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Read and write data from anywhere with redirection in the Linux terminal)
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[#]: via: (https://opensource.com/article/20/6/redirection-bash)
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[#]: author: (Seth Kenlon https://opensource.com/users/seth)
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通过重定向在 Linux 终端任意读写
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======
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重定向是一种十分高效的数据流动方式，它能帮你减少很多鼠标和键盘上的操作。
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![Hands programming][1]
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对于任何编程或脚本语言，输入与输出重定向都是很自然的功能。严格来说，当你使用电脑时，数据自然而然地在发生着重定向。从 `stdin`（标准输入，通常是你的键盘或者鼠标）读取输入，输入则发往 `stdout`（标准输出，一段文本或者数据流），最后错误信息送至 `stderr`。如果你使用 [Bash][2] 或 [Zsh][3] 之类的 shell，那么理解这些数据流能够让你更好地控制信息流向。
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标准输入，标准输出以及标准错误输出存在于 Linux 文件系统中。你可以在 `/dev` 查看：
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```
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$ ls /dev/std*
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/dev/stderr@ /dev/stdin@ /dev/stdout@
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```
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你可能没法直接使用它们，但将它们想象成你能传递数据的元位置，会很有帮助。
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重定向的基础很简单：用一些 `>` 符号重定向输出，然后用另外一些 `<` 符号重定向输入。
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### 重定向输出
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将 [ls][4] 命令的输出写入一个文件：
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```
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`$ ls > list.txt`
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```
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你没法像平常那样看到 `ls` 的输出，因为它们并没有被发送到屏幕，而是被写入 `list.txt` 文件了，这个功能用处太多了，事实上，你甚至可以用它来将文件内容拷贝到另一个文件。不一定是文本文件，你也可以用将重定向用于二进制数据：
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```
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`$ cat image.png > picture.png`
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```
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你可能会好奇为什么要这样做，有时候用来获取 [文件权限信息][5] 比较实用。
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### 重定向输入
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你也能将输入重定向“到”一个命令。可以说，它没有重定向输出那么有用，因为许多命令已经被硬编码，只从你的参数中接收输入。但是，如果某个命令需要一系列参数，而且你把这些参数写在文件里，想要快速“复制粘贴”到终端的时候（除非你并不想复制粘贴），它就帮得上忙了。
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```
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`$ sudo dnf install $(<package.list)`
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```
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重定向输入得常规用法是 **here-document** (简写成 **here-doc**) 和 **here-string** 技巧。这种输入方法将一整块文本重定向至标准输入流，直到碰见一个特殊的 end-of-file 标记（许多人习惯用 `EOF`，实际上你可以使用任何字符串，只要它是唯一的）。试着把这些（在第二个 `EOF` 标记之前）敲进你的终端：
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```
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$ echo << EOF
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> foo
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> bar
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> baz
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> EOF
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```
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输出结果：
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```
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foo
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bar
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baz
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```
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使用 [Bash][2] 编写脚本的人常常用这个技巧，将数行文本一次性写入文件或者打印到屏幕上。只要你别忘了末尾的 end-of-file 标记，这会是一个帮你避免大量繁琐 `echo` 或 `printf` 语句的好办法。
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一句 **here-string** 类似于一段 **here-doc**，但是它只含有一个字符串（或者用引号包裹的几个字符串，同样会被当成一个字符串）
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```
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$ cat <<< "foo bar baz"
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foo bar baz
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```
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### 重定向错误信息
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错误信息流叫做 `stderr`，通过 `2>` 实现这个目的。下面这个命令把错误信息定向到 `output.log` 文件：
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```
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`$ ls /nope 2> output.log`
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```
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### 将数据送往 /dev/null
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既然标准输入、标准输出和错误输出都有自己的位置，那么 _nowhere_ 也应该在 Linux 文件系统占有一席之地。没错，它叫做 `null`，位于 `/dev`，频繁使用的人懒得说 `slash dev slash null`，于是索性叫它 "devnull"。
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通过重定向，你可以把数据发送到 `/dev/null`。比如，`find` 命令常常会输出很多具体信息，而且在搜索文件遇到权限冲突时，会事无巨细地报告：
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```
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$ find ~ -type f
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/home/seth/actual.file
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find: `/home/seth/foggy': Permission denied
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find: `/home/seth/groggy': Permission denied
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find: `/home/seth/soggy': Permission denied
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/home/seth/zzz.file
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```
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`find` 命令把那些当作错误，所以你可以只把错误信息重定向至 `/dev/null`：
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```
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$ find ~ -type f 2> /dev/null
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/home/seth/actual.file
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/home/seth/zzz.file
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```
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### 使用重定向
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在 Bash 中，重定向是转移数据的有效方法。你可能不会频繁使用重定向，但是学会如何使用它，能帮你在打开文件、复制粘贴数据这类需要移动鼠标、大量按键操作上，节省很多不必要的时间。不要做如此浪费时间的事情。使用重定向，好好享受生活。
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/20/6/redirection-bash
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作者：[Seth Kenlon][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/nophDog)
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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://opensource.com/users/seth
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/programming-code-keyboard-laptop.png?itok=pGfEfu2S (Hands programming)
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[2]: https://opensource.com/resources/what-bash
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[3]: https://opensource.com/article/19/9/getting-started-zsh
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[4]: https://opensource.com/article/19/7/master-ls-command
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[5]: https://opensource.com/article/19/8/linux-permissions-101
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