输入/输出重定向

输入/输出重定向 - I/O Redirection

https://linuxcommand.org/lc3_lts0070.php

In this lesson, we will explore a powerful feature used by command line programs called input/output redirection. As we have seen, many commands such as ls print their output on the display. This does not have to be the case, however. By using some special notations we can redirect the output of many commands to files, devices, and even to the input of other commands.

​ 在这个课程中，我们将探索命令行程序中使用的一项强大功能，称为输入/输出重定向。正如我们所见，许多命令（如 ls）将它们的输出打印到显示器上。但是，情况并非总是如此。通过使用一些特殊的符号，我们可以将许多命令的输出重定向到文件、设备，甚至其他命令的输入。

标准输出 Standard Output

Most command line programs that display their results do so by sending their results to a facility called standard output. By default, standard output directs its contents to the display. To redirect standard output to a file, the “>” character is used like this:

​ 大多数命令行程序将它们的结果显示出来是通过将结果发送到一个称为标准输出的设备。默认情况下，标准输出将其内容定向到显示器上。要将标准输出重定向到文件，可以使用 “>” 符号，像这样：

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[me@linuxbox me]$ ls > file_list.txt

In this example, the ls command is executed and the results are written in a file named file_list.txt. Since the output of ls was redirected to the file, no results appear on the display.

​ 在这个例子中，执行了 ls 命令，并将结果写入名为 file_list.txt 的文件中。由于 ls 的输出被重定向到文件，所以在显示器上没有显示任何结果。

Each time the command above is repeated, file_list.txt is overwritten from the beginning with the output of the command ls. To have the new results appended to the file instead, we use “>>” like this:

​ 每次重复执行上述命令时，file_list.txt 将被从头开始覆盖为 ls 命令的输出。如果要将新结果追加到文件而不是覆盖，可以使用 “>>"，像这样：

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[me@linuxbox me]$ls >> file_list.txt

When the results are appended, the new results are added to the end of the file, thus making the file longer each time the command is repeated. If the file does not exist when we attempt to append the redirected output, the file will be created.

​ 当结果被追加时，新的结果会添加到文件的末尾，从而使得文件每次重复执行命令时都会变长。如果在尝试追加重定向输出时文件不存在，文件将会被创建。

标准输入 Standard Input

Many commands can accept input from a facility called standard input. By default, standard input gets its contents from the keyboard, but like standard output, it can be redirected. To redirect standard input from a file instead of the keyboard, the “<” character is used like this:

​ 许多命令可以从一个称为标准输入的设备接受输入。默认情况下，标准输入从键盘获取内容，但是像标准输出一样，它也可以被重定向。要将标准输入从文件而不是键盘重定向，可以使用 “<” 符号，像这样：

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[me@linuxbox me]$ sort < file_list.txt

In the example above, we used the sort command to process the contents of file_list.txt. The results are output on the display since the standard output was not redirected. We could redirect standard output to another file like this:

​ 在上面的例子中，我们使用了 sort 命令来处理 file_list.txt 的内容。由于没有重定向标准输出，结果会输出到显示器上。我们可以像这样将标准输出重定向到另一个文件：

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[me@linuxbox me]$ sort < file_list.txt > sorted_file_list.txt

As we can see, a command can have both its input and output redirected. Be aware that the order of the redirection does not matter. The only requirement is that the redirection operators (the “<” and “>”) must appear after the other options and arguments in the command.

​ 如我们所见，一个命令可以同时重定向其输入和输出。需要注意的是，重定向的顺序不重要。唯一的要求是重定向操作符（"<” 和 “>"）必须出现在命令中其他选项和参数之后。

管道 Pipelines

The most useful and powerful thing we can do with I/O redirection is to connect multiple commands together to form what are called pipelines. With pipelines, the standard output of one command is fed into the standard input of another. Here is a very useful example:

​ 使用I/O重定向最有用和强大的功能之一是将多个命令连接在一起，形成所谓的管道。通过管道，一个命令的标准输出被发送到另一个命令的标准输入。下面是一个非常有用的例子：

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[me@linuxbox me]$ ls -l | less

In this example, the output of the ls command is fed into less. By using this "| less" trick, we can make any command have scrolling output.

​ 在这个例子中，ls 命令的输出被发送到 less 命令。通过使用这个 "| less" 的技巧，我们可以使任何命令都具有滚动输出。

By connecting commands together, we can accomplish amazing feats. Here are some examples to try:

​ 通过将命令连接在一起，我们可以完成令人惊奇的任务。以下是一些可以尝试的例子：

过滤器 Filters

One kind of program frequently used in pipelines is called a filter. Filters take standard input and perform an operation upon it and send the results to standard output. In this way, they can be combined to process information in powerful ways. Here are some of the common programs that can act as filters:

​ 在管道中经常使用的一种程序称为过滤器。过滤器接受标准输入并对其进行操作，然后将结果发送到标准输出。通过这种方式，它们可以结合在一起以强大的方式处理信息。以下是一些常见的可以作为过滤器的程序：

使用管道执行任务 Performing tasks with pipelines

1. Printing from the command line. Linux provides a program called lpr that accepts standard input and sends it to the printer. It is often used with pipes and filters. Here are a couple of examples:

2. 从命令行打印。 Linux提供了一个名为lpr的程序，可以接受标准输入并将其发送到打印机。它经常与管道和过滤器一起使用。以下是一些示例：

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   cat poorly_formatted_report.txt | fmt | pr | lpr cat unsorted_list_with_dupes.txt | sort | uniq | pr | lpr

   In the first example, we use cat to read the file and output it to standard output, which is piped into the standard input of fmt. fmt formats the text into neat paragraphs and outputs it to standard output, which is piped into the standard input of pr. pr splits the text neatly into pages and outputs it to standard output, which is piped into the standard input of lpr. lpr takes its standard input and sends it to the printer.

   在第一个示例中，我们使用cat读取文件并将其输出到标准输出，然后通过管道将其传递给fmt的标准输入。fmt将文本格式化为整齐的段落，并将其输出到标准输出，然后通过管道将其传递给pr的标准输入。pr将文本整齐地分页，并将其输出到标准输出，然后通过管道将其传递给lpr的标准输入。lpr接受标准输入并将其发送到打印机。

   The second example starts with an unsorted list of data with duplicate entries. First, cat sends the list into sort which sorts it and feeds it into uniq which removes any duplicates. Next pr and lpr are used to paginate and print the list.

   第二个示例以一个包含重复条目的未排序数据列表开始。首先，cat将列表发送到sort，sort对其进行排序并将其传递给uniq，uniq删除任何重复项。然后使用pr和lpr进行分页和打印列表。

3. Viewing the contents of tar files Often you will see software distributed as a gzipped tar file. This is a traditional Unix style tape archive file (created with tar) that has been compressed with gzip. You can recognize these files by their traditional file extensions, “.tar.gz” or “.tgz”. You can use the following command to view the directory of such a file on a Linux system:

4. 查看tar文件的内容 经常会看到软件以gzipped tar文件的形式分发。这是一个传统的Unix风格的磁带归档文件（使用tar创建），经过gzip压缩。你可以通过文件的传统扩展名”.tar.gz"或".tgz"来识别这些文件。你可以使用以下命令在Linux系统上查看此类文件的目录：

   1	tar tzvf name_of_file.tar.gz | less

进一步阅读 Further Reading

• Chapter 6 of The Linux Command Line covers this topic in more detail.
• The Linux Command Line的第6章更详细地介绍了这个主题。
• Chapters 19 through 21 of The Linux Command Line provide an in-depth look at the text processing tools available in Linux.
• 《The Linux Command Line》的第19至21章深入介绍了Linux中可用的文本处理工具。
• To learn more about the AWK programming language, consider the AWK adventure.
• 如果想更多了解AWK编程语言，请参考AWK adventure。