许多文本处理命令（过滤器）可以从标准输入流或从文件中获取输入。

Many text processing commands (filters) can take input either from the standard input stream or from a file.

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可以将它们用作过滤器，或者将文件名作为命令的参数。

They can be used as filters, or they can take a filename as an argument.

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由于tr是纯粹的过滤器，因此您将使用cat命令为其生成输入。

Since tr is purely a filter, you generate input for it using the cat command.

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