如果希望将错误或标准输出组合为单个文件，请使用编号重定向(请参见清单4)。
If you want to combine error and standard output into a single file, use numbered redirection (see Listing 4).
即使找不到文件，它也不会向标准输出返回错误。
It will not return an error to standard output if the file is not found. One way to test the return of the command is to use substitution, like shown in Listing 8.
在命令管道中作为组件工作(通过读取标准输入或文件名参数，以及写至标准输出和标准错误)。
Working as a component in a pipeline of commands (by reading either standard input or a filename argument, and writing to standard output and standard error).
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