该变量用于说明在脚本中如何处理，所需环境变量的验证必须在脚本执行前定义。

This variable is used to illustrate how to handle in the script, the verification that required environment variables must be defined prior to running the script.

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程序的man页面中通常包含对自定义应用程序环境变量的说明。

Custom application environment variables are typically documented in the program's man pages.

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事实上，它只是说明变量定义在程序的其他地方。

In effect, it claims that a definition of the variable exists elsewhere in the program.

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