检查的环境变量而没有命令扩展，不使用批处理文件中定义的？ [英] Check if an environment variable is defined without command extensions and without using a batch file?

问题描述

我需要使用CMD.EXE命令行（CMD.EXE正在从石膏构建工具的称呼）来确定一个环境变量是否定义或没有。我怎样才能做到这一点？我还好假设变量的值不包含单或双引号，但不能假定启用了命令扩展。

I need to use a cmd.exe command line (cmd.exe is being called from the gyp build tool) to determine whether an environment variable is defined or not. How can I do this? I am okay assuming that the variable value does not contain single or double quotes, but cannot assume that command extensions are enabled.

我试过以下，这在.bat文件的伟大工程，但在命令行上直接输入失败：

I've tried the following, which works great in a .bat file, but fails when typed directly on the command line:

IF "%UNDEFINED%" == "" (echo yes)

在这确切的行是一个.bat文件并执行，我看是作为输出。当我在命令行中键入它，输出是空的。我在Windows XP SP3测试这一点，虽然我的同事看到在Windows 7中相同的结果这是 HTTP建议的方法：// support.microsoft.com/kb/121170 和<一个href=\"http://www.robvanderwoude.com/battech_defined.php\">http://www.robvanderwoude.com/battech_defined.php.我不想使用如果定义未定义（回声是），因为如果命令扩展被禁用，将无法正常工作。

When that exact line is in a .bat file and executed, I see yes as the output. When I type it on the command line, the output is empty. I am testing this on Windows XP SP3, though my coworker sees the same results on Windows 7. This is the method suggested by http://support.microsoft.com/kb/121170 and http://www.robvanderwoude.com/battech_defined.php. I do not want to use IF DEFINED UNDEFINED (echo yes) because that won't work if command extensions are disabled.

在下面的帖子中得票最多的回答使我相信，这个问题涉及到如何％的膨胀在CmdLineParser不同的处理主场迎战BatchLineParser，但仍然没有导致我一个解：
<一href=\"http://stackoverflow.com/questions/4094699/how-does-the-windows-command-inter$p$pter-cmd-exe-parse-scripts\">How不Windows命令国米preTER（CMD.EXE）解析脚本？

The top-voted answer in the following post has led me to believe that this issue is related to how percent-expansion is handled differently in the "CmdLineParser" vs. the "BatchLineParser," but still has not led me to a solution: How does the Windows Command Interpreter (CMD.EXE) parse scripts?

推荐答案

OK，这花了一点，但我想我已经想通了。试试这个：

OK, this took a bit, but I think I've figured it out. Try this:

SET UNDEFINED 2>Nul | Findstr/I "."
IF ERRORLEVEL 1  ECHO Not Defined.

这适用于所有AFAIK情况下，不依赖于任何命令扩展功能。

This works for all cases AFAIK, and does not rely on any command extension features.

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