Cmd 文件中% 和%% 的区别是什么？

我最近在一个.cmd 文件中包含了一行类似的代码:

for /f %%f in ('dir /b .\directory\*.sql') DO sqlcmd -b -o ".\directory\output\%%f.txt" -i ".\directory\%%f"

最初我只使用了% f，它在命令行上运行时可以正常工作，但是在文件中运行时就不行了。当我切换到% f 时，它在文件中起作用了。我只是想知道有什么区别。

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In DOS you couldn't use environment variables on the command line, only in batch files, where they used the % sign as a delimiter. If you wanted a literal % sign in a batch file, e.g. in an echo statement, you needed to double it.

This carried over to Windows NT which allowed environment variables on the command line, however for backwards compatibility you still need to double your % signs in a .cmd file.

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(Explanation in more details can be found in an archived Microsoft KB article.)

Three things to know:

  1. The percent sign is used in batch files to represent command line parameters: %1, %2, ...

  2. Two percent signs with any characters in between them are interpreted as a variable:

    echo %myvar%

  3. Two percent signs without anything in between (in a batch file) are treated like a single percent sign in a command (not a batch file): %%f

Why's that?

For example, if we execute your (simplified) command line

FOR /f %f in ('dir /b .') DO somecommand %f

in a batch file, rule 2 would try to interpret

%f in ('dir /b .') DO somecommand %

as a variable. In order to prevent that, you have to apply rule 3 and escape the % with an second %:

FOR /f %%f in ('dir /b .') DO somecommand %%f

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