What is the difference between % and %% in a cmd file?

Question

I recently included a line similar to this in a .cmd file:

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

Originally I had only used %f, and it would work fine when run on the command line, but not when run through the file. When I switched to %%f, it worked in the file. Just wondering what the difference is.

Answer 1

(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

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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