ECHO 5%%

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MS-DOS uses %1, %2, ... %9 as replaceable command line parameters. For example, before executing the command ECHO %1, %1 will be replaced with the first parameter passed to the batch file. %0 is replaced with the command used to execute the batch file.

A single percent sign on a line is treated as a "nul" character in a batch file. For example:

   ECHO %     is processed as ECHO
   ECHO a%b   is processed as ECHO ab

If a command contains two percent signs, MS-DOS will treat any characters between them as an environment variable to be expanded. For example, if the SET command shows that the current environment variables are

   COMSPEC=C:\COMMAND.COM
   PATH=C:\DOS
   PROMPT=$P$G
   B=C

then

   ECHO %PATH%     is processed as ECHO C:\DOS
   ECHO a%b%       is processed as ECHO aC
   ECHO a%b b%a    is processed as ECHO aa

If there are no characters between the two percent signs, one percent sign is stripped off and the other will remain. This is why a FOR command that echos the name of each file with a .COM extension would be

   FOR %V IN (*.COM) DO ECHO %V

but if the same command is placed in a batch file, the following is required:

   FOR %%V IN (*.COM) DO ECHO %%V

Reference(s):

"Microsoft MS-DOS Batch File Quick Reference," Microsoft Press.