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A Polyglot for jscript in batch
0</* :hello
@ECHO Hello, batch!
@cscript /nologo /E:jscript %~f0 %*
@goto :EOF
WScript.Echo('Hello, jscript!');
Hello, batch!
Hello, jscript!

Windows lacks anything quite like Unix's shebang, which provides a way for a script to tell its interpreter how to interpret it, but polyglots can be used to provide similar functionality.

A polyglot is a script that works in multiple interpreters, hiding functionality and syntax specific to one language from another language's interpreter by clever use of each language/interpreter's specific features.

Let's look at how each of our two interpreters deal with the above script:

  • BATCH allows redirection to occur anywhere in a command line, not just a the tail. It does this by actually rearranging the command in pre-parsing, turning our first line 0<*/ :hello into :hello 0<*/, which is a label and is not executed. Line 2 is a hello from batch, Line 3 executes the called script in cscript with the appropriate arguments (subtitute your interpreter here), and line 4 sends us to :EOF, ignoring the rest of the script.

  • JAVASCRIPT: Lines 1-5 are reduced to a single instruction once the multi-line comment is ignored: 0<0;. This adds minimal weight and no memory overhead, and the script continues as normal.

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