Useful Polyglot Code

Four years ago I blogged about an incantation that would allow the Windows command interpreter (cmd) to execute Unix shell scripts written inside plain batch files. Time for an update.

The original trick I posted involved starting the shell script with the line

@#!sh %0.bat %* 2>nul

What we need in order to have a Windows command interpreter batch file (a file with a .BAT suffix) executed through the Unix shell is a Windows command that pass the file as an argument to the Unix shell. The following code will do this trick for its first argument.

@echo off
if "%~$PATH:1"=="" (
	rem Not found in PATH; try to run it as specified using the current
	rem directory or an absolute path
	sh %1 %2 %3 %4 %5 %6 %7 %8 %9
) else (
	rem Run the script from the path
	sh %~$PATH:1 %2 %3 %4 %5 %6 %7 %8 %9
)

We then need a polyglot command to invoke the above code: a line that will be valid both in the Windows command interpreter and the Unix shell, because both will execute our file's first line. A polyglot program is one that is simultaneously valid in more than one programming language. Such programs have a long history as interesting curiosities. For instance, the following code by Jack Applin, which won in the 1986 International Obfuscated C Code Contest, is portable to the C and Fortran 77 compilers as well as executable by the Unix Bourne shell.

cat =13 /*/ >/dev/null 2>&1; echo "Hello, world!"; exit
*
*  This program works under cc, f77, and /bin/sh.
*
*/; main() {
      write(
cat-~-cat
     /*,'(
*/
     ,"Hello, world!"
     ,
cat); putchar(~-~-~-cat); } /*
     ,)')
      end
*/

In our case starting a batch / shell script file with the following polyglot line does the trick.

@#!sh %0.bat %*

1. A batch file named #!sh.bat with these contents. (Download the file by right-clicking on the link; note that the file name starts with #!).
2. An empty file named @#!sh.

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