coderay --help states this:

  coderay file.rb > file.html          # highlight file to HTML page

However, that command generates terminal output. The file.html page contains ANSI escape sequences and no HTML. For an HTML page, only this works:

coderay credits.cpp -page > test.html

This is on ArchLinux 64bit with Ruby ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-linux] and CodeRay 1.1.0.

Vale,
Quintus