jkozuch,
robots.txt by their very nature need to be readable. thats how they work. And guess what, if Googlebot can read it, I can too, and so can everyone else.
robots.txt are NOT intended to hide/keep private/etc.. directories, it’s intended to keep a directory from being indexed. and thats NOT the same thing. Additionally, since a robots.txt file needs to be readable, by design, putting the name of a directory that you want to be “hidden” (and I use that term loosely) inside it, defeats the purpose.
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In a nutshell:
a directory which you desire to be “hidden” (and I use that term loosely) should not be linked to. NOT just because a searchbot will find it, but because a human will, inevitably.
a directory that you dont want indexed is probably going to be one with dynamic content, files that are downloadable, perhaps a directory that once contained ‘things’ but no longer does, and google (for instance) keeps going back to check on from time to time.
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apples and oranges.