Is there any reason you're not wanting to use the Rename procedure that's
part of the fpc system library?

www.freepascal.org/docs-html/rtl/system/rename.html

One problem with your code is that it would depend on a *nix style operating
system, and that the 'mv' command is available at your specified path.
Obviously that makes your code less portable. That might not matter, but if
it's potentially a concern it's worth noting.

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