Replace percent-escaped characters in string ( , [, …) with bash

Question

I have strings containing percent-escaped characters like %20 and %5B, and I would like to transform it to \"normal\" characters like \\ f

Answer 1

The builtin printf in bash has a special format specifier (i.e. %b) which converts \x** to the corresponding value:

$ str='foo%20%5B12%5D'
$ printf "%b\n" "${str//%/\\x}"
foo [12]