You seem to be confused about how interpolation works. In your first example, $cmd is interpolated in the double quotes, and $cmd happens to contain a string that contains some things that look like variables. If you wanted those variables to then be interpolated, you'd need to eval that code somehow. This however would not be considered exactly safe. But to directly answer the question something to the effect of:

$cmd = eval "\"$cmd\"";

will give you the string that you want to use in your system command.

In reply to Re: problem with string literal in system() by cephas
in thread problem with string literal in system() by cvg2mco

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