asking for help (checking md5 before updating)

[seventhorama]

I wanna make a patcher script, which do scan the target folder before applying patch (that use xdelta), but i have no idea on how to perform md5 check before the update process began (to make sure the destination folder mached the version, the setupwas intended to..)

I want to make it like this one, to make the setup stop whenever the md5 check fails, and shows which file doesnt match the md5