repackage android phonegap application from command line

I'd like to do the following (and yes, I've read the posts warning against this etc., and am aware of the caveats):

1. unzip my .apk
2. change the contents of one of the files in assets/www/
3. repackage everything into a valid (unsigned is fine) .apk file

I want to do this fro开发者_如何学JAVAm the command-line (no Eclipse etc) but I don't know how to do 3) above - ideally something like "aapt blah-de-blah" or even "ant some-other-options" would do the trick, but all suggestions welcome.

Thanks,

Paul