Now some time ago I saw a patch for pbuilder itself to “fix” this, but for the time beeing I used to set DEBIAN_FRONTEND to readline in my pbuilderrc. But today Michael Koch came up with much better suggestions: Preset the debconf value in the CHROOT and save it.
It's pretty easy:
$ sudo pbuilder login --save-after-login # echo "sun-java5-jdk shared/accepted-sun-dlj-v1-1 boolean true" | debconf-set-selections # echo "sun-java6-jdk shared/accepted-sun-dlj-v1-1 boolean true" | debconf-set-selections # exit
and voila, problem solved. Thanks to Michael for the tip.
Manual Prinz suggested a slightly different way, that realizes the same debconf setting, but with hooks.