It took me like an hour to go through all the respective man pages.
Also FYI, the mans where far more helpful than Google.
In light of the "shell shock" of this week, I assume I need to upgrade bash. Can I merely execute you could selectively backport bash to those systems. And bear in mind this security vulnerability is only one of many.
To check a particular system for the vulnerabilities (or see if the upgrade works) you can check the bash versions that you are using and see if the version is affected (it probably is) or there are numerous shell test scripts available on the web.
Done W: There is no public key available for the following key IDs: 9AA38DCD55BE302B W: GPG error: Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9AA38DCD55BE302B W: You may want to run apt-get update to correct these problems$ apt-get install debian-archive-keyring Reading package lists... Done The following packages will be upgraded: debian-archive-keyring 1 upgraded, 0 newly installed, 0 to remove and 51 not upgraded. After unpacking 4096B of additional disk space will be used.