Maybe cutting off network access could be done too? That would reduce some potentially useful features, but it could be a parameter to enable it (hey, we're talking Linux, never enough parameters! ;). That is, assuming that a syscalls/capabilities trick like that is possible at all. (Anyone can say more here? Some kernel hackers? This is HN, ain't it?)
Yeah, I think almost any ability to produce an effect other than printing to stdout and stderr should probably be restricted by default, with a flag to enable it. If people adopt this and frequently use specific capabilities, they can add a shell alias to add enable their preferred set of capabilities.
Or, one possible alternative would be to always prevent any side-effects other than printing output, and then display a warning saying something like "This command tried to access the network/modify the filesystem/be naughty in some other way. To allow this and re-run the command, press Control+Shift+Enter"
capabilities should certainly work. for portability, setting a non-existant proxy should also help. most commandline tools should honor shell proxy variables and fail the request.
