Interestingly enough, on plan 9 environment variables are also files in a directory, /env. This
makes commands like printenv obsolete (ls /env).

Basically, they are inherited into the new namespace when the process changes (unless one
configures the namespace with rfork).

There are some weird rules how environment variables with special names are inherited,
but unfortunately, I can't remember the exact rules.

Edit: See also http://man.cat-v.org/plan_9/3/env