read and env variables

[isabella parakiss]

$ a= read a <<< x       # this creates a variable in the current shell
$ declare -p a
declare -- a="x"

$ b= mapfile b <<< x    # this doesn't
$ declare -p b
bash: declare: b: not found


Other shells don't seem to agree on what should happen in this case, but
it'd be nice to have a more consistent behavior.

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xoxo iza