Question

I expect

set -- "a b"

to set $1 = "a" and $2 = "b", but instead it’s setting $1 = "a b" and $2 = "" what gives?

Answer 1

The double quotes indicate that "a b" should be treated as a single parameter. Use

set -- a b

to treat them as two parameters.

Edit in response to your comment: Like this?

$ a="a b"
$ set -- $a
$ echo $1
a
$ echo $2
b