$ FOO=BAR
$ env | grep FOO
$ set | grep FOO
FOO=BAR

$ export BAR=FOO
$ env | grep FOO
BAR=FOO
$ set | grep FOO
BAR=FOO
FOO=BAR

$ FOO=BAR
$ bash
$$ echo $FOO             # Note the empty line

$$ exit
$ export FOO
$ bash
$$ echo $FOO
BAR
$$

$ export FOO
$ FOO=BAR
$ bash
$$ echo $FOO
BAR
$$