unset a b
#
readonly a=A
b=B
readonly b
# readonly on already readonly var is harmless:
readonly b a
readonly | grep '^readonly [ab]='
# this should work:
export a b
export -n a b
echo Ok:$?
env | grep -e^a= -e^b=  # shows nothing

echo
# these should all fail (despite the same value being assigned)
# bash does not abort even in non-interactive more (in script)
true; a=A
echo Fail:$?
true; readonly a=A
echo Fail:$?

echo
# in bash, assignment in export fails, but export succeeds! :)
# we don't mimic that!
true; export a=Z
echo Fail:$?
#env | grep '^a='
#echo "^^^a is exported"
export -n a  # undo that bashism, if it happens

echo
export b
# this fails to both set and export a:
a=Z env | grep '^[ab]='
echo "^^^a is not exported"
# but external command does get executed, and $? is not mangled (stays 42):
(exit 42); a=Z env echo Visible:$?

echo
true; unset a
echo Fail:$?