Why cp: missing file operand? What "cp" parameter i need to add or whats wrong please?

domlistexp=domlistexp
domlistexp_bak=domlistexp_bak
touch $domlistexp
rm -rf $domlistexp_bak
cp $domlistexp $domlistexp_bak

output:
+ rm -rf domlist_bak
+ cp
cp: missing file operand
Try `cp --help' for more information.

Check carefully the names of the variables. The line
tells us that the variables domlistexp and domlistexp_bak are empty. Moreover, the output you have shown doesn't match the lines of code posted above: the line
should be
instead.

colucix, no i think it should not.

sed /^\$domain$/d $domlist_bak > $domlist;rm -rf $domlist_bak
cp $domlistexp $domlistexp_bak

i think its correct and that is the place where the cp: error happening..? so error is in: cp $domlistexp $domlistexp_bak command?

He's saying your variable names are wrong, and the code you posted does not match the output you posted. Therefore you messed something up in copying things into your post, which, since the likely cause is a typo, makes it impossible to diagnose the problem.

will NOT expand to
Which means you did not post your actual code or output, and we can do nothing to help you.

As colucix said,
indicates that your variables $domlistexp $domlistexp_bak being used in the cp command (or whatever you ARE using, since we've already established there's a discrepancy here) are not expanding, they're empty variables. This likely points to a typo in the code that you did not post here.