    $ awk -vPRIMER="ACTG" '{if ($0~/^>/) { print $0; } else { print PRIMER$0; }}' in.fa > out.fa

(by  Alex Reynolds)

Can this code be modified to add primer sequence in the end of the complete sequence, what we have generated in the previous code? And I assume we will already use the reverse complement sequence so that we dont have to include that thing in the code.
Reposition `PRIMER$0` to `$0PRIMER` to move the string to the end.