Difference between quote and double quote on bash

sed -e '/^$$/d' 
sed -e "/^$$/d" 
`
echo '$${c}'
echo "$${c}"

awk  wants the single quote 

when you use single quotes the variables aren’t going to be expanded.http://www.howtogeek.com/howto/29980/whats-the-difference-between-single-and-double-quotes-in-the-bash-shell/