If an attribute value is enclosed between single quotes, only character escaping is performed on it.
If an attribute value is enclosed between double quotes, both character escaping and variables expansion are performed on it.
To preserve the literal value of a special character (\, ", ', , {, } or $), precede it with a backslash \ character.
Parts of the attribute value string that are of the form $VARNAME are substituted with the actual value of variable VARNAME. If VARNAME is not declared, it is replaced by an empty string. The variable name must be enclosed between braces as in ${VARNAME} if the characters following it may be considered as part of the name.