Let me be more specific here:

I have two variables in a shell script that I assign by reading contents of a file
The contents are as such
a="$var1=xyz"
b="$var1=abc"

I need to search a second file for occurences of $var1=xyz and replace it with
$var1=abc

I have used sed -s 's/'"$a"'/'"$b"'/g' which works fine but whenever the contents of variable a contain some special characters for example a="$var1=fdh&%4"
the replace is not working. If I change the value to just a="$var1=", the above sed works successfully to replace $var1= in the file to $var1=abc.

Can anybody please help?