I have a file containing values (config.txt):

Toto=1
Tutu=42

And another file with fields without values (file.txt):

Toto=
Tata=
Tutu=
Titi=

I want to write a script (maybe with regex ^(.*)=) to replace fields in file.txt by values from config.txt. For this example, result will be :

Toto=1
Tata=
Tutu=42
Titi=