xxxxxxxxxx
Just concatenate var '"$var"' to replace values string in sed command:
$ sed -i 's/string_to_replace/'"$var"'/g' file_to_replace.txt
xxxxxxxxxx
Use double quotes to make the shell expand variables while preserving whitespace:
sed -i "s/$var1/ZZ/g" "$file"
xxxxxxxxxx
You can try this sed command
sed 's/,\(.*china\)/,Tomas_proxy.lt\/\1/' FileName
or
sed 's/,\(.*china\)/,Tomas_proxy.lt\/\1/' FileName > NewFile
or
sed -i.bak 's/,\(.*china\)/,Tomas_proxy.lt\/\1/' FileName