a=null
b=null

#!/bin/bash
while : do
    .............
    val_a=1
    sed -i "s/^(a=).*/1$val_a/" file.cfg
    .............
done

#!/bin/bash
while : do
    .............
    val_b=2
    sed -i "s/^(b=).*/1$val_b/" file.cfg
    .............
done