#a function gets the names of the variables,
#local -n turns them into references (bash 4.3 and newer)
swapStrings() {
local -n v1=$1
local -n v2=$2
local tmp=$v1
v1=$v2
v2=$tmp
}
s1="A"
s2="B"
swapStrings s1 s2
#s1 is "B", s2 is "A"
echo "s1 is $s1"
echo "s2 is $s2"