Question

I’d like to know why this works:

arr=()
fun() { arr[$1]=$2; }
fun 1 2
echo ${arr[1]}
# echoes '2'

but this doesn’t:

arr=()
fun() { arr[$1]=$2; }
fun 1 2 &
wait
echo ${arr[1]}
# echoes a blank line

Answer 1

By running fun in the background in your second example, you run it in a subshell. Changes to the array made in the subshell are not visible to the parent shell, where you echo the value of arr[1].