What does declare -A do in Linux shell?

Question

I'm confused by what declare -A does here, could somebody explain?

declare -A deploy
roles="test1 test2 test3 test4"

for role in $roles; do
    deploy[$role]=${!role}
done

More confuse about ${!role}

Answer 1

declare -A defines an associative array, one that can map a string to another string.

For example:

pax> declare -A mymap

pax> mymap[washington]=george
pax> mymap[lincoln]=abe

pax> echo ${!mymap[*]}
washington lincoln

pax> echo ${mymap[no_such_key]}

pax> echo ${mymap[washington]}
george

pax> echo ${mymap[lincoln]}
abe

---

In terms of the ${!role} bit, this is indirect expansion. Normally, a variable will have one level of expansion as you can see below:

pax> plugh=xyzzy
pax> xyzzy=zorkmid
pax> echo ${plugh}
xyzzy

However, you can also treat the expansion of the name as another variable which is subsequently expanded:

pax> echo ${!plugh}
zorkmid

What happens there is that the plugh is expanded to xyzzy, then that itself is expanded again to zorkmid.

In your specific case where, for example, the variable role is set to test1, the following lines are equivalent:

deploy[$role]=${!role}
deploy[test1]=${test1}