#!/usr/bin/env bash

declare -A users=([first1.last1@example.com]=user1 [firt2.last2@example.com]=user2 [first3.last3@example.com]=user3 [first4.last4@example.com]=user4)

for i in ${!users[@]}
    do
        echo mail: ${i}  user: ${users[$i]}
    done

echo "###"

case ${1} in
    ${!users[$1]})
    echo ${!users[$1]}
esac

echo "###"

case ${1} in
    ${users[$1]})
    echo ${users[$1]}
esac

echo "###"


---> sh -x ./associative_array.sh constantin.roman@oracle.com
+ users=([first1.last1@example.com]=user1 [firt2.last2@example.com]=user2 [first3.last3@example.com]=user3 [first4.last4@example.com]=user4)
+ declare -A users
+ for i in '${!users[@]}'
+ echo mail: first3.last3@example.com user: user3
mail: first3.last3@example.com user: user3
+ for i in '${!users[@]}'
+ echo mail: first4.last4@example.com user: user4
mail: first4.last4@example.com user: user4
+ for i in '${!users[@]}'
+ echo mail: first1.last1@example.com user: user1
mail: first1.last1@example.com user: user1
+ for i in '${!users[@]}'
+ echo mail: firt2.last2@example.com user: user2
mail: firt2.last2@example.com user: user2
+ echo '###'
###
+ case $1 in
+ echo '###'
###
+ case $1 in
+ echo '###'
###