On Ubuntu 16.04.1:

test.sh script:

#!/bin/bash

IAM=$(whoami)
ID=$(id -u $IAM)
echo "${IAM}"
echo "${ID}"


Andromeda $ ls -al | grep test.sh 
-rwsrwxrwx  1 pkaramol pkaramol        71 Δεκ   8 07:52 test.sh

As pkaramol (uid=1000)

Andromeda $ ./test.sh 
pkaramol
1000

As testuser (uid=1001)

Andromeda $ su testuser 
Password: 
testuser@Andromeda:/home/pkaramol/Desktop$ ./test.sh 
testuser
1001

Given that the suid has been set, why, in the second run, do we not see pkaramol and 1000 in the output? Isn't the script supposed to be executed with the file owner's id?