Shell Programming and Scripting

if [ $uid != 'root' ] 
then 
        echo "You are not a superuser, please login as a superuser"
        exit1; 
fi

if [ $USERNAME != 'root' ]
 then
  echo "you are not root"
 else
  echo "you are root"
 fi

id | sed 's/).*$//;s/^.*(//'

$ ps -p $$
   PID TTY      TIME CMD
 10387 pts/4    0:00 bash
$ id
uid=0(root) gid=1(other)
$ echo $UID
0
$ [ "$UID" -eq 0 ]||{ echo 'Go away!';exit 1;}
$ su - oracle
$  [ "$UID" -eq 0 ]||{ echo 'Go away!';exit 1;}
Go away!
logout