Shell Programming and Scripting

for i in W X Y Z
do
if [ -n "$i" ] ; ## [ "$i" ] ## [ $i ] 
then read -p "Please enter a value for i: " i
fi
done

#!/bin/bash
arr[0]=$W;arr[1]=$X;arr[2]=$Y;arr[3]=$Z
ar[0]=W; ar[1]=X; ar[2]=Y; ar[3]=Z
for i in `seq 0 3`
do
if [ -z ${arr[$i]} ];then
  read -p "Please enter value for ${ar[$i]} : " arr[$i]
fi
done
W=${arr[0]};X=${arr[1]};Y=${arr[2]};Z=${arr[3]};

# ./loop.sh
first values W='1' Y='2' X='' Z=''
Please enter a value for empty string X: 3
Please enter a value for empty string Z: 4
after loops W='1' Y='2' X='3' Z='4'

## loop.sh ##
W=1;Y=2
echo first values "W='$W' Y='$Y' X='$X' Z='$Z'"
for i in W X Y Z
do
x=$(eval echo \$$i)
if [ -z $x ] ; ## [ "$i" ] ## [ $i ]
then read -p "Please enter a value for empty string $i: " $i
fi
done
echo after loops "W='$W' Y='$Y' X='$X' Z='$Z'"