Bash scripts troubles with Tomsrtbt... Help!

Hi,

I have troubles with Bash scripts an Tomsrtbt:
I don't succeed to use "counter".
And I don't succed with zero-length-string tests:

#!/bin/sh
I=0
echo "Initial value:$I"
echo "Numbers from 0 to 9:"
while [ $I -lt 10 ] ; do
echo $I
#I=$(($I+1))
I=$[$I+1]
#Both solutions are OK for Mdk8.2
done
echo "1st stage ended"
echo "-----------------"
#This first stage doesn't work:
#With I=$(($I+1))
#I get :
#0+1: not found
#[: 10:unknown operand

#With I=$[$I+1]
#I get :
#[:$[0+1]:bad number


#-----------------------------------------------
liste="1 2 3 4 5 6"
for I in $liste
do
echo $I
done
echo "2nd stage ended"
echo "-----------------"
#This second stage is OK.


#-----------------------------------------------
A=''
while true; do
echo "Hit 'Quit' to quit, or not-empty-string to follow up."
echo "Hit value for A:"
read A
if [ ! -z $A ]; then
	if [ $A = "Quit" ]; then
		exit
	else
		break
	fi
fi
done
echo "You hit: $A"
echo "3rd stage ended"
echo "-----------------"
#With 'Quit' and 'not-empty-strings' it works,
#but with empty-string, I get:
#[: -z: argument expected


#-----------------------------------------------
B=''
while true; do
echo "Hit value for B:"
read B
if [ ! -z $B ]; then
break
fi
done
echo "You hit: $B"
echo "4rth stage ended"
echo "-----------------"
#With 'not-empty-strings' it works,
#but with empty-string, I get:
#[: -z: argument expected


#-----------------------------------------------
C=''
while [ -z $C ]; do
echo "Hit value for C:"
read C
done
echo "You hit: $C"
echo "5th stage ended"
echo "-----------------"
#I can't do anything.
#An error occurs and I can't hit anything
#I get:
#[: -z: argument expected


#-----------------------------------------------
while [ ! $D ]; do
echo "Saisir une valeur pour D:"
read D
done
echo "Vous avez saisi: $D"
echo "C'est fini!"
#I can't do anything.
#An error occurs and I can't hit anything
#I get:
#[: argument expected


What should I do?
What is the syntax with Tomsrtbt?
In what way does the shell differ from the one I use with Mdk8.2
(my script is OK with Mdk8.2)

Thank you for all.