if conditions for checking variable was set or exist on shell script

conditional.sh

#!/bin/sh
a=1
b=2
  
# #Check whether they are equal
# if [ $a = $b ]
# then
#     echo "a is equal to b"
# fi

if [ ! -z "$a" ] && [ ! -z "$b" ];
then
    echo "a & b exist"
elif  [ ! -z "$a" ]; then
	echo "a exist"
elif  [ ! -z "$b" ]; then
	echo "b exist"
else
	echo "a and/or b not exist"
fi
  
#Check whether they are not equal
# if [ $a != $b ]
# then
#     echo "a is not equal to b"
# fi