line 39: [: ==: unary operator expected - arrays

Question

I am trying to generate a password with certain requirements.
When I enter the while loop to generate a random character from the array it is fine until I add a count for my index "$i"
With the following code:
#!/bin/bash
#SET ARRAY VALUES
all=( 0 1 2 3 4 5 6 7 8 9 a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z )
echo
#SET COUNT VALUES TO 0
numc=0
lowc=0
upc=0
i=0
while true;
do
#GENERATE PASSWORD
phrase[$i]=${all[$RANDOM%62]}
#CHECK IF PASSWORD MEETS REQUIREMENTS
for ((n=0; n<10; n++))
do
if [ ${phrase[$i]} == ${all[$n]} ]
then
echo num ${all[$n]}
let "numc++"
let "i++"
fi
done
I get the error "line 39: [: ==: unary operator expected"
but if I remove the let "i++" line then there is no error. But I need to increase my index in order to exit the loop and check the minimum length of the password

Answer 1

If ${phrase[$i]} (by the way you don't need $ in [$i] context ${phrase[i]} works too) is ever the empty string that if test will become [ == value-of-all-n ] which isn't a valid test.
Either quote the variables (which is almost always the right thing to do) or prevent that from ever being the empty string. (Was that i++ supposed to happen outside the inner loop?)