Linux, increment variable always getting reset inside sub loop

process_substitution.sh

main="test_subtitution"
a="$main/folder_a"
b="$main/folder_b"

mkdir -p $a $b

touch $a/file_1 $a/file_5 $a/file_9 $a/file_10 $a/file_8
touch $b/file_1 $b/file_5 $b/file_9 $b/file_10 $b/file_8



######################
echo "ls | while; do index++"
index=1
ls $a | while read i; do echo "$index"_"$(basename $i)"; index=$(($index + 1)); done
# This will get
# 1_file_a
# 2_file_b
# 3_file_c
# 4_file_d
# 5_file_e
echo



######################
echo "find | while read; do index++"
index=1
find $main/* -type f | while read i
	do 
		echo "$index"_"$(basename $i)"
		index=$(($index + 1))
	done
# will get
# 1_file_1
# 2_file_10
# 3_file_5
# 4_file_8
# 5_file_9
# 6_file_1
# 7_file_10
# 8_file_5
# 9_file_8
# 10_file_9
echo



#######################
echo "ls -R | while; do index++"
index=1
ls -R $main | while read i; do echo "$index"_"$(basename $i)"; index=$(($index + 1)); done
# Will get
# 1_test_subtitution:
# 2_folder_a
# 3_folder_b
# basename: missing operand
# Try 'basename --help' for more information.
# 4_
# 5_folder_a:
# 6_file_1
# 7_file_10
# 8_file_5
# 9_file_8
# 10_file_9
# basename: missing operand
# Try 'basename --help' for more information.
# 11_
# 12_folder_b:
# 13_file_1
# 14_file_10
# 15_file_5
# 16_file_8
# 17_file_9
echo



#######################
echo "find -type d | while; do ls | while; do index++"
index=1
find $main/* -type d | while read i
	do
		ls $i | while read x
			do
				echo "$index"_"$(basename $x)"
				index=$(($index + 1))
			done
	done	

# will get
# 1_file_1
# 2_file_10
# 3_file_5
# 4_file_8
# 5_file_9
# 1_file_1
# 2_file_10
# 3_file_5
# 4_file_8
# 5_file_9
echo



####################### (the solution)
echo "find -type d | while; do while; do index++ < <( )" 
index=1
find $main/* -type d | while read i
	do
		while read x
			do
				echo "$index"_"$(basename $x)"
				index=$(($index + 1))
			done < <(ls $i)
	done
	
# Will get
# 1_file_1
# 2_file_10
# 3_file_5
# 4_file_8
# 5_file_9
# 6_file_1
# 7_file_10
# 8_file_5
# 9_file_8
# 10_file_9
echo



####################### (same, but I just adding -v parameter to the ls command)
echo "find -type d | while; do while; do index++ < <(ls -v)" 
index=1
find $main/* -type d | while read i
	do
		while read x
			do
				echo "$index"_"$(basename $x)"
				index=$(($index + 1))
			done < <(ls -v $i)
	done
	
# Will get
# 1_file_1
# 2_file_5
# 3_file_8
# 4_file_9
# 5_file_10
# 6_file_1
# 7_file_5
# 8_file_8
# 9_file_9
# 10_file_10