Split single string into character array using ONLY bash

s="hello"
declare -a a   # define array a
for ((i=0; i<${#s}; i++)); do a[$i]="${s:$i:1}"; done
declare -p a   # print array a in a reusable form

Output:

declare -a a='([0]="h" [1]="e" [2]="l" [3]="l" [4]="o")'

or (please note the comments)

s="hello"
while read -n 1 c; do a+=($c); done  <<< "$s"
declare -p a

Output:

declare -a a='([0]="h" [1]="e" [2]="l" [3]="l" [4]="o")'

To split string into array of characters, with null delimiter, you can:

str='hello'
arr=()
i=0
while [ "$i" -lt "${#str}" ]; do
  arr+=("${str:$i:1}")
  i=$((i+1))
done

printf '%s\n' "${arr[@]}"

With delimiter other than null, you can:

set -f
str='1,2,3,4,5'
IFS=',' arr=($str)
printf '%s\n' "${arr[@]}"

Just for fun (and other shells) other variant:

word=hello
unset letter
while [ ${#word} -gt 0 ]
do
    rest=${word#?}
    letter[${#letter[*]}]=${word%$rest}
    word=$rest
done

And check

for l in "${!letter[@]}"
do
    echo "letter [$l] = ${letter[l]}"
done

will print

letter [0] = h
letter [1] = e
letter [2] = l
letter [3] = l
letter [4] = o

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