Bash: ${string:$i:1} what does this mean? - linux

Question

This is the script. It reverses a string entered by the user:
#!/bin/bash
read -p "Enter string:" string
len=${#string}
for (( i=$len-1; i>=0; i-- ))
do
# "${string:$i:1}"extract single single character from string.
reverse="$reverse${string:$i:1}"
done
echo "$reverse"
I don't understand the following part of the script. What is this? Looks like some kind of extended variable interpolation.
${string:$i:1}

Answer 1

in bash doing something lik this: ${string:3:1} means: take substring starting from the character at pos 3 (0-based, so the 4th character), and length = 1 character.
for example:
string=abc
then ${string:0:1} equals a and ${string:2:1} equals c.
This script takes the value of the variable $i: so it just takes the character at position $i.

Answer 2

It's substring expansion.
from the man pages:
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of parameter starting at the character specified by offset. If length is omitted, expands to the
substring of parameter starting at the character specified by offset. length and offset are arithmetic expressions (see ARITHMETIC EVALUATION below). If
offset evaluates to a number less than zero, the value is used as an offset from the end of the value of parameter. Arithmetic expressions starting with a -
must be separated by whitespace from the preceding : to be distinguished from the Use Default Values expansion. If length evaluates to a number less than
zero, and parameter is not # and not an indexed or associative array, it is interpreted as an offset from the end of the value of parameter rather than a
number of characters, and the expansion is the characters between the two offsets. If parameter is #, the result is length positional parameters beginning
at offset. If parameter is an indexed array name subscripted by # or *, the result is the length members of the array beginning with ${parameter[offset]}.
A negative offset is taken relative to one greater than the maximum index of the specified array. Substring expansion applied to an associative array proâ
duces undefined results. Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the :- expansion.
Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1 by default. If offset is 0, and the posiâ
tional parameters are used, $0 is prefixed to the list.