CMD: piping ECHO to SET/ expanding variables in variables

Question

%x:~12,3% Returns 3 characters starting at the 12:th character in x variable. What I have been trying to accomplish is using variables instead of 12 an

Answer 1

@ECHO OFF
SETLOCAL

SET "x=abcdefghijklmmopqrstuvwxyz"

SET /a start=12
SET /a length=3

CALL SET "var=%%x:~%start%,%length%%%"
ECHO var=%var%
CALL echo %%x:~%start%,%length%%%

SETLOCAL ENABLEDELAYEDEXPANSION
SET /a start=6
SET /a length=4

SET "var=!x:~%start%,%length%!"
ECHO var=%var%
echo !x:~%start%,%length%!

GOTO :EOF

Two methods - the first in standard mode and the second using delayedexpansion. There are hundreds of examples on SO about delayedexpansion.