Learn batch script

strings

get substring

SET s=abcdefghijklmnopqrst

ECHO %s:~3,1%

REM # %<var_name>:~<start_index>,<length>%

REM # what if with variable index and length?

REM # copied from https://ss64.com/nt/syntax-substring.html 

SET _startchar=2

SET _length=1

SET _donor=884777

CALL SET _substring=%%_donor:~%_startchar%,%_length%%%

ECHO (%_substring%)