fix(user/bkdatev):Make compatible with Bash 3.2.57

- Note: In Bash 5.1, a negative length argument in a parameter
  substitution (i.e. the `$len` in `${myVar:$start_idx:$len}`) causes
  the last characters of the $myVar string to be truncated which is
  convenient in various applications. However, a negative length in Bash
  3.2.57 (a macOS machine I happen to have) throws an error; for
  compatibility, I've instead added an additional arithemetical
  operation to calculate the positive length that corresponds to a
  truncation by 2 of the total string length.