exec 3<binary.file     # open the file for reading on file descriptor 3
read -N1 -u3 num       # read 1 character into variable "num"
read -N$num -u3 str    # read "num" chars
exec 3<&-              # close fd 3

exec 3<binary.file     # open the file for reading on file descriptor 3
IFS=                   #
read -N1 -u3 char      # read 1 character into variable "char"

# to obtain the ordinal value of the char "char"
num=$(printf %s "$char" | od -An -vtu1 | sed 's/^[[:space:]]*//')

read -N$num -u3 str    # read "num" chars
exec 3<&-              # close fd 3