default_port_file=.default_port
default_port_value=56

get_default_port() {
   if [ -f $default_port_file ]
   then
      default_port=$(<$default_port_file)
   else
      default_port=$default_port_value
   fi
}

set_default_port() {
   if [[ $1 > ${default_port:-$default_port_value} ]]
   then
      echo $1 > $default_port_file
   fi
}

demo()
{
   get_default_port
   echo "Enter port number [$default_port]:"
   read port

   port=${port:-$default_port}
   echo "Port number = $port"
   echo

   set_default_port $((port+1))
}

until [[ "${ans:-y}" != "y" ]]
do
   demo
   echo "do u want re-run it (y/n) [y]:"
   read ans
done
exit

$ ls .default_port
/bin/ls: cannot access .default_port: No such file or directory
$ demo.sh
Enter port number [56]:

Port number = 56

do u want re-run it (y/n) [y]:
y
Enter port number [57]:
12
Port number = 12

do u want re-run it (y/n) [y]:
y
Enter port number [57]:
68
Port number = 68

do u want re-run it (y/n) [y]:
y
Enter port number [69]:

Port number = 69

do u want re-run it (y/n) [y]:
n
$ cat .default_port
70
$ demo.sh
Enter port number [70]:

Port number = 70

do u want re-run it (y/n) [y]:
n
$