Save file content from editbox in BASH

I want to learn shell-scripting. I want to create simple tool to show file content with edit option but I can't get value from

#!/bin/bash

BACKTITLE="Some backtitle"

FILENAME="filename.txt"



touch $FILENAME



INPUT=/tmp/menu.sh.$$



ret=0



while [ $ret -eq 0 ]

do

    dialog --title "Menu" \

        --backtitle "$BACKTITLE"  \

        --menu "Wybierz" 10 60 3 \

        1 "Pokaz menu" \

        2 "Edytuj" \

        2>"${INPUT}"



    ret=$?

    option=$(<"${INPUT}")



    if [ $ret -eq 0 ]

    then

        if [ $option -eq 1 ]

        then

            dialog --title "File content" \

                --backtitle "$BACKTITLE" \

                --textbox $FILENAME 10 60

        elif [ $option -eq 2 ]

        then

            dialog --title "Edit file content" \

                --backtitle "$BACKTITLE" \

                --editbox $FILENAME 10 60



            editboxret=$?

            echo $editboxret

            ret=0

        fi

    fi

done