Validate user response in shell script

My shell script is working fine and the files are copying to the remote directory. However I need to check if my INPUT_STRING is starting with S and also check if the files are present in the directory before I ftp it.

#!/bin/bash
echo "Enter if the tag is present in
        Dev
        Test
        Prod
        "
while :
do
  read -r INPUT_STRING
  INPUT_STRING=`echo $INPUT_STRING | tr '[:lower:]' '[:upper:]'`
  case $INPUT_STRING in
    test | TEST)
      echo "Please enter  Tag no : "
      read -r input_variable
      if [[ ${#input_variable} -ne "7" ]]
      then
        echo "Please check the  Tag no"
        exit 1
      fi
      HOST=xxxx
      USER=xxxx
      PASSWORD=xxxx
      mypath="/path/to/$input_variable/"
      ftp -inv $HOST <<- EOF > FTPLOG
        user $USER $PASSWORD
        cd "$mypath"
        pwd
        mput x
        mput y.csv
        mput x.csv
        mput a.csv
        mput b.out
        EOF
      fgrep "550 Failed to change directory" FTPLOG >& /dev/null
      if [[ $? -eq 0 ]]
      then
        echo "File is not transfered to the  tag $input_variable. Please check the
        tag no given"
      else
        echo "File is transfered to the  tag $input_variable"
      fi
      exit 1
      ;;
    *)
      echo "Error: Invalid option..."
      exit 1
      ;;
  esac
done