#!/bin/bash

local    opt=""                      # when will you learn to declare what you use?
local    file_in=""                  # input file
local -i lFUsed=0                    # flag f-option was used = false

while getopts ":f:" opt; do
  case $opt in
    f) 
       file_in="$OPTARG"
       # a test for the file being readable/existing would fit nicely here, no?
       lFUsed=1               # change flag when -f was indeed used
       ;;

    \?)
       echo "Invalid option -$OPTARG" >&2
       ;;

  esac
done

if ! (( lFUsed )) ; then
     echo "no f-option was used, Aborting!." >&2
     exit 1
fi

exit 0