shell_script_if.sh

#!/usr/bin/env bash

ARG_PATH="/"
CONFIG_PATH="/"

if [ ! -z "$ARG_PATH" ]; then # if path was provided as argument
  if [ -d "$ARG_PATH" ]; then # check if it exists
    echo File exists!
    if [ -r $ARG_PATH ] && [ -s $ARG_PATH ]; then
      echo This file readable and not empty.
    fi
  else
    echo Provided path not found: $ARG_PATH
    exit 1
  fi
elif [ -d "$CONFIG_PATH" ]; then
  echo At least there is some config file
else 
  echo Path not provided
  exit 1
fi