#!/bin/bash

 mkdir lower.readonly upper.writeable merged work
 mkdir lower.readonly/subdir1
 echo "unteres" >lower.readonly/eins
 echo "unteres" >lower.readonly/zwei
 echo "unteres" >lower.readonly/drei
 echo "oberes" >upper.writeable/drei
 echo "oberes" >upper.writeable/vier

 sudo mount -t overlay -o \
   lowerdir=lower.readonly,upperdir=upper.writeable,workdir=work \
   overlay merged

 ls -l lower.readonly upper.writeable merged work