How to Overlay two files with UnionFs in a Linux System

UnionFS creates a unified seamless filesystem by transparently overlaying files and directories from separate filesystems. Each participant directory is referred to as a branch, and when mounting branches, we can set priorities and access modes. Thus, in this short guide, we will consider how to get it installed in Ubuntu 22.04. Furthermore, you’ll learn how to overlay two files with UnionFs.

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Prerequisites to Overlaying Files With UnionFs

To overlay two files using UnionFs, you need the following:

A Linux Machine (Ubuntu 18 or Higher)
A user account with sudo privileges

Step 1: Update the package index

you can use the command below :

apt-get update

Step 2: Install Unionfs

Install the appropriate UnionFs packages specific to your distribution. Hence, you can use the command below:

sudo apt install -y unionfs-fuse

Step 3: Create two separate directories (branches) with two files each respectively

Once you’ve installed UnionFs on your Linux system, proceed to overlay the two files by creating directories. So, the two directories will serve as a source for the overlay.

mkdir dir1
touch dir1/f1
touch dir1/f2
mkdir dir2
touch dir2/f3
touch dir2/f4

How to Overlay two files with UnionFs-mkdir — directories and files

Step 4: Let’s create an empty directory to overlay files

To overlay the two directories, we need to mount them using UnionFs. Meanwhile, this empty directory is where union will be mounted and we can do this with the command as shown below:

mkdir union

How to Overlay two files with UnionFs-union — dir union

Step 5: Mount the two branches

To successfully overlay files with UnionFs, you need to ensure you mount them. Therefore, we can overlay the two branches on top of each with the command :